RBSE Solutions Class 10 Science Chapter 6 Chemical Reaction and Catalyst

I. Multiple Choice Questions

Question 1. Which change happens when \( FeCl_3 \) changes to \( FeCl_2 \)?
(a) Oxidation
(b) Reduction
(c) Decomposition
(d) Combination
Answer: (b) Reduction
In simple words: When \( FeCl_3 \) turns into \( FeCl_2 \), it means the iron atom has gained an electron. This process of gaining electrons is called reduction.

🎯 Exam Tip: Understanding changes in oxidation states (e.g., iron changing from +3 to +2) is key to identifying oxidation and reduction reactions correctly.

Question 2. When a substance breaks down into simple molecules, then which type of reaction happens?
(a) Decomposition
(b) Displacement
(c) Oxidation
(d) Combination
Answer: (a) Decomposition
In simple words: When something complex splits into smaller, simpler parts, we call that a decomposition reaction. It's like breaking a big LEGO model into individual bricks.

🎯 Exam Tip: Look for a single reactant breaking into multiple products to identify a decomposition reaction, which often requires energy input.

Question 3. Substances which donate electrons are known as?
(a) Oxidizing agent
(b) Catalyst
(c) Reducing agent
(d) None of the options
Answer: (c) Reducing agent
In simple words: Things that give away electrons are called reducing agents. They cause other things to gain electrons, and in doing so, they themselves get oxidized.

🎯 Exam Tip: Remember that reducing agents donate electrons, while oxidizing agents accept electrons; these are opposite roles in a redox reaction.

Question 4. Which of the following reaction is an example of reversible reaction?
(a) \( N_2 + O_2 \leftrightarrow 2NO \)
(b) \( H_2 + I_2 \leftrightarrow 2HI \)
(c) \( PCl_5 \leftrightarrow PCl_3 + Cl_2 \)
(d) All of the options
Answer: (d) All of the options
In simple words: A reversible reaction is one where the chemicals can turn into new things, and those new things can also turn back into the original chemicals. All the given examples show reactions that can go in both directions.

🎯 Exam Tip: Look for the double-headed arrow (\(\leftrightarrow\) or \(\rightleftharpoons\)) in an equation, which always indicates a reversible reaction where equilibrium can be established.

Question 5. Substance which increases the speed of reaction is known as?
(a) Catalyst
(b) Oxidizing Agent
(c) Reducing agent
(d) None of the options
Answer: (a) Catalyst
In simple words: A catalyst is like a helper that makes a chemical reaction go faster, but it doesn't get used up itself and can be recovered at the end.

🎯 Exam Tip: Catalysts are crucial in many industrial processes to make reactions more efficient by lowering the activation energy.

Question 6. Enzymes are which type of catalyst?
(a) Negative catalyst
(b) Positive catalyst
(c) Autocatalyst
(d) Bio-catalyst
Answer: (d) Bio-catalyst
In simple words: Enzymes are special types of catalysts found in living things like plants and animals. They help speed up body reactions, such as digestion.

🎯 Exam Tip: Remember that all enzymes are catalysts, but not all catalysts are enzymes; enzymes are biological in nature and highly specific.

Question 7. What is happening to magnesium in this reaction: \( 2Mg + O_2 \rightarrow 2MgO \)?
(a) Oxidation
(b) Reduction
(c) Decomposition
(d) Displacement
Answer: (a) Oxidation
In simple words: In this reaction, magnesium combines with oxygen to form magnesium oxide. When a substance gains oxygen, we call that oxidation.

🎯 Exam Tip: Oxidation can be broadly defined as the gain of oxygen, the loss of hydrogen, or the loss of electrons by a substance.

Question 8. Which symbol is used for reversible reaction?
(a) \( \leftrightarrow \)
(b) \( \rightarrow \)
(c) \( \leftarrow \)
(d) \( \rightleftharpoons \)
Answer: (d) \( \rightleftharpoons \)
In simple words: The symbol with two half-arrows pointing in opposite directions means a reaction can go both forwards and backwards. This shows it is a reversible reaction where equilibrium is reached.

🎯 Exam Tip: Always use the correct arrow symbol: \( \rightarrow \) is for irreversible reactions, while \( \leftrightarrow \) or \( \rightleftharpoons \) is for reversible reactions, with \( \rightleftharpoons \) often indicating equilibrium.

Question 9. A reaction which is catalyzed by its own product is known by which name?
(a) Biochemical
(b) Reversible
(c) Autocatalysis
(d) Irreversible
Answer: (c) Autocatalysis
In simple words: When a reaction creates a product that then helps the reaction go even faster, it's called autocatalysis. The reaction effectively speeds itself up.

🎯 Exam Tip: Autocatalysis is a special type of catalysis where a product of the reaction itself acts as a catalyst, often leading to a rapid acceleration of the reaction.

Question 10. What happens in exothermic reaction?
(a) Heat is released
(b) Heat is absorbed
(c) Heat is distributed
(d) None of the options
Answer: (a) Heat is released
In simple words: In an exothermic reaction, heat energy is given out to the surroundings. This makes the area around the reaction feel warmer.

🎯 Exam Tip: Remember 'exo' means 'out', so exothermic means heat goes out, making the surroundings feel hot; conversely, 'endo' means 'in' for endothermic reactions.

Chemical Reaction And Catalyst Very Short Answer Type Questions

Question 11. What is a chemical change?
Answer: A chemical change is a process where one or more new substances are formed from the original substances. Unlike physical changes, it involves the rearrangement of atoms and the formation of new chemical bonds.
In simple words: A chemical change is when a completely new thing is made from old things, and you can't easily turn it back into what it was before.

🎯 Exam Tip: Key indicators of a chemical change include the formation of a new substance, a change in color, heat/light release, or gas production.

Question 12. Name the catalyst which facilitates change of vegetable oil to vegetable ghee.
Answer: Nickel is the catalyst used to facilitate the change of vegetable oil to vegetable ghee. Nickel is commonly used in the hydrogenation process, converting unsaturated fats (oils) into saturated fats (ghee).
In simple words: Nickel is the catalyst that helps turn liquid vegetable oil into solid vegetable ghee by adding hydrogen to the oil.

🎯 Exam Tip: Remember nickel is crucial for hydrogenation, a process used to solidify liquid oils and produce products like margarine and vegetable ghee.

Question 13. Name the product obtained when lime water reacts with carbon dioxide.
Answer: Calcium carbonate is the product obtained when lime water reacts with carbon dioxide. This reaction is often used as a test for carbon dioxide, as the calcium carbonate formed is a white precipitate that turns lime water milky.
In simple words: When lime water (calcium hydroxide solution) meets carbon dioxide, a white solid called calcium carbonate is formed. This is why lime water turns milky.

🎯 Exam Tip: The formation of a white precipitate (calcium carbonate) and the turning of lime water milky is a classic and definitive test for the presence of carbon dioxide gas.

Question 14. This is an example of which type of reaction: \( Zn + CuSO_4 \rightarrow ZnSO_4 + Cu \)?
Answer: This is a displacement reaction. In this reaction, zinc, being more reactive than copper, replaces copper from its salt solution.
In simple words: When zinc replaces copper in a chemical mixture, it's called a displacement reaction. It's like one element pushing another out of its spot because it's stronger.

🎯 Exam Tip: Displacement reactions occur when a more reactive metal displaces a less reactive metal from its compound, often seen as one element 'swapping' places with another.

Question 15. Give an example of redox reaction.
Answer: An example of a redox reaction is: \( 2Mg + O_2 \rightarrow 2MgO \). Here, magnesium (Mg) gets oxidized because it gains oxygen, and oxygen (\( O_2 \)) gets reduced because it combines with magnesium. Redox reactions always involve both oxidation (loss of electrons) and reduction (gain of electrons) happening simultaneously.
In simple words: When magnesium burns with oxygen to make magnesium oxide, magnesium is oxidized (gains oxygen) and oxygen is reduced (combines with magnesium). This is a redox reaction because both actions happen together.

🎯 Exam Tip: A key characteristic of redox reactions is the transfer of electrons between reactants, even if it's primarily defined by oxygen or hydrogen transfer.

Question 16. What is a reversible reaction?
Answer: A reversible reaction is a chemical reaction that can proceed in both the forward and reverse directions. This means that as products are formed, they can also react to form back the original reactants. The reaction eventually reaches a state of dynamic equilibrium.
In simple words: A reversible reaction is like a two-way street for chemicals; they can turn into new things and then turn back into the old things again.

🎯 Exam Tip: Reversible reactions are always indicated by a double arrow (\(\leftrightarrow\) or \(\rightleftharpoons\)) in the chemical equation, distinguishing them from irreversible reactions.

Question 17. What is the purpose of catalyst promoter and catalyst poison?
Answer: A catalyst promoter is a substance that enhances the activity or efficiency of a catalyst, making it work better. Conversely, a catalyst poison is a substance that suppresses or reduces the function of a catalyst, often by binding to its active sites. Promoters and poisons are used to fine-tune catalytic activity, optimizing industrial processes for desired outcomes.
In simple words: A catalyst promoter helps a catalyst work even better, making reactions faster. A catalyst poison, however, slows down or stops a catalyst from working at all.

🎯 Exam Tip: Think of promoters as 'boosters' and poisons as 'inhibitors' for catalysts; they affect the catalyst's performance rather than participating directly in the main reaction.

Question 18. Which type of reaction happens between an acid and a base?
Answer: A neutralization reaction happens between an acid and a base. Neutralization reactions typically produce salt and water, bringing the pH of the solution closer to 7.
In simple words: When an acid and a base mix, they do a neutralization reaction. This reaction often makes salt and water, making the final mixture less acidic or basic.

🎯 Exam Tip: The defining feature of a neutralization reaction is the reaction between an acid and a base to form salt and water, usually resulting in a neutral solution if the strengths are balanced.

Question 19. How many types of reactions are there on the basis of speed?
Answer: Chemical reactions can be classified into three types based on their speed:
1. **Very Fast Reactions:** These reactions occur almost instantaneously, often within fractions of a second. An example is the precipitation of silver chloride when silver nitrate reacts with sodium chloride.
2. **Moderate Reactions:** These reactions proceed at a measurable rate, taking minutes to hours. Examples include the hydrolysis of esters or the inversion of cane sugar.
3. **Very Slow Reactions:** These reactions take a very long time, sometimes days, months, or even years, to complete. A classic example is the rusting of iron.
Understanding reaction speed is vital in fields like chemical engineering, enabling control over manufacturing processes.
In simple words: Reactions can be grouped into three types by how fast they happen: very fast (instant), moderate (take some time), or very slow (take a long time). Each type describes how quickly chemicals change into new ones.

🎯 Exam Tip: When classifying reactions by speed, be ready to give examples that clearly demonstrate instantaneous, measurable, and prolonged reaction times.

Question 20. Give an example of a decomposition reaction caused by heat.
Answer: When calcium carbonate is heated up to \( 473K \), it breaks down to form calcium oxide and carbon dioxide.
\( CaCO_3 \xrightarrow{\text{heat}} CaO + CO_2 \)
This thermal decomposition is a common industrial process for producing lime (calcium oxide) from limestone (calcium carbonate).
In simple words: If you heat up calcium carbonate, it splits into two simpler things: calcium oxide and carbon dioxide. This is a breakdown reaction that needs heat to start.

🎯 Exam Tip: Thermal decomposition reactions are identified by the use of heat to break down a single compound into two or more simpler substances.

Question 21. What is the function of catalyst in a chemical reaction?
Answer: A catalyst's primary function in a chemical reaction is to alter the speed of the reaction. It can either increase (positive catalyst) or decrease (negative catalyst) the reaction rate without being consumed in the process. Catalysts achieve this by providing an alternative reaction pathway with a lower activation energy.
In simple words: A catalyst's job is to change how fast a chemical reaction goes. It can make it faster or sometimes slower, but it doesn't get used up itself.

🎯 Exam Tip: Remember, a catalyst changes the reaction rate but is not consumed, does not change the overall energy change, and does not shift the equilibrium position of a reversible reaction.

Question 22. What is the fundamental principle for balancing a chemical equation?
Answer: The fundamental principle for balancing a chemical equation is the Law of Conservation of Mass. This law states that mass cannot be created or destroyed in a chemical reaction, meaning the total mass of reactants must equal the total mass of products. This ensures that the number of atoms for each element remains the same on both sides of the equation.
In simple words: The main idea for balancing equations is the Law of Conservation of Mass. This law says that during a reaction, no mass is lost or gained; atoms are just rearranged, so the total number of atoms of each element must be the same on both sides.

🎯 Exam Tip: Balancing chemical equations is essential to uphold the Law of Conservation of Mass, ensuring that the number of atoms of each element is the same on both sides of the equation.

Question 23. What is a redox reaction?
Answer: A redox reaction is a chemical reaction where both oxidation and reduction processes occur simultaneously. Oxidation involves the loss of electrons (or gain of oxygen/loss of hydrogen), while reduction involves the gain of electrons (or loss of oxygen/gain of hydrogen). These two processes are always coupled.
In simple words: A redox reaction is when one chemical loses electrons (oxidizes) and another chemical gains electrons (reduces) all at once. They always happen together in the same reaction.

🎯 Exam Tip: Always identify both the oxidized species (the one losing electrons) and the reduced species (the one gaining electrons) to confirm a redox reaction.

Question 24. Burning of coal is which type of reaction?
Answer: Burning of coal is an oxidation reaction. Specifically, it is a combustion reaction, which is a rapid form of oxidation that produces heat and light.
In simple words: Burning coal is an oxidation reaction because coal combines with oxygen from the air. This process quickly releases energy as heat and light.

🎯 Exam Tip: Combustion reactions are a specific type of exothermic oxidation reaction that release significant amounts of energy.

Question 25. What will be the pH of a solution obtained after reaction between a strong acid and a strong base?
Answer: The pH of a solution obtained after a complete reaction between a strong acid and a strong base will be 7. A pH of 7 indicates a neutral solution, which is the expected result when a strong acid fully neutralizes a strong base.
In simple words: When a strong acid and a strong base react completely, the solution made will have a pH of 7. This means it is neutral, not acidic or basic.

🎯 Exam Tip: Remember that strong acid-strong base neutralization always results in a neutral solution (pH 7) at the equivalence point, assuming complete reaction.

Chemical Reaction And Catalyst Short Answer Type Questions

Question 26. Distinguish between physical and chemical changes.
Answer:

Understanding these differences helps us categorize everyday transformations, from melting ice (physical) to burning wood (chemical).
In simple words: A physical change only alters how something looks, like its shape or size, but it's still the same chemical. A chemical change makes a brand new substance that you can't easily turn back into the original.

🎯 Exam Tip: Focus on whether new substances are formed or only the appearance changes to differentiate clearly between chemical and physical changes.

Auto Catalyst

When a product of the reaction enhances the speed of the reaction, it is known as an auto catalyst. For example: \( CH_3COOC_2H_5 + H_2O \rightarrow CH_3COOH + C_2H_5OH \)

Bio-Catalyst

A bio-catalyst is a substance that catalyzes biochemical reactions. For example: \( \text{Maltose} \xrightarrow{\text{Maltase}} \text{Glucose} \)

Question 27. Explain combination and decomposition reactions with suitable examples.
Answer: A **combination reaction** happens when two or more substances join together to form a single, new product. For example, when carbon burns in air, it combines with oxygen to make carbon dioxide: \( C + O_2 \rightarrow CO_2 \). On the other hand, a **decomposition reaction** is when one complex substance breaks down into two or more simpler products. For instance, water can be broken down into hydrogen and oxygen gas using electricity (electrolysis): \( 2H_2O (l) \xrightarrow{\text{electric current}} 2H_2(g) + O_2(g) \). These reactions are opposites, one building up and one breaking down.
In simple words: A combination reaction puts two or more things together to make one new thing. A decomposition reaction breaks one big thing into two or more smaller things. They are opposite types of reactions.

🎯 Exam Tip: Combination reactions typically involve simpler reactants forming a more complex product, while decomposition reactions involve a complex reactant breaking down into simpler ones, often requiring energy.

Question 28. Following is an example of which type of reaction: \( AgNO_3 + KCl \rightarrow AgCl + KNO_3 \)? Explain.
Answer: This is an example of a **double displacement reaction**. In this type of reaction, the ions of two reactant compounds switch places. Here, silver from silver nitrate switches with potassium from potassium chloride to form silver chloride and potassium nitrate. Specifically, the silver ion replaces the potassium ion, and the nitrate ion replaces the chloride ion. This reaction often results in the formation of a precipitate, like silver chloride.
In simple words: This is a double displacement reaction. It means the parts of two chemicals swap partners. Silver and potassium exchange places, creating two new substances.

🎯 Exam Tip: Double displacement reactions are characterized by the exchange of ions between two reactant compounds, often resulting in the formation of a precipitate, gas, or water.

Oxidation:

\( Na \rightarrow Na^+ + e^- \); \( 2Cl^- \rightarrow Cl_2 + 2e^- \)

Reduction:

\( Cl + e^- \rightarrow Cl^- \); \( Mg^{2+} + 2e^- \rightarrow Mg \)

Question 30. Explain different types of catalyst.
Answer: Catalysts can be grouped in different ways based on their physical state compared to the reactants and products, and also by their action.
**Based on physical state:**
1. **Homogeneous Catalyst:** When the catalyst, reactants, and products are all in the same physical state (e.g., all gases or all liquids), it is a homogeneous catalyst. Example: In the reaction \( 2SO_2 (g) + O_2(g) \xrightarrow{NO (g)} 2SO_3 (g) \), nitric oxide (NO) is a gaseous catalyst for gaseous reactants.
2. **Heterogeneous Catalyst:** When the catalyst, reactants, and products are in different physical states, it is a heterogeneous catalyst. Example: In the synthesis of ammonia \( N_2 (g) + 3H_2(g) \xrightarrow{Fe (s)} 2NH_3 (g) \), nitrogen and hydrogen are gases, but iron (Fe) is a solid catalyst.
**Based on action:**
1. **Positive Catalyst:** A positive catalyst enhances or increases the speed of a reaction. Example: \( 2KClO_3 \xrightarrow{MnO_2} 2KCl + 3O_2 \), where manganese dioxide speeds up the decomposition of potassium chlorate.
2. **Negative Catalyst:** A negative catalyst (also known as an inhibitor) reduces or slows down the speed of a reaction. Example: \( 2H_2O_2 \xrightarrow{glycerol} 2H_2O + O_2 \), where glycerol slows the decomposition of hydrogen peroxide.
Understanding these classifications helps in selecting the right catalyst for specific industrial processes.
In simple words: Catalysts are of different kinds. Some are called **homogeneous** when they are in the same physical state (like all gas or all liquid) as the other chemicals. Others are **heterogeneous** when they are in a different physical state. There are also **positive catalysts** that speed up reactions, and **negative catalysts** that slow them down.

🎯 Exam Tip: Remember to classify catalysts based on both their physical state relative to reactants/products and their effect on reaction speed (positive or negative), providing a clear example for each type.

Question 31. Explain different types of decomposition reaction.
Answer: Decomposition reactions, where a single compound breaks down into two or more simpler substances, can be classified into different types based on the energy source that causes the breakdown:
1. **Thermal decomposition:** This type occurs when heat energy is used to break a compound into two or more simpler substances. For example, heating calcium carbonate to get calcium oxide and carbon dioxide: \( CaCO_3 \xrightarrow{\Delta} CaO + CO_2 \).
2. **Electrolytic decomposition:** This involves using electrical energy to break down a compound. An example is passing electricity through water to separate it into hydrogen and oxygen gas: \( 2H_2O (l) \xrightarrow{\text{electric current}} 2H_2(g) + O_2(g) \).
3. **Photochemical decomposition:** This reaction uses light energy to break down a compound. An example is the decomposition of silver bromide into silver and bromine when exposed to sunlight, often used in photography: \( 2AgBr (s) \xrightarrow{\text{sunlight}} 2Ag(s) + Br_2(g) \). Each type requires a specific form of energy to initiate the breaking of chemical bonds.
In simple words: Decomposition reactions, where one thing breaks into many, can happen in three ways: by **heat** (thermal), by **electricity** (electrolytic), or by **light** (photochemical). Each way uses a different kind of energy to split the substance.

🎯 Exam Tip: When discussing types of decomposition, always provide a specific energy source (heat, electricity, or light) and a relevant chemical equation as an example for each type of reaction.

Question 32. Chloroform is stored with a small amount of ethyl alcohol. Why?
Answer: Chloroform has a tendency to react with oxygen from the air, especially in the presence of light. This reaction results in the production of phosgene, which is a highly poisonous gas. A small amount of ethanol (ethyl alcohol) is added to chloroform to prevent this unwanted oxidation reaction. Ethanol acts as a negative catalyst, inhibiting the formation of phosgene and ensuring the stability of chloroform.
In simple words: Chloroform reacts with air and light to make a dangerous gas called phosgene. So, a tiny bit of alcohol (ethanol) is added to chloroform to stop this bad reaction from happening and keep it safe.

🎯 Exam Tip: Remember that storing reactive chemicals properly, often with stabilizers or inhibitors like ethanol, is crucial for safety and maintaining their chemical integrity.

Question 33. When a weak acid reacts with a strong base, the resulting salt solution is alkaline. Why?
Answer: When a weak acid reacts with a strong base, the resulting salt solution becomes alkaline (basic). This happens because a weak acid does not fully break apart (ionize) in water. However, the strong base completely ionizes, releasing many hydroxide (\( OH^- \)) ions into the solution. Since the strong base provides more \( OH^- \) ions than the weak acid provides \( H^+ \) ions, the final solution will have an excess of hydroxide ions, making it alkaline.
In simple words: When a weak acid mixes with a strong base, the final liquid is alkaline (basic). This is because the strong base releases more "basic" particles than the weak acid releases "acidic" particles, making the mixture slightly basic.

🎯 Exam Tip: Understand that the strength of the acid and base determines the pH of the resulting salt solution; a strong component will dominate the pH if the other component is weak.

Question 34. Are the following reactions possible? Justify your answer.
(i) \( Cu + ZnSO_4 \rightarrow CuSO_4 + Zn \)
(ii) \( Fe + CuSO_4 \rightarrow FeSO_4 + Cu \)
Answer:
(i) This reaction is **not possible** because copper is less reactive than zinc. According to the reactivity series of metals, a less reactive metal cannot displace a more reactive metal from its salt solution. Therefore, copper cannot displace zinc from zinc sulfate.
(ii) This reaction **is possible** because iron is more reactive than copper. In the reactivity series, iron comes before copper, indicating it can displace copper from its salt solution.
In simple words:
(i) No, copper cannot replace zinc because copper is not as active as zinc. Only a more active metal can push a less active metal out of its compound.
(ii) Yes, iron can replace copper because iron is more active than copper. So, iron can push copper out of its compound.

🎯 Exam Tip: To determine if a displacement reaction is possible, always refer to the reactivity series of metals; a more reactive metal will displace a less reactive one from its compound.

Question 35. Find oxidation and reduction in the following reactions:
(i) \( C + O_2 \rightarrow CO_2 \)
(ii) \( Mg + Cl_2 \rightarrow MgCl_2 \)
(iii) \( ZnO + C \rightarrow Zn + CO \)
(iv) \( Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2 \)
Answer:
(i) In \( C + O_2 \rightarrow CO_2 \): Carbon (C) undergoes **oxidation** (gains oxygen), and Oxygen (\( O_2 \)) undergoes **reduction** (combines with carbon).
(ii) In \( Mg + Cl_2 \rightarrow MgCl_2 \): Magnesium (Mg) undergoes **oxidation** (loses electrons to chlorine), and Chlorine (\( Cl_2 \)) undergoes **reduction** (gains electrons from magnesium).
(iii) In \( ZnO + C \rightarrow Zn + CO \): Zinc oxide (ZnO) undergoes **reduction** (loses oxygen), and Carbon (C) undergoes **oxidation** (gains oxygen).
(iv) In \( Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2 \): Iron oxide (\( Fe_2O_3 \)) undergoes **reduction** (loses oxygen), and Carbon monoxide (CO) undergoes **oxidation** (gains oxygen).
In all these redox reactions, the substance that undergoes oxidation loses electrons, while the substance undergoing reduction gains electrons.
In simple words:
(i) Carbon is oxidized, oxygen is reduced.
(ii) Magnesium is oxidized, chlorine is reduced.
(iii) Zinc oxide is reduced, carbon is oxidized.
(iv) Iron oxide is reduced, carbon monoxide is oxidized.

🎯 Exam Tip: Remember that oxidation involves gaining oxygen, losing hydrogen, or losing electrons, while reduction involves losing oxygen, gaining hydrogen, or gaining electrons.

Question 29. Explain different types of chemical reactions.
Answer: Chemical reactions can be categorized into several types based on how reactants transform into products:
1. **Combination reaction:** This occurs when two or more reactants combine to form a single, more complex product.
Example: \( A + B \rightarrow AB \).
A specific example is when carbon burns in air: \( C + O_2 \rightarrow CO_2 \). Another example is the burning of magnesium ribbon: \( 2Mg + O_2 \rightarrow 2MgO \).
2. **Decomposition reaction:** This is the opposite of a combination reaction, where a single complex reactant breaks down into two or more simpler products.
Example: \( AB \rightarrow A + B \).
A specific example is when calcium carbonate is heated: \( CaCO_3 \xrightarrow{\Delta} CaO + CO_2 \). Another example is the electrolysis of water: \( 2H_2O (l) \xrightarrow{\text{electric current}} 2H_2(g) + O_2(g) \).
3. **Displacement reaction:** In this reaction, a more reactive element displaces (replaces) a less reactive element from its compound.
Example: \( AB + C \rightarrow AC + B \).
A specific example is when zinc granules are placed in copper sulfate solution, where zinc displaces copper: \( Zn + CuSO_4 \rightarrow ZnSO_4 + Cu \).
4. **Double displacement reaction:** This type involves an exchange of ions between two reactant compounds, forming two new compounds.
Example: \( AB + CD \rightarrow AD + CB \).
A specific example is the reaction between sodium hydroxide and copper sulfate: \( CuSO_4 + 2NaOH \rightarrow Cu(OH)_2 + Na_2SO_4 \), where hydroxide and sulfate ions switch partners.
5. **Oxidation-Reduction Reaction (Redox Reaction):** These reactions involve both oxidation and reduction occurring simultaneously.
* **Based on Oxygen/Hydrogen Transfer:** Oxidation is the addition of oxygen or removal of hydrogen. Reduction is the removal of oxygen or addition of hydrogen. Example: \( 2Mg + O_2 \rightarrow 2MgO \) (Mg is oxidized; O is reduced). Example: \( 2H_2S + O_2 \rightarrow 2H_2O + 2S \) (H2S is oxidized; O2 is reduced).
* **Based on Electropositive/Electronegative Element Transfer:** Oxidation involves the removal of an electropositive element or the addition of an electronegative element. Reduction involves the addition of an electropositive element or the removal of an electronegative element. Example: \( 2KI + Cl_2 \rightarrow 2KCl + I_2 \) (KI is oxidized, Cl2 is reduced). Example: \( 2FeCl_3 + H_2 \rightarrow 2FeCl_2 + 2HCl \) (FeCl3 is reduced, H2 is oxidized).
* **Based on Electron Transfer:** Oxidation is the loss of electrons. Reduction is the gain of electrons. Example: \( Na \rightarrow Na^+ + e^- \) (Na is oxidized). Example: \( 2Cl^- \rightarrow Cl_2 + 2e^- \) (Cl- is oxidized). Example: \( Cl + e^- \rightarrow Cl^- \) (Cl is reduced). Example: \( Mg^{2+} + 2e^- \rightarrow Mg \) (Mg2+ is reduced).
In simple words: There are several types of chemical reactions:
**Combination reaction:** Two or more things join to make one new thing.
**Decomposition reaction:** One big thing breaks into two or more smaller things.
**Displacement reaction:** One part of a chemical replaces another part.
**Double displacement reaction:** Parts of two chemicals swap places with each other.
**Redox reaction:** One chemical loses electrons (oxidizes) and another gains electrons (reduces) at the same time.

🎯 Exam Tip: For each type of reaction, clearly define it, provide a general form (like A+B->AB), and give a specific, balanced chemical equation as an example for full marks.

Question 38. What do you know about types of catalyst and characteristics of catalyst?
Answer: Catalysts are chemical substances that change the rate of a chemical reaction without being consumed themselves. They can be classified in several ways:
**Types of Catalysts (based on physical state):**
1. **Homogeneous Catalyst:** In this type, the catalyst, reactants, and products all exist in the same physical state (e.g., all being gases or all liquids). For example, \( CH_3COOCH_3 (l) + H_2O(l) \xrightarrow{HCl (aq)} CH_3COOH (aq) + CH_3OH (aq) \), where HCl is a liquid catalyst for liquid reactants. Another example is \( 2SO_2 (g) + O_2(g) \xrightarrow{NO (g)} 2SO_3 (g) \), where NO is a gaseous catalyst.
2. **Heterogeneous Catalyst:** Here, the catalyst is in a different physical state from the reactants and products. For example, in the synthesis of ammonia: \( N_2 (g) + 3H_2(g) \xrightarrow{Fe (s)} 2NH_3 (g) \), where gaseous reactants react on a solid iron catalyst. Another example is the hydrogenation of vegetable oil: \( \text{vegetable oil (l) + H_2 (g)} \xrightarrow{Ni (s)} \text{vegetable ghee (s)} \), with solid nickel as the catalyst.
Other types include **Autocatalysts** (where a reaction product acts as a catalyst) and **Bio-catalysts** (enzymes in biological systems).
**Characteristics of Catalyst:**
* A catalyst primarily alters the speed of a reaction (either speeding it up or slowing it down) but does not participate as a reactant or product.
* Only a minute quantity of a catalyst is usually sufficient to influence a large amount of reactants.
* Most reactions require a specific catalyst; one catalyst cannot work effectively for all reactions.
* A catalyst does not initiate a reaction that would otherwise not occur; it only affects the rate of existing reactions.
* A catalyst affects the rate of the forward and backward reactions equally, thus it does not change the position of equilibrium in a reversible reaction.
In simple words: Catalysts are grouped by their state compared to the other chemicals: **homogeneous** (same state) or **heterogeneous** (different state).
**Key features of catalysts:** They only change how fast a reaction goes, but they don't get used up. You only need a tiny bit. Each reaction needs its own special catalyst, and a catalyst can't start a reaction from nothing. They speed up both ways of a reversible reaction equally.

🎯 Exam Tip: When describing catalysts, remember to cover both their classification (homogeneous/heterogeneous) and their fundamental properties, such as not being consumed and affecting reaction rates without changing equilibrium.

Question 39. Write the steps of writing a chemical equation. What are the characteristics of a chemical equation?
Answer: To write a chemical equation, follow these steps:
1. First, write the symbols or formulas for the reactants (the starting chemicals) on the left side. Use a plus sign (+) between different reactants.
2. Draw an arrow (→) after the reactants. This arrow means "reacts to form" or "yields."
3. Next, write the symbols or formulas for the products (the new chemicals formed) on the right side of the arrow. Use a plus sign (+) between different products.
4. Show the physical state of each reactant and product using small letters in brackets: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous (dissolved in water).
5. If there are special conditions for the reaction, like heat (often indicated by \( \Delta \) or 'heat' above the arrow), pressure, or a catalyst, write them above or below the arrow.
6. Finally, make sure the number of atoms for each element is the same on both sides of the arrow. This is called balancing the equation, and it follows the Law of Conservation of Mass.
**Characteristics of a Chemical Equation:**
A chemical equation provides a lot of important information about a reaction:
* It shows all the reactants and products involved.
* It tells you the number of molecules and the relative masses of the substances participating.
* It indicates the physical state (solid, liquid, gas, aqueous) of each substance.
* It can show the conditions needed for the reaction to happen, like temperature, pressure, or catalysts involved.
* A balanced chemical equation reflects the conservation of atoms and mass during the reaction.
In simple words: When you write a chemical equation, you put the starting chemicals on the left, then an arrow, then the new chemicals on the right. You also show if they are solid, liquid, or gas, and if you need heat or anything special.
**What an equation tells you:** It gives details about the chemicals involved, how many there are, their relative weights, and what conditions are needed for the reaction to take place.

🎯 Exam Tip: Ensure your chemical equations are balanced to reflect the conservation of mass, and always include state symbols and reaction conditions for clarity and completeness.

Question 40. Differentiate between the following:
(a) Reversible and irreversible reactions
(b) Catalyst promoter and catalyst poison
(c) Homogeneous and heterogeneous catalysts
(d) Oxidation and reduction
Answer:
(a) **Reversible and irreversible reactions:** A **reversible reaction** is one that can proceed in both the forward and reverse directions, eventually reaching an equilibrium state. It is indicated by a double arrow (\(\leftrightarrow\) or \(\rightleftharpoons\)). An **irreversible reaction** proceeds in only one direction, from reactants to products, and continues until one reactant is completely consumed. It is indicated by a single arrow (\(\rightarrow\)).
(b) **Catalyst promoter and catalyst poison:** A **catalyst promoter** is a substance that enhances the activity or efficiency of a catalyst, making the reaction faster or more selective. A **catalyst poison** is a substance that suppresses or reduces the activity of a catalyst, often by blocking its active sites.
(c) **Homogeneous and heterogeneous catalysts:** A **homogeneous catalyst** exists in the same physical state (phase) as the reactants and products. For example, a liquid catalyst in a liquid reaction mixture. A **heterogeneous catalyst** exists in a different physical state (phase) from the reactants. For example, a solid catalyst used for gaseous reactants.
(d) **Oxidation and reduction:** **Oxidation** is a process that involves the gain of oxygen, the loss of hydrogen, or the loss of electrons. **Reduction** is a process that involves the loss of oxygen, the gain of hydrogen, or the gain of electrons. Oxidation and reduction always occur together in a redox reaction.
In simple words:
(a) **Reversible reactions** can go forwards and backwards; **irreversible reactions** only go forwards.
(b) A **catalyst promoter** helps a catalyst work better; a **catalyst poison** stops a catalyst from working well.
(c) A **homogeneous catalyst** is in the same physical state as the other chemicals; a **heterogeneous catalyst** is in a different physical state.
(d) **Oxidation** means gaining oxygen or losing electrons; **reduction** means losing oxygen or gaining electrons.

🎯 Exam Tip: For differentiation questions, always provide a clear, concise contrast for each pair, highlighting the key distinguishing features and, if possible, brief examples.

Chemical Reaction And Catalyst Additional Questions Solved

I. Multiple Choice Questions

Question 1. In the reaction, \( SO_2 (g) + 2H_2S (g) \rightarrow 2H_2O(l) + S(s) \), the reducing agent is
(a) \( SO_2 \)
(b) \( H_2O \)
(c) \( H_2S \)
(d) \( S \)
Answer: (c) \( H_2S \)
In simple words: In this reaction, \( H_2S \) is the reducing agent because it gives away hydrogen to \( SO_2 \), causing \( SO_2 \) to become \( H_2O \). In the process, \( H_2S \) itself gets oxidized to \( S \).

🎯 Exam Tip: Identify the reducing agent as the substance that loses electrons or hydrogen, or gains oxygen, while causing another substance to be reduced.

Question 2. Which of the following statements about a chemical equation is false?
(a) Physical states of reactants and products
(b) Symbols and formula of all the substances involved in a particular reaction
(c) Number of atoms/molecules of the reactants and products formed
(d) Whether a particular reaction is actually feasible or not
Answer: (d) Whether a particular reaction is actually feasible or not
In simple words: A chemical equation tells you what chemicals are involved, how many of them, and their states (like solid or gas). But it doesn't automatically tell you if the reaction can actually happen easily or at all, which is called its feasibility.

🎯 Exam Tip: A chemical equation primarily provides stoichiometric and state information; thermodynamic feasibility requires additional considerations like Gibbs free energy, which are not directly shown in the equation.

Question 3. Chemically rust is
(a) hydrated ferrous oxide
(b) only ferric oxide
(c) hydrated ferric oxide
(d) none of the options
Answer: (c) hydrated ferric oxide
In simple words: Rust is technically called hydrated ferric oxide. It's the brownish material that forms when iron gets wet and mixes with air, and it includes water molecules.

🎯 Exam Tip: Remember that rust is not just ferric oxide (\( Fe_2O_3 \)) but also contains water molecules (\( \cdot xH_2O \)), hence the term 'hydrated'.

Question 4. Which of the following are exothermic processes?
(i) Evaporation of water
(ii) Dilution of an acid \( (H_2SO_4) \)
(iii) Reaction of water with quick lime
(iv) Sublimation of camphor (crystals)
(a) (i) and (ii)
(b) (iii) and (iv)
(c) (i) and (iv)
(d) (ii) and (iii)
Answer: (d) (ii) and (iii)
In simple words: Exothermic processes give out heat to the surroundings. Diluting a strong acid and mixing water with quick lime both release heat, making them exothermic reactions. Evaporation and sublimation, however, absorb heat.

🎯 Exam Tip: Exothermic reactions typically cause the temperature of the surroundings to rise, while endothermic reactions cause it to drop; use this observable change as a key indicator.

Question 5. The following reaction is an example of
\( 4Fe(s) + 3O_2(g) \rightarrow 2Fe_2O_3(s) \)
(a) combination reaction
(b) redox reaction
(c) exothermic reaction
(d) All of the options
Answer: (d) All of the options
In simple words: This reaction, where iron combines with oxygen to make iron oxide, is a combination reaction (things joining), a redox reaction (electrons moving), and an exothermic reaction (it gives out heat). So, all these options are correct.

🎯 Exam Tip: Many reactions fit multiple classifications; analyze each aspect (combination, redox, energy change) to choose the most comprehensive answer when applicable.

Question 6. Methane on combustion gives
(a) \( CO_2 \)
(b) \( H_2O \)
(c) both \( CO_2 \) and \( H_2O \)
(d) neither \( CO_2 \) nor \( H_2O \)
Answer: (c) both \( CO_2 \) and \( H_2O \)
In simple words: When methane burns completely, it always produces both carbon dioxide and water. This is a common outcome for burning fuels that contain carbon and hydrogen.

🎯 Exam Tip: Remember that complete combustion of hydrocarbons always yields carbon dioxide and water as products.

Question 7. The electrolytic decomposition of water gives \( H_2 \) and \( O_2 \) in the ratio of
(a) 1: 2 by volume
(b) 2:1 by volume
(c) 8:1 by mass
(d) 1: 2 by mass
Answer: (b) 2:1 by volume
In simple words: When water is broken down by electricity, hydrogen gas is produced twice as much by volume as oxygen gas because water's formula is \( H_2O \).

🎯 Exam Tip: The ratio of hydrogen to oxygen by volume (2:1) is directly related to the subscript numbers in the chemical formula for water, \( H_2O \).

Question 8. In the decomposition of lead (II) nitrate to give lead (II) oxide, nitrogen dioxide and oxygen gas, the coefficient of nitrogen dioxide (in the balanced equation) is
(a) 1
(b) 2
(c) 3
(d) 4
Answer: (d) 4
In simple words: When lead (II) nitrate breaks down, the balanced chemical equation shows that there are four molecules of nitrogen dioxide produced.

🎯 Exam Tip: Balancing chemical equations involves ensuring the same number of atoms of each element on both sides of the reaction. For lead (II) nitrate, \( 2Pb(NO_3)_2(s) \rightarrow 2PbO(s) + 4NO_2(g) + O_2(g) \), you can count the atoms to verify the coefficients.

Question 10. Fatty foods become rancid due to the process of
(a) oxidation
(b) corrosion
(c) reduction
(d) hydrogenation
Answer: (a) oxidation
In simple words: Fatty foods spoil and get a bad smell or taste (become rancid) when they react with oxygen from the air, a process called oxidation.

🎯 Exam Tip: Rancidity is a key example of oxidation affecting food quality. Understanding this helps in food preservation techniques.

Question 11. We store silver chloride in a dark coloured bottle because it is
(a) a white solid
(b) undergoes redox reaction
(c) to avoid action by sunlight
(d) none of the options
Answer: (c) to avoid action by sunlight
In simple words: Silver chloride is kept in dark bottles because sunlight can break it down into silver and chlorine, changing its properties.

🎯 Exam Tip: Many light-sensitive chemicals, like silver halides used in photography, are stored in dark bottles to prevent unwanted decomposition reactions.

Question 12. Silver article turns black when kept in the open for a few days due to formation of
(a) \( H_2S \)
(b) \( AgS \)
(c) \( AgSO_4 \)
(d) \( Ag_2S \)
Answer: (d) \( Ag_2S \)
In simple words: Silver items turn black because they react with hydrogen sulfide gas in the air, forming a black layer of silver sulfide.

🎯 Exam Tip: This blackening is called tarnishing, and it's a chemical reaction specific to silver in the presence of sulfur compounds.

Question 13. Based on the reaction given below, what is the correct increasing order of reactivity of metals?
(a) Ag < Cu < Fe
(b) Cu > Fe > Ag
(c) Fe < Cu < Ag
(d) Cu < Ag < Fe
Answer: (a) Ag < Cu < Fe
In simple words: This order correctly shows that silver is the least reactive, copper is more reactive than silver, and iron is the most reactive among these three metals.

🎯 Exam Tip: The reactivity series helps predict whether a metal can displace another from its salt solution; a more reactive metal will displace a less reactive one.

Question 14. Dilute hydrochloric acid is added to granulated zinc taken in a test tube. The following observations are recorded. Point out the correct observation.
(a) The surface of metal becomes shining
(b) The reaction mixture turns milky
(c) Odour of a pungent smelling gas is recorded
(d) A colourless and odourless gas is evolved
Answer: (d) A colourless and odourless gas is evolved
In simple words: When zinc reacts with dilute hydrochloric acid, hydrogen gas is produced, which is colorless and has no smell.

🎯 Exam Tip: The evolution of a gas is a common sign of a chemical reaction. A simple test like a lit splint can confirm if the gas is hydrogen (it will make a 'pop' sound).

Question 15. When carbon dioxide is passed through lime water,
(a) calcium hydroxide is formed
(b) white precipitate of \( CaO \) is formed
(c) lime water turns milky
(d) colour of lime water disappears.
Answer: (c) lime water turns milky
In simple words: When carbon dioxide gas is bubbled through clear lime water, the solution turns cloudy or milky due to the formation of insoluble calcium carbonate.

🎯 Exam Tip: This milky appearance is a classic test to confirm the presence of carbon dioxide gas.

Question 16. In which of the following chemical equations, the abbreviation represent the correct states of the reactants and products involved at reaction temperature?
(a) \( 2H_2(g) + O_2(g) \rightarrow 2H_2O(l) \)
(b) \( 2H_2(g) + O_2(I) \rightarrow 2H_2O(I) \)
(c) \( 2H_2(g) + O_2(g) \rightarrow 2H_2O(g) \)
(d) \( H_2O(g) \)
Answer: (c) \( 2H_2(g) + O_2(g) \rightarrow 2H_2O(g) \)
In simple words: The correct equation shows hydrogen and oxygen as gases, and water also as a gas (vapor) at the typical high temperatures of this reaction.

🎯 Exam Tip: Always consider the reaction conditions when assigning physical states; combustion reactions typically produce water in its gaseous state.

Question 17. What is true about the following equation?
\( 2KClO_3 \xrightarrow{\text{Heat}} 2KCl (s) + 3O_2(g) \)
(a) It is a combination reaction.
(b) It is a decomposition reaction and is accompanied by release of heat.
(c) It is a photochemical decomposition reaction and exothermic in nature.
(d) It is a decomposition reaction and is endothermic in nature.
Answer: (d) It is a decomposition reaction and is endothermic in nature.
In simple words: This equation shows potassium chlorate breaking down when heated, meaning it needs heat to happen, making it an endothermic decomposition reaction.

🎯 Exam Tip: A reaction requiring heat (indicated by "Heat" above the arrow) is endothermic, and one substance breaking into simpler ones is decomposition.

Question 18. What is true about the following equation?
\( 3Fe(s) + 4H_2O(g) \rightarrow Fe_3O_4(s) + 4H_2(g) \)
(i) Iron metal is being oxidised
(ii) Water is being reduced
(iii) Water is acting as reducing agent
(iv) Water is acting as oxidising agent
(a) (i), (ii) and (iii)
(b) (ii) and (iv)
(c) (i), (ii) and (iv)
(d) (ii) and (iv)
Answer: (c) (i), (ii) and (iv)
In simple words: In this reaction, iron loses electrons (oxidized), and water gains electrons (reduced). Because water causes iron to oxidize, it is an oxidizing agent.

🎯 Exam Tip: To identify oxidation and reduction, track the gain or loss of oxygen/hydrogen, or changes in oxidation states/electron transfer.

Question 19. When a magnesium ribbon is burnt in air, the ash formed is
(a) black
(b) white
(c) yellow
(d) pink
Answer: (b) white
In simple words: When magnesium burns, it creates a bright white light and leaves behind a white powdery substance called magnesium oxide.

🎯 Exam Tip: Magnesium combustion is a classic demonstration of a combination reaction producing a distinct white ash.

Question 21. Rancidity can be prevented by
(a) adding antioxidants
(b) storing food away from light
(c) keeping food in refrigerator
(d) all of the options
Answer: (d) all of the options
In simple words: You can stop fatty foods from going bad (rancid) by adding special chemicals, keeping them out of light, or storing them in a cold place like a fridge.

🎯 Exam Tip: Antioxidants, refrigeration, and light protection are all methods that slow down the oxidation process responsible for rancidity.

Question 22. Which of the following is not an example of single displacement reaction?
(a) \( CuO + H_2 \rightarrow H_2O + Cu \)
(b) \( Zn + CuSO_4 \rightarrow ZnSO_4 + Cu \)
(c) \( 4NH_3 + 5O_2 \rightarrow 4NO + 6H_2O \)
(d) \( Zn + 2HCl \rightarrow H_2 + ZnCl_2 \)
Answer: (c) \( 4NH_3 + 5O_2 \rightarrow 4NO + 6H_2O \)
In simple words: The reaction between ammonia and oxygen is not a single displacement because it is a complex redox reaction, not a simple element swapping places in a compound.

🎯 Exam Tip: In a single displacement reaction, one element replaces another element in a compound. If multiple elements change partners, it's typically a more complex redox or double displacement reaction.

Question 23. The reaction of \( H_2 \) gas with oxygen gas to form water is an example of
(a) combination reaction
(b) redox reaction
(c) exothermic reaction
(d) all of these reactions
Answer: (a) combination reaction
In simple words: When hydrogen and oxygen gases combine to form water, it is called a combination reaction because two simpler substances join to make one new substance. It is also a redox and exothermic reaction.

🎯 Exam Tip: While it is a combination reaction, it is also a redox (due to electron transfer) and exothermic (releases heat) reaction, showing how reactions can fit into multiple categories.

Question 25. The reaction in which two compounds exchange their ions to form two new compounds is called
(a) displacement reaction
(b) combination reaction
(c) double displacement reaction
(d) redox reaction
Answer: (c) double displacement reaction
In simple words: When two different compounds swap their parts (ions) with each other to create two entirely new compounds, it's known as a double displacement reaction.

🎯 Exam Tip: Double displacement reactions often result in the formation of a precipitate, gas, or water.

Question 26. On immersing an iron nail in \( CuSO_4 \) solution for few minutes, you will observe
(a) no reaction takes place
(b) the colour of solution fades away
(c) the surface of iron nails acquire a black coating
(d) the colour of solution changes to green
Answer: (d) the colour of solution changes to green
In simple words: When an iron nail is placed in blue copper sulfate solution, the iron displaces copper, turning the solution green as iron sulfate is formed.

🎯 Exam Tip: This is a classic single displacement reaction where a more reactive metal (iron) displaces a less reactive metal (copper).

Question 27. An element X on exposure to moist air turns reddish-brown and a new compound Y is formed. The substance X and Y are
(a) X = Fe, Y = \( Fe_2O_3 \)
(b) X = Ag, Y = \( Ag_2S \)
(c) X = Cu, Y = \( CuO \)
(d) X = Al, Y = \( Al_2O_3 \)
Answer: (a) X = Fe, Y = \( Fe_2O_3 \)
In simple words: Element X is iron, which rusts in damp air to form reddish-brown rust, known as iron (III) oxide, which is substance Y.

🎯 Exam Tip: The reddish-brown color is a key indicator for rust, which is primarily hydrated iron (III) oxide, meaning iron is the original element.

Question 1. Give the formula for lime.
Answer: The chemical formula for lime, also known as calcium oxide, is \( CaO \). Lime is commonly used in agriculture and construction.
In simple words: Lime has the chemical formula \( CaO \).

🎯 Exam Tip: Knowing common names and their chemical formulas, like lime and calcium oxide, is crucial for chemistry basics.

Question 2. Give the formula for iron(II) oxide and iron (III) oxide.
Answer: The chemical formula for iron (II) oxide is \( FeO \), and for iron (III) oxide, it is \( Fe_2O_3 \). The Roman numerals indicate the valency or charge of the iron in each compound.
In simple words: Iron (II) oxide is \( FeO \), and iron (III) oxide is \( Fe_2O_3 \).

🎯 Exam Tip: The Roman numeral in the name (II or III) tells you the oxidation state of the metal, which helps determine the correct formula with oxygen.

Question 3. Give the formula for rust.
Answer: The chemical formula for rust is \( Fe_2O_3 \cdot 2H_2O \). This represents hydrated iron (III) oxide, meaning iron oxide that has water molecules loosely attached to its structure.
In simple words: Rust is made of iron, oxygen, and water, and its formula is \( Fe_2O_3 \cdot 2H_2O \).

🎯 Exam Tip: Rust is not just iron oxide; it's hydrated iron oxide, emphasizing the role of water in its formation.

Question 4. Name the conditions necessary for rusting.
Answer: The two essential conditions required for rusting to occur are the presence of oxygen (from air) and moisture (water). Both must be present for iron to corrode and form rust.
In simple words: Rusting needs both air (oxygen) and water (moisture) to happen.

🎯 Exam Tip: To prevent rusting, you need to block either oxygen or moisture, or both, from reaching the iron surface.

Question 5. Name the reaction seen during rancidity of food.
Answer: The reaction responsible for rancidity in food is called oxidation. This happens when fats and oils in food react with oxygen in the air, leading to unpleasant smells and tastes.
In simple words: When food goes bad and smells awful, it's due to a process called oxidation.

🎯 Exam Tip: Rancidity is a chemical change, not just physical spoilage, and it's a type of oxidation reaction.

Question 6. Name the different forms of energy required for breaking down the reactants in decomposition reaction.
Answer: Decomposition reactions, which break down a single compound into simpler substances, often require energy in the form of heat, light, or electricity. For example, heat can decompose calcium carbonate, light can decompose silver halides, and electricity can decompose water.
In simple words: To break things apart in a decomposition reaction, you might need energy from heat, light, or electricity.

🎯 Exam Tip: Remember that energy input is characteristic of endothermic reactions, which many decomposition reactions are.

Question 7. What is the insoluble substance formed in a chemical reaction called?
Answer: The insoluble substance that forms during a chemical reaction and separates from the solution is called a precipitate. Precipitation reactions are often visible as a cloudy suspension or a solid settling at the bottom.
In simple words: An undissolved solid that forms during a chemical reaction is called a precipitate.

🎯 Exam Tip: The formation of a precipitate is one of the key indicators that a chemical reaction has occurred.

Question 9. What are noble metals?
Answer: Noble metals are a group of metals that are highly resistant to corrosion and oxidation, meaning they do not easily react with other substances, even at high temperatures. Gold is a classic example of a noble metal.
In simple words: Noble metals are special metals like gold that do not rust or react easily, even when heated.

🎯 Exam Tip: Their low reactivity makes noble metals valuable for jewelry and electronics.

Question 10. Name two metals which do not corrode?
Answer: Two metals that are known for their resistance to corrosion are gold and platinum. These precious metals are highly unreactive and do not easily rust or tarnish when exposed to air or moisture.
In simple words: Gold and platinum are two metals that do not corrode or rust easily.

🎯 Exam Tip: Gold and platinum are excellent examples of noble metals due to their extreme resistance to chemical attack.

Question 11. Name the products obtained when silver bromide is exposed to sunlight.
Answer: When silver bromide is exposed to sunlight, it undergoes a decomposition reaction and breaks down into silver metal and bromine gas. This reaction is used in black and white photography.
In simple words: Sunlight breaks silver bromide into silver and bromine.

🎯 Exam Tip: This photochemical decomposition reaction is the basis for traditional photographic film and paper.

Question 12. Name the gas which burns with pop sound.
Answer: Hydrogen gas burns with a characteristic "pop" sound when a lit splint is brought near it. This sound is produced due to the rapid burning of hydrogen in the presence of oxygen.
In simple words: Hydrogen gas makes a popping sound when you light it with a match.

🎯 Exam Tip: The 'pop' sound is a distinctive test for hydrogen gas, confirming its presence in experiments.

Question 13. Name the compound used to test the evolution of carbon dioxide gas.
Answer: Freshly prepared calcium hydroxide solution, commonly known as lime water, is used to test for carbon dioxide gas. When carbon dioxide is bubbled through lime water, it turns milky due to the formation of insoluble calcium carbonate.
In simple words: You can use fresh lime water to see if carbon dioxide gas is present, as it will turn cloudy.

🎯 Exam Tip: Remember that lime water turning milky is a specific and reliable test for carbon dioxide gas.

Question 14. Name the gas evolved when lead nitrate is heated.
Answer: When lead nitrate is heated, it breaks down and releases nitrogen dioxide gas. This gas is recognized by its reddish-brown color and pungent smell.
In simple words: Heating lead nitrate gives off a reddish-brown gas called nitrogen dioxide.

🎯 Exam Tip: The reddish-brown fumes are a characteristic sign of nitrogen dioxide gas in decomposition reactions involving nitrates.

Question 15. Name two oxides of sulphur.
Answer: Two common oxides of sulfur are sulfur dioxide (\( SO_2 \)) and sulfur trioxide (\( SO_3 \)). These compounds are known for their role in air pollution and acid rain.
In simple words: Two kinds of sulfur oxides are sulfur dioxide and sulfur trioxide.

🎯 Exam Tip: Both sulfur dioxide and sulfur trioxide are significant air pollutants that contribute to acid rain formation.

Question 16. Give the examples of exothermic reaction.
Answer: Respiration, the process by which living organisms convert food into energy, is an exothermic reaction as it releases heat. Another example is when water is added to quicklime, which also releases a significant amount of heat.
In simple words: Breathing (respiration) and mixing water with lime are examples of reactions that give off heat.

🎯 Exam Tip: Exothermic reactions are important in everyday life, from simple heating processes to biological energy production.

Question 17. Give one example of decomposition reaction that occurs in nature,
Answer: The rotting of fruits and vegetables is a common example of a decomposition reaction occurring in nature. During this process, complex organic substances in the fruits and vegetables break down into simpler compounds due to the action of microorganisms.
In simple words: When fruits and vegetables rot, it's a natural breaking-down reaction.

🎯 Exam Tip: Natural decomposition reactions play a vital role in nutrient cycling and waste breakdown in ecosystems.

Question 18. Name the type of reaction in which two or more than two reactants form a single compound.
Answer: This type of reaction is called a combination reaction. In a combination reaction, multiple reactants come together to form a single, more complex product.
In simple words: When two or more things combine to make one new thing, it is a combination reaction.

🎯 Exam Tip: Combination reactions simplify multiple reactants into a single, more complex product, like \( A + B \rightarrow AB \).

Question 19. Name the chemical used in black and white photography.
Answer: Silver bromide is the chemical commonly used in traditional black and white photography. It is sensitive to light and undergoes a chemical change when exposed, forming the image.
In simple words: Silver bromide is the main chemical in old black and white photos.

🎯 Exam Tip: The light sensitivity of silver halides, like silver bromide, is fundamental to how photography works.

Question 20. Name the ions present in barium sulphate.
Answer: Barium sulfate is an ionic compound, and it consists of barium ions (\( Ba^{2+} \)) and sulfate ions (\( SO_4^{2-} \)). These ions are held together by electrostatic forces.
In simple words: Barium sulfate has barium ions (\( Ba^{2+} \)) and sulfate ions (\( SO_4^{2-} \)) in it.

🎯 Exam Tip: Correctly identifying the constituent ions in a compound is key to understanding its chemical behavior and solubility.

Question 21. Name the reaction which forms insoluble salts.
Answer: The reaction that forms insoluble salts, which often appear as a solid precipitate, is called a precipitation reaction. These reactions are typically double displacement reactions where two soluble ionic compounds react to form an insoluble product.
In simple words: A reaction that makes a salt that won't dissolve is called a precipitation reaction.

🎯 Exam Tip: The formation of a precipitate is a visible sign of a chemical reaction, often used in qualitative analysis.

Question 22. State the reaction in which hydrogen acts as a reducing agent.
Answer: In the reaction where copper oxide is heated with hydrogen gas, \( CuO(s) + H_2(g) \xrightarrow{\text{heat}} Cu(s) + H_2O(l) \), hydrogen acts as a reducing agent. It removes oxygen from copper oxide, reducing it to copper metal. This shows hydrogen's ability to donate electrons or add to a substance.
In simple words: When hydrogen reacts with copper oxide to form copper and water, hydrogen takes away oxygen, so it acts as a reducing agent.

🎯 Exam Tip: A reducing agent is itself oxidized, meaning it gains oxygen or loses hydrogen/electrons in the process.

Question 1. What happens when carbon dioxide and water react in the same ratio?
Answer: When six molecules of carbon dioxide and six molecules of water react together, they produce glucose (\( C_6H_{12}O_6 \)) and release oxygen gas (\( O_2 \)). This process is known as photosynthesis, where plants convert light energy into chemical energy.
\( 6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2 \)
In simple words: If six carbon dioxide molecules and six water molecules react, they make sugar (glucose) and oxygen gas. This is like how plants make their food.

🎯 Exam Tip: Photosynthesis is a vital biochemical reaction that converts light energy into chemical energy, sustaining life on Earth.

Question 2. How can you chemically remove the black coating of copper oxide?
Answer: The black coating of copper oxide can be chemically removed by passing hydrogen gas over it while it is being heated. As hydrogen passes over the hot copper oxide, it reacts with the oxygen to form water, leaving behind reddish-brown copper metal.
In simple words: You can get rid of the black copper oxide coating by heating it and passing hydrogen gas over it; the hydrogen takes away the oxygen, leaving pure copper.

🎯 Exam Tip: This is a reduction reaction where hydrogen acts as a reducing agent, removing oxygen from the copper oxide.

Question 3. Product formed when A and B react together are zinc chloride and hydrogen gas. Find reactant A and B.
Answer: Reactant A is zinc metal (\( Zn \)), and reactant B is hydrochloric acid (\( HCl \)). When zinc metal reacts with hydrochloric acid, it displaces hydrogen to form zinc chloride (\( ZnCl_2 \)) and hydrogen gas (\( H_2 \)). This is a single displacement reaction.
\( Zn + 2HCl \rightarrow ZnCl_2 + H_2 \)
In simple words: If zinc chloride and hydrogen gas are made, then the starting materials (reactants) were zinc metal and hydrochloric acid.

🎯 Exam Tip: Knowing common reactions of metals with acids is essential; they typically produce a salt and hydrogen gas.

Question 4. Name the product obtained and type of reaction given below:
\( Na_2 SO_4 + BaCl_2 \rightarrow \)
Answer: When sodium sulfate (\( Na_2SO_4 \)) reacts with barium chloride (\( BaCl_2 \)), the products formed are barium sulfate (\( BaSO_4 \)) and sodium chloride (\( NaCl \)). This type of reaction, where ions are exchanged between two compounds, is called a double displacement reaction.
\( Na_2SO_4 + BaCl_2 \rightarrow BaSO_4 + 2NaCl \)
In simple words: When sodium sulfate and barium chloride react, they make barium sulfate and sodium chloride. This is a double displacement reaction, meaning the parts swap places.

🎯 Exam Tip: Double displacement reactions often lead to precipitation, especially when barium sulfate, an insoluble compound, is formed.

Question 6. Three test tubes are taken and labelled as A, B and C. In test tube A iron nail is dipped in water. Invest tube B iron nail is dipped in mixture of water and oil and in test tube C iron nail is added with dry calcium chloride. Name the test tube in which the iron nail will rust and why?
Answer: The iron nail will rust in test tube A. This is because test tube A is the only one that provides both necessary conditions for rusting: the presence of moisture (water) and air (oxygen). In test tube B, the oil layer prevents air from reaching the nail, and in test tube C, the dry calcium chloride absorbs all moisture.
In simple words: The iron nail in test tube A will rust because it has both water and air, which are needed for rusting. The other test tubes are missing one of these things.

🎯 Exam Tip: This experiment clearly demonstrates that both oxygen and water are essential for rusting to occur.

Question 7. Metal X becomes green when left in air, turns black when heated in air. Name the metal and the compounds formed in both the cases.
Answer: Metal X is copper (\( Cu \)). When copper is exposed to moist air, it reacts to form a green coating of basic copper carbonate (\( CuCO_3 \cdot Cu(OH)_2 \)). When this green compound, or copper itself, is heated in air, it reacts with oxygen to form a black compound, which is copper (II) oxide (\( CuO \)).
In simple words: The metal X is copper. When left out, it turns green (copper carbonate), and when heated, it turns black (copper oxide).

🎯 Exam Tip: Copper's distinct color changes (green patina, black oxide) are important indicators of its chemical reactions with air and heat.

Question 9. Explain why most of the metal articles become dull when kept exposed to air?
Answer: Most metal objects become dull when exposed to air because their surfaces react with various gases and moisture present in the atmosphere. This reaction forms a thin layer of a new compound, such as an oxide or sulfide, which covers the shiny metal surface, causing it to lose its luster. For example, aluminum forms a dull layer of aluminum oxide.
In simple words: Metals become dull in the air because they react with gases and moisture, forming a new, dull layer on their surface.

🎯 Exam Tip: This process, known as corrosion or tarnishing, is a chemical reaction that changes the surface properties of the metal.

Question 11. Define displacement reaction. Give one example of it, how is it different from double displacement reaction?
Answer: A displacement reaction occurs when a more reactive element displaces a less reactive element from its compound. For example, iron (\( Fe \)) displaces copper (\( Cu \)) from copper sulfate (\( CuSO_4 \)): \( Fe + CuSO_4 \rightarrow FeSO_4 + Cu \). In contrast, a double displacement reaction involves the exchange of ions between two different compounds, where two new compounds are formed. For example, \( Na_2SO_4 + BaCl_2 \rightarrow BaSO_4 + 2NaCl \). The key difference is that single displacement has one element replacing another, while double displacement has two parts swapping.
In simple words: A displacement reaction is when one element takes the place of another in a compound. A double displacement reaction is when two compounds swap their parts.

🎯 Exam Tip: Distinguishing between single and double displacement depends on whether one element replaces another or if two compounds exchange ionic partners.

Question 12. Give differences between the exothermic and endothermic reactions.
Answer: An exothermic reaction is a chemical process that releases energy, usually in the form of heat, into its surroundings. This makes the surroundings feel warmer. Conversely, an endothermic reaction is a chemical process that absorbs energy from its surroundings, often making the surroundings feel colder. Endothermic reactions require energy input to break bonds and form new ones.
In simple words: Exothermic reactions give off heat, making things warmer. Endothermic reactions take in heat, making things cooler.

🎯 Exam Tip: The change in temperature of the surroundings is the most direct indicator of whether a reaction is exothermic or endothermic.

Question 13. Explain and name the type of reaction seen when iron reacts with hydrochloric acid.
Answer: When iron reacts with hydrochloric acid, the iron displaces hydrogen from the acid, producing hydrogen gas and iron (II) chloride, which is a salt of iron. The balanced equation is \( Fe(s) + 2HCl(aq) \rightarrow FeCl_2(aq) + H_2(g) \). This reaction is classified as a single displacement reaction because one element (iron) replaces another element (hydrogen) in a compound.
In simple words: When iron and hydrochloric acid mix, hydrogen gas comes out, and a new iron salt is formed. This is a displacement reaction.

🎯 Exam Tip: Reactions of active metals with acids are typically single displacement reactions that produce hydrogen gas.

Question 14. Why is photosynthesis considered as endothermic reaction?
Answer: Photosynthesis is considered an endothermic reaction because it requires an input of energy, specifically light energy from the sun, to occur. This energy is used to convert carbon dioxide and water into glucose (a sugar) and oxygen, making it a process that absorbs energy from its surroundings.
In simple words: Photosynthesis is an endothermic reaction because it needs energy from sunlight to make sugar from carbon dioxide and water.

🎯 Exam Tip: Endothermic reactions store energy within the products, making them essential for processes like plant growth.

Question 16. Give one example for each of the following reactions:
(a) Combination reaction
(b) Decomposition reaction
(c) Displacement reaction
Answer:
(a) Combination reaction: When magnesium burns in oxygen, it forms magnesium oxide.
\( 2Mg(s) + O_2(g) \rightarrow 2MgO(s) \)
(b) Decomposition reaction: When calcium carbonate is heated, it breaks down into calcium oxide and carbon dioxide.
\( CaCO_3(s) \xrightarrow{\text{heat}} CaO(s) + CO_2(g) \)
(c) Displacement reaction: When zinc is added to copper sulfate solution, zinc displaces copper.
\( Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s) \)
In simple words: (a) Magnesium and oxygen combining to make magnesium oxide. (b) Heating calcium carbonate to get calcium oxide and carbon dioxide. (c) Zinc replacing copper in copper sulfate solution.

🎯 Exam Tip: Understanding the basic patterns of these reaction types helps in predicting products and balancing equations.

Question 18. Define corrosion, rusting and rancidity.
Answer:
Corrosion: This is a natural process that gradually destroys materials, usually metals, by chemical or electrochemical reactions with their environment. It happens when metals are exposed to air, moisture, acids, and other gases.
Rusting: This is a specific type of corrosion that applies only to iron and its alloys. It occurs when iron is exposed to both oxygen and moisture, forming a reddish-brown flaky substance called rust (hydrated iron oxide).
Rancidity: This refers to the spoilage of food, especially those containing fats and oils, when they are exposed to air for a long time. The fats and oils undergo oxidation, leading to unpleasant smells and tastes.
In simple words: Corrosion is when metals get damaged by reacting with their surroundings. Rusting is a type of corrosion specific to iron, making it turn reddish-brown. Rancidity is when fatty foods spoil and start to smell or taste bad because they react with air.

🎯 Exam Tip: While all rusting is corrosion, not all corrosion is rusting. Rancidity is a food spoilage process, distinct from metal corrosion.

Question 20. How can one make an equation more informative?
Answer: To make a chemical equation more informative, you can add details like:
(i) The physical state of each reactant and product: use (g) for gas, (l) for liquid, (aq) for aqueous solution (dissolved in water), and (s) for solid.
(ii) The reaction conditions: mention things like temperature, pressure, or the presence of a catalyst above or below the reaction arrow.
These additions help to understand the reaction more completely.
In simple words: You can make an equation more informative by showing if things are solid, liquid, gas, or dissolved in water, and by writing conditions like heat or catalysts above the arrow.

🎯 Exam Tip: Including physical states and reaction conditions provides a complete picture of the chemical change occurring.

Question 21. Give the chemical equations (balanced) for the following:
(1) Reaction used in black and white photography.
(2) Reaction when glucose is oxidised.
(3) Formation of water from \( H_2 \) and \( O_2 \).
Answer:
(1) Black and white photography uses the decomposition of silver bromide under light:
\( 2AgBr(s) \xrightarrow{\text{Sunlight}} 2Ag(s) + Br_2(g) \)
(2) When glucose is oxidized (as in respiration), it reacts with oxygen to produce carbon dioxide, water, and energy:
\( C_6H_{12}O_6(aq) + 6O_2(aq) \rightarrow 6CO_2(aq) + 6H_2O(l) + \text{Energy} \)
(3) Water is formed from the combination of hydrogen and oxygen gases:
\( 2H_2(g) + O_2(g) \rightarrow 2H_2O(l) \)
In simple words: (1) Silver bromide breaks down into silver and bromine with sunlight. (2) Glucose reacts with oxygen to make carbon dioxide, water, and energy. (3) Hydrogen gas and oxygen gas combine to make water.

🎯 Exam Tip: Always balance chemical equations to ensure the law of conservation of mass is upheld, where atoms are neither created nor destroyed.

Question 22. Give equations to show the chemical reactions of zinc and lead where it displaces copper from its compound.
Answer:
Zinc displacing copper from copper sulfate solution:
\( Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s) \)
Lead displacing copper from copper chloride solution:
\( Pb(s) + CuCl_2(aq) \rightarrow PbCl_2(aq) + Cu(s) \)
These are both single displacement reactions because zinc and lead are more reactive than copper.
In simple words: Zinc metal can push copper out of copper sulfate solution to make zinc sulfate and copper. Lead metal can also push copper out of copper chloride solution to make lead chloride and copper.

🎯 Exam Tip: For displacement reactions, remember that a more reactive metal will always displace a less reactive metal from its salt solution.

Reduction reaction: The reaction in which hydrogen is gained or oxygen is lost, is called reduction reaction.

\( \text{e.g., } \text{CuO} + \text{H}_2 \xrightarrow{\text{heat}} \text{Cu} + \text{H}_2\text{O} \)

Question 24. What happens chemically when quick lime is added to water?
Answer: Quick lime is calcium oxide (CaO). When water is added to it, a large amount of heat is released, and calcium hydroxide (Ca(OH)2) is formed. This reaction is highly exothermic, meaning it gives off heat.
\( \text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 \)
In simple words: When quick lime mixes with water, it gets hot and forms a new substance called slaked lime.

🎯 Exam Tip: Remember that adding water to quick lime is a common example of an exothermic reaction, which releases a significant amount of heat.

Question 25. How will you test for the gas which is liberated when hydrochloric acid reacts with an active metal?
Answer: The gas liberated when hydrochloric acid reacts with an active metal is hydrogen. This gas can be tested by bringing a burning matchstick near the mouth of the test tube. Hydrogen gas will burn with a distinctive 'pop' sound.
In simple words: Bring a burning matchstick near the gas. If you hear a "pop" sound, it's hydrogen.

🎯 Exam Tip: The 'pop' sound is a classic indicator for hydrogen gas, distinguishing it from other gases like oxygen (which rekindles a glowing splint) or carbon dioxide (which extinguishes a flame).

Question 26. Identify in the following reactions: (i) the substance oxidized and (ii) the substance reduced. \( \text{ZnO} + \text{C} \rightarrow \text{Zn} + \text{CO} \)
Answer: In this reaction, carbon (C) gains oxygen, so carbon is oxidized. Zinc oxide (ZnO) loses oxygen, so zinc oxide is reduced. This type of reaction is an example of a redox reaction, where both oxidation and reduction occur simultaneously.
(i) Carbon (C) is the substance oxidized (it gains oxygen).
(ii) Zinc oxide (ZnO) is the substance reduced (it loses oxygen).
In simple words: In the given reaction, carbon takes oxygen, so it's oxidized. Zinc oxide loses its oxygen, so it's reduced.

🎯 Exam Tip: For simple reactions, remember that oxidation often means gaining oxygen or losing hydrogen, while reduction means losing oxygen or gaining hydrogen.

Question 27. Give an example of photochemical reaction.
Answer: A common example of a photochemical reaction is the decomposition of silver bromide (AgBr) in black and white photography. When exposed to light, silver bromide breaks down to form silver (Ag) and bromine gas (Br2). This process uses light energy to drive the chemical change.
In simple words: In old black and white photos, light made a chemical change happen. Silver bromide changed into silver metal and bromine gas when light hit it.

🎯 Exam Tip: Photochemical reactions are those that are initiated or driven by light energy, like photosynthesis or the fading of colored dyes in sunlight.

Question 28. Describe an activity to illustrate a decomposition reaction.
Answer: Take a test tube and add calcium carbonate (CaCO3) to it. Heat the test tube strongly. A gas will be evolved, which you can pass through lime water. The lime water will turn milky, indicating the formation of carbon dioxide gas. This activity shows that calcium carbonate decomposes into calcium oxide and carbon dioxide when heated.
In simple words: Heat a test tube with chalk powder (calcium carbonate). It will break down into a new solid and a gas. If you pass the gas through lime water, it will turn cloudy.

🎯 Exam Tip: When describing an activity, always mention the materials, procedure, observations, and the conclusion to make it complete.

Question 29. Balance the following chemical equation: \( \text{Fe(s)} + \text{H}_2\text{O(g)} \rightarrow \text{Fe}_3\text{O}_4\text{(s)} + \text{H}_2\text{(g)} \)
Answer: To balance the equation, we need to ensure that the number of atoms of each element is the same on both sides of the reaction. We start by balancing the most complex molecule first, which is \( \text{Fe}_3\text{O}_4 \). This requires 3 Fe atoms and 4 O atoms. The balanced equation is:
\( 3\text{Fe(s)} + 4\text{H}_2\text{O(g)} \rightarrow \text{Fe}_3\text{O}_4\text{(s)} + 4\text{H}_2\text{(g)} \)
In simple words: To balance this equation, we need 3 iron atoms and 4 water molecules on the left to make \( \text{Fe}_3\text{O}_4 \) and 4 hydrogen molecules on the right.

🎯 Exam Tip: Always start balancing equations by identifying elements that appear only once on each side first, then handle hydrogen and oxygen, and finally check all elements.

Question 30. Why is respiration considered as an exothermic process?
Answer: Respiration is considered an exothermic process because, during this process, glucose (a sugar) reacts with oxygen to produce carbon dioxide, water, and energy. This energy is released in the form of heat, which is vital for maintaining body temperature and powering various bodily functions. The chemical equation for respiration shows this energy release:
\( \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy} \)
In simple words: Respiration is like burning food in our bodies. It releases heat and energy, which is why it's called an exothermic process.

🎯 Exam Tip: Exothermic reactions release energy (often as heat or light), while endothermic reactions absorb energy from their surroundings.

Question 31. Balance the following chemical equation: \( \text{FeSO}_4 \xrightarrow{\Delta} \text{Fe}_2\text{O}_3 + \text{SO}_2 + \text{SO}_3 \)
Answer: The unbalanced equation shows 1 Fe on the left and 2 Fe on the right, and 1 S on the left with 2 S on the right. To balance the iron and sulfur atoms, we need to put a coefficient of 2 in front of \( \text{FeSO}_4 \) on the left side. This ensures the atoms are equal on both sides.
\( 2\text{FeSO}_4 \xrightarrow{\Delta} \text{Fe}_2\text{O}_3 + \text{SO}_2 + \text{SO}_3 \)
In simple words: To balance this equation, you need two units of \( \text{FeSO}_4 \) on the left side to match the iron and sulfur atoms on the right side.

🎯 Exam Tip: When balancing equations, start with elements other than oxygen and hydrogen, then balance oxygen, and finally hydrogen if present, or vice versa.

Question 32. Balance the following chemical equation: \( \text{MnO}_2 + \text{HCl} \rightarrow \text{MnCl}_2 + \text{Cl}_2 + \text{H}_2\text{O} \)
Answer: To balance the equation, we need to ensure the number of atoms for each element is equal on both sides. We see 1 Mn on both sides, but the Cl atoms and H atoms are not balanced. By adding a coefficient of 4 in front of HCl on the left, we can balance the hydrogen and chlorine atoms. The balanced equation is:
\( \text{MnO}_2 + 4\text{HCl} \rightarrow \text{MnCl}_2 + \text{Cl}_2 + 2\text{H}_2\text{O} \)
In simple words: To balance this equation, you need four units of hydrochloric acid (HCl) on the left side to match the manganese, chlorine, and hydrogen atoms on the right.

🎯 Exam Tip: Balancing chlorine atoms in equations like this often involves considering both combined and elemental forms of chlorine on the product side.

Question 33. On what basis is a chemical equation balanced?
Answer: A chemical equation is balanced on the basis of the Law of Conservation of Mass. This fundamental law states that mass can neither be created nor destroyed in a chemical reaction. Therefore, the total number of atoms of each element on the left-hand side of the reaction (reactants) must be equal to the total number of atoms of each element on the right-hand side of the reaction (products). This ensures that the total mass of reactants equals the total mass of products.
In simple words: Chemical equations are balanced because of a rule called the Law of Conservation of Mass. This law says that no atoms are lost or gained during a reaction; they just rearrange. So, the number of atoms of each element must be the same on both sides.

🎯 Exam Tip: Understanding the Law of Conservation of Mass is key to grasping why balancing chemical equations is necessary; it's a core principle in chemistry.

Question 34. State any two observations in an activity, which may suggest that a chemical reaction has taken place. Given examples to support your answer.
Answer: The following observations in an activity may suggest that a chemical reaction has taken place:

• Change in state: For example, when solid lead nitrate reacts with potassium iodide solution, a yellow precipitate (solid) of lead iodide is formed, indicating a change from liquid to solid state.
• Change in colour: For instance, when iron nails are left exposed to air and moisture, they react to form rust, which is a reddish-brown coating. This change from shiny grey iron to reddish-brown rust indicates a chemical reaction.
• Evolution of a gas: When zinc metal reacts with dilute hydrochloric acid, hydrogen gas is evolved, which can be observed as bubbles.
• Change in temperature: Adding quick lime to water causes the mixture to become hot, showing that heat is released (an exothermic reaction).

In simple words: You can tell a chemical reaction happened if you see things like a new solid forming, a color change, gas bubbles appearing, or the temperature changing.

🎯 Exam Tip: These observable changes are the primary evidence of a chemical reaction, distinguishing it from a physical change.

Question 35. Identify the type of reaction in the following example: \( \text{Na}_2\text{SO}_4\text{(aq)} + \text{BaCl}_2\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)} + 2\text{NaCl(aq)} \)
Answer: This reaction is a double displacement reaction. In this type of reaction, the ions of two reactants exchange places to form two new products. Here, sodium (Na+) exchanges with barium (Ba2+), and sulfate (SO42-) exchanges with chloride (Cl-), leading to the formation of barium sulfate precipitate and sodium chloride solution. This is also a precipitation reaction because an insoluble product (\( \text{BaSO}_4 \)) is formed.
In simple words: This is a double displacement reaction because the parts of the two starting chemicals swap places to make two new chemicals.

🎯 Exam Tip: Double displacement reactions often result in the formation of a precipitate, a gas, or water.

Question 36. Identify the type of reaction in the following example: \( \text{Fe(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{FeSO}_4\text{(aq)} + \text{Cu(s)} \)
Answer: This is a displacement reaction. In this reaction, iron (Fe), which is more reactive than copper (Cu), displaces copper from its compound, copper sulfate (\( \text{CuSO}_4 \)). As a result, iron sulfate (\( \text{FeSO}_4 \)) is formed, and copper metal is precipitated out.
In simple words: This is a displacement reaction because iron is stronger than copper and kicks copper out of its compound to take its place.

🎯 Exam Tip: To identify displacement reactions, refer to the reactivity series of metals; a more reactive metal will displace a less reactive metal from its salt solution.

Question 37. Identify the type of reaction in the following example: \( 2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{H}_2\text{O(g)} \)
Answer: This is a combination reaction. In a combination reaction, two or more reactants combine to form a single product. Here, hydrogen gas and oxygen gas combine to form water vapor.
In simple words: This is a combination reaction because hydrogen and oxygen join together to make just one thing: water.

🎯 Exam Tip: Combination reactions typically have multiple reactants but only one product, simplifying the overall chemical structure.

Question 38. Balance the given chemical equation: \( \text{Al(s)} + \text{CuCl}_2\text{(aq)} \rightarrow \text{AlCl}_3\text{(aq)} + \text{Cu(s)} \)
Answer: To balance this equation, we need to adjust coefficients so that the number of atoms for each element is equal on both sides. Aluminum (Al) has 1 atom on the left and 1 on the right. Copper (Cu) has 1 atom on both sides. Chlorine (Cl) has 2 atoms on the left and 3 on the right. We find the least common multiple for 2 and 3, which is 6. So, we place 3 in front of \( \text{CuCl}_2 \) and 2 in front of \( \text{AlCl}_3 \). Then, we balance Al and Cu atoms.
\( 2\text{Al(s)} + 3\text{CuCl}_2\text{(aq)} \rightarrow 2\text{AlCl}_3\text{(aq)} + 3\text{Cu(s)} \)
In simple words: To balance this equation, we need 2 aluminum atoms and 3 copper chloride units on the left side to match the products on the right side.

🎯 Exam Tip: When dealing with polyatomic ions or uneven numbers of atoms for an element (like chlorine here), using the least common multiple often helps streamline the balancing process.

Question 40. Balance the following chemical equation: \( \text{Pb(NO}_3\text{)}_2\text{(s)} \xrightarrow{\text{heat}} \text{PbO(s)} + \text{NO}_2\text{(g)} + \text{O}_2\text{(g)} \)
Answer: This is a decomposition reaction where lead nitrate breaks down upon heating. To balance it, we need to ensure all atoms are equal on both sides. Start by balancing the nitrogen and oxygen atoms carefully. The balanced equation requires 2 molecules of lead nitrate to decompose.
\( 2\text{Pb(NO}_3\text{)}_2\text{(s)} \xrightarrow{\text{heat}} 2\text{PbO(s)} + 4\text{NO}_2\text{(g)} + \text{O}_2\text{(g)} \)
In simple words: When lead nitrate is heated, it breaks apart. We need to start with two lead nitrate molecules to make sure all the lead, nitrogen, and oxygen atoms match on both sides of the reaction.

🎯 Exam Tip: For decomposition reactions with multiple gaseous products (like \( \text{NO}_2 \) and \( \text{O}_2 \)), balancing oxygen atoms can be tricky and should be done last, often by trial and error.

Question 41. Name a reducing agent that may be used to obtain manganese from manganese dioxide.
Answer: Coke or carbon is a suitable reducing agent that can be used to obtain manganese from manganese dioxide. In this reaction, carbon removes oxygen from manganese dioxide, leaving behind pure manganese and forming carbon dioxide or carbon monoxide in the process. This is a common method for extracting metals from their oxides.
In simple words: Carbon, like coke, can be used to pull oxygen away from manganese dioxide, leaving manganese metal behind.

🎯 Exam Tip: Reducing agents are substances that cause reduction in another substance by donating electrons or removing oxygen. Carbon is a strong reducing agent for many metal oxides.

Question 42. In electrolysis of water, why is the volume of gas collected over one electrode double that of gas collected over the other electrode?
Answer: In the electrolysis of water (\( \text{H}_2\text{O} \)), water molecules break down into hydrogen gas (\( \text{H}_2 \)) and oxygen gas (\( \text{O}_2 \)). The chemical formula of water is \( \text{H}_2\text{O} \), which indicates that each water molecule contains two hydrogen atoms for every one oxygen atom. Therefore, when water is split, twice as much hydrogen gas (by volume) is produced compared to oxygen gas. The hydrogen gas collects at the cathode, and oxygen gas collects at the anode. This 2:1 ratio by volume directly reflects the atomic composition of water.
In simple words: Water is made of two parts hydrogen and one part oxygen. So, when water is broken apart using electricity, you get twice as much hydrogen gas as oxygen gas.

🎯 Exam Tip: Remember the 2:1 volume ratio of hydrogen to oxygen in water electrolysis, which is a direct consequence of water's chemical formula (\( \text{H}_2\text{O} \)).

Question 43. What change in colour is observed when silver chloride is left exposed to sunlight? What type of chemical reaction is this?
Answer: When silver chloride (AgCl) is left exposed to sunlight, it changes colour from white to greyish-black. This colour change is due to the decomposition of silver chloride into silver metal and chlorine gas. The type of chemical reaction is a decomposition reaction, specifically a photochemical decomposition, as it is driven by light energy.
In simple words: White silver chloride turns black in sunlight. This is a decomposition reaction where light breaks it down.

🎯 Exam Tip: This reaction is a classic example of a photochemical decomposition and was historically used in black and white photography.

Question 1. Different types of chemical reactions are: (a) Combination reactions, (b) Decomposition reaction, (c) Displacement reaction, (d) Double displacement reaction, (e) Oxidation-reduction reaction.
Answer:
(a) Combination reactions: These reactions occur when two or more substances combine to form a single, more complex compound. An example is the formation of magnesium oxide when magnesium burns in oxygen.
\( 2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \)

(b) Decomposition reaction: This type of reaction involves a single compound breaking down into two or more simpler substances. For instance, when calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide.
\( \text{CaCO}_3 \xrightarrow{\Delta} \text{CaO} + \text{CO}_2 \)

(c) Displacement reaction: In these reactions, a more reactive element displaces a less reactive element from its compound. An example is when iron displaces copper from copper sulfate solution.
\( \text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu} \)

(d) Double displacement reaction: These reactions involve the exchange of ions between two compounds, typically in aqueous solutions, to form two new compounds. Often, one of the products is a precipitate, gas, or water. An example is the reaction between sodium sulfate and barium chloride.
\( \text{Na}_2\text{SO}_4\text{(aq)} + \text{BaCl}_2\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)} + 2\text{NaCl(aq)} \)

(e) Oxidation-reduction reaction: Also known as redox reactions, these involve the transfer of electrons between species, or a change in oxidation states. Oxidation is the loss of electrons or gain of oxygen, and reduction is the gain of electrons or loss of oxygen. For example, when copper reacts with oxygen, copper is oxidized, and oxygen is reduced.
\( \text{Oxidation: } 2\text{Cu} + \text{O}_2 \xrightarrow{\text{heat}} 2\text{CuO} \)
\( \text{Reduction: } \text{CuO} + \text{H}_2 \xrightarrow{\text{heat}} \text{Cu} + \text{H}_2\text{O} \)
In simple words: Chemical reactions can combine things, break things apart, swap parts between chemicals, or involve one chemical replacing a part of another. They can also involve elements gaining or losing oxygen or electrons.

🎯 Exam Tip: Be able to identify and provide an example for each major type of chemical reaction, as they are fundamental concepts in chemistry.

Question 2. Give an activity to prove that water contains H : O in the ratio of 2 :1.
Answer: To prove that water contains hydrogen and oxygen in a 2:1 ratio by volume, you can perform the electrolysis of water:
1. Take a plastic mug. Drill two holes at its base and fit rubber stoppers into these holes. Insert carbon electrodes into the rubber stoppers.
2. Connect these electrodes to a 6-volt battery.
3. Fill the mug with water so that the electrodes are fully immersed. Add a few drops of dilute sulfuric acid to the water to make it conduct electricity better.
4. Take two test tubes, fill them with water, and carefully invert them over the two carbon electrodes.
5. Switch on the current and leave the apparatus undisturbed for some time.
6. Observation: You will observe gas bubbles forming at both electrodes. The volume of gas collected in the test tube over the cathode (negative electrode) will be approximately double the volume of gas collected in the test tube over the anode (positive electrode).
7. Conclusion: The gas collected at the cathode is hydrogen, and the gas collected at the anode is oxygen. Since the volume of hydrogen is double that of oxygen, it proves that hydrogen and oxygen are present in water in a 2:1 ratio by volume.
In simple words: By passing electricity through water (electrolysis), you can collect hydrogen and oxygen gas. You'll see twice as much hydrogen gas as oxygen gas, showing that water has twice as much hydrogen as oxygen.

🎯 Exam Tip: When setting up electrolysis, ensure the water is acidified to allow current to flow, and that the test tubes are inverted correctly to collect the gases without mixing with air.

Question 3. (i) What is rancidity? (ii) Suggest two methods to reduce the problem of rancidity. (iii) How is corrosion different from rusting?'
Answer:
(i) Rancidity: Rancidity is the process where fat and oil-containing food items spoil due to oxidation when exposed to air. This results in an unpleasant smell and taste, making the food inedible.
(ii) Two methods to reduce rancidity:
1. Storing food in closed containers: Limiting exposure to air slows down the oxidation process.
2. Using antioxidants: These are substances that prevent or slow down oxidation. Nitrogen gas, for instance, is often flushed into packets of chips to replace oxygen.
3. Keeping food in a refrigerator: Low temperatures slow down the rate of oxidation.
(iii) Corrosion vs. Rusting:
* Corrosion: This is a general term for the slow deterioration of metals due to their reaction with air, moisture, acids, or other chemicals in the environment. It applies to all metals.
* Rusting: This is a specific type of corrosion that only applies to iron and its alloys (like steel). It occurs when iron reacts with oxygen and moisture to form hydrated iron(III) oxide, commonly known as rust (a reddish-brown flaky substance). Rusting is a common example of corrosion, but not all corrosion is rusting.
In simple words: (i) Rancidity is when oily foods go bad and smell funny because of air. (ii) You can stop it by keeping food in airtight boxes or adding special chemicals. (iii) Corrosion is when any metal gets damaged by its surroundings, but rusting is only when iron metal gets damaged by air and water.

🎯 Exam Tip: Remember that rusting is a specific form of corrosion, applicable only to iron, while corrosion is a broader term for the degradation of metals.

Question 4. What is meant by exothermic and endothermic reaction? Give examples to explain the same.
Answer:
* Exothermic reactions: These are chemical reactions that release energy, usually in the form of heat, into the surroundings. This causes the temperature of the surroundings to increase. A common example is the burning of natural gas (methane) or the process of respiration, where energy is released.
\( \text{CH}_4\text{(g)} + 2\text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} + 2\text{H}_2\text{O(g)} + \text{heat} \)
* Endothermic reactions: These are chemical reactions that absorb energy from the surroundings, usually in the form of heat. This causes the temperature of the surroundings to decrease. An example is the decomposition of calcium carbonate upon heating, which requires heat input to proceed, or photosynthesis, which absorbs light energy.
\( \text{CaCO}_3\text{(s)} \xrightarrow{\text{heat}} \text{CaO(s)} + \text{CO}_2\text{(g)} \)
In simple words: Exothermic reactions give off heat and make things warmer, like a fire. Endothermic reactions take in heat and make things colder, like an ice pack.

🎯 Exam Tip: Think of "exo" as "exit" (energy exits) and "endo" as "enter" (energy enters) to easily distinguish between exothermic and endothermic reactions.

Question 5. Solid calcium oxide was taken in a container and water was added slowly to it. (i) State the two observations made in the experiment. (ii) Write the name and chemical formula of the product formed.
Answer:
(i) Observations:
1. The mixture becomes very hot, indicating that a significant amount of heat is released. This shows it's an exothermic reaction.
2. The solid calcium oxide reacts vigorously with water and a new white, powdery substance is formed, which slowly settles down. The solid often produces a hissing sound as it reacts.
(ii) Product formed: The product formed is calcium hydroxide, commonly known as slaked lime. Its chemical formula is \( \text{Ca(OH)}_2 \). The reaction is:
\( \text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{Heat} \)
In simple words: (i) When water is added to solid quick lime, it gets very hot, and a new white substance is made. (ii) The new substance is called slaked lime, and its formula is \( \text{Ca(OH)}_2 \).

🎯 Exam Tip: Remember that the reaction of quick lime with water is a classic example of both a combination reaction and an exothermic reaction.

Question 6. What is an oxidation reaction? Give an example of oxidation reaction. Is oxidation an exothermic or an endothermic reaction?
Answer: An oxidation reaction is a chemical process where a substance gains oxygen, loses hydrogen, or loses electrons. An example of an oxidation reaction is the burning of copper in air to form copper oxide. In this reaction, copper combines with oxygen.
\( 2\text{Cu} + \text{O}_2 \xrightarrow{\Delta} 2\text{CuO} \)
Oxidation reactions, particularly combustion reactions (like burning), are typically exothermic, meaning they release heat into the surroundings.
In simple words: Oxidation is when a chemical takes in oxygen or loses hydrogen. When copper burns and turns into copper oxide, that's oxidation. Many oxidation reactions, like burning, give off heat.

🎯 Exam Tip: While many oxidation reactions are exothermic, it's important to note that not all are. However, combustion is always an exothermic oxidation reaction.

Question 8. (i) What is observed when a solution of potassium iodide is added to a solution of lead nitrate taken in a test tube? (ii) What type of reaction is this? (iii) Write a balanced chemical equation to represent the above reaction.
Answer:
(i) When a solution of potassium iodide is added to a solution of lead nitrate, a bright yellow precipitate is formed. This yellow solid is lead iodide.
(ii) This type of reaction is a double displacement reaction. It is also a precipitation reaction because an insoluble product (the precipitate) is formed.
(iii) The balanced chemical equation for the reaction is:
\( 2\text{KI(aq)} + \text{Pb(NO}_3\text{)}_2\text{(aq)} \rightarrow \text{PbI}_2\text{(s)} + 2\text{KNO}_3\text{(aq)} \)
In simple words: (i) When you mix potassium iodide and lead nitrate solutions, a bright yellow powder forms. (ii) This is a double displacement reaction. (iii) Two units of potassium iodide react with one unit of lead nitrate to make one unit of lead iodide and two units of potassium nitrate.

🎯 Exam Tip: The formation of a precipitate is a strong indicator of a double displacement reaction, especially when combining two clear solutions.

Question 9. What is a redox reaction? When a magnesium ribbon burns in air with a dazzling flame and forms a white ash, is magnesium oxidised or reduced? Why?
Answer: A redox reaction (reduction-oxidation reaction) is a chemical reaction that involves both oxidation and reduction occurring simultaneously. Oxidation is the loss of electrons or gain of oxygen, while reduction is the gain of electrons or loss of oxygen. When a magnesium ribbon burns in air, it combines with oxygen from the air to form magnesium oxide, which is a white ash. In this process, magnesium gains oxygen, so it is oxidized. Oxygen, in turn, gains electrons from magnesium and is reduced.
\( 2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \)
Therefore, magnesium is oxidized because it combines with oxygen.
In simple words: A redox reaction is when one chemical gains something (like oxygen) and another loses something at the same time. When magnesium burns, it takes oxygen, so it gets oxidized.

🎯 Exam Tip: In a combustion reaction, the substance that burns is always oxidized because it reacts with oxygen.

Question 10. Distinguish between an exothermic and an endothermic reaction. Amongst the following reactions, identify the exothermic and the endothermic reaction. (i) Heating coal in air to form carbon dioxide. (ii) Heating lime-stone in a lime kiln to form quick lime.
Answer:

Regarding the given reactions:
(i) Heating coal in air to form carbon dioxide is a combustion reaction, which releases heat. Therefore, it is an exothermic reaction.
(ii) Heating limestone in a lime kiln to form quick lime (thermal decomposition of \( \text{CaCO}_3 \)) requires continuous input of heat. Therefore, it is an endothermic reaction.
In simple words: Exothermic reactions give off heat, making things warmer. Endothermic reactions take in heat, making things colder. Burning coal is exothermic, and heating limestone to break it down is endothermic.

🎯 Exam Tip: Classify reactions based on whether they release (exothermic) or absorb (endothermic) energy. Combustion is a prime example of an exothermic process.

Question 11. Distinguish between a displacement reaction and a double displacement reaction. Identify the displacement and the double displacement reaction from the following reactions. (i) \( \text{HCl(aq)} + \text{NaOH(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)} \) (ii) \( \text{Fe(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{FeSO}_4\text{(aq)} + \text{Cu(s)} \)
Answer:

Identifying the given reactions:
(i) \( \text{HCl(aq)} + \text{NaOH(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)} \) is a double displacement reaction (specifically, a neutralization reaction). The hydrogen ion from HCl and the sodium ion from NaOH exchange places, forming NaCl and \( \text{H}_2\text{O} \).
(ii) \( \text{Fe(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{FeSO}_4\text{(aq)} + \text{Cu(s)} \) is a displacement reaction. Iron (Fe) is more reactive than copper (Cu), so it displaces copper from copper sulfate solution.
In simple words: A displacement reaction is when one element pushes out another weaker element from a compound. A double displacement reaction is when two compounds swap their parts with each other.

🎯 Exam Tip: Pay attention to how many elements are involved in the "swapping" or "replacing" to distinguish between single and double displacement reactions.