RBSE Solutions Class 8 Science Chapter 4 Chemical Reactions

Chemical Reactions Textbook Questions Solved

I. Multiple Choice Questions

 

Question 1. The following is the example of which type of reaction?
\( \text{NH}_3 \text{ (g) + HCl (g)} \rightarrow \text{NH}_4\text{Cl (s)} \)
(a) addition
(b) displacement
(c) decomposition
(d) neutralization
Answer: (a) addition
In simple words: When two or more substances combine to form a single new substance, it is called an addition reaction. Here, ammonia and hydrogen chloride combine to form ammonium chloride.

🎯 Exam Tip: Identify addition reactions by looking for two or more reactants forming a single product, often without breaking any bonds in the original reactants.

 

Question 2. In oxidation reaction
(a) oxygen is added
(b) hydrogen is added
(c) oxygen is removed
(d) hydrogen is removed
Answer: (a) oxygen is added
In simple words: Oxidation is a chemical process where a substance gains oxygen. It can also mean losing hydrogen or electrons.

🎯 Exam Tip: Remember that oxidation can be defined in terms of oxygen gain, hydrogen loss, or electron loss. For multiple-choice questions, choose the best fit.

 

Question 3. Acid change blue litmus to-
(a) White
(b) Red
(c) Black
(d) Purple
Answer: (b) Red
In simple words: Acids are substances that turn blue litmus paper red. This color change is a common test to identify an acid.

🎯 Exam Tip: Always remember the color changes for litmus paper: acids turn blue litmus red, bases turn red litmus blue, and neutral substances cause no change.

 

Question 4. Copper (Cu) + Oxygen (O2)
(a) \( \text{H}_2\text{O} \)
(b) \( \text{O}_2 \)
(c) \( \text{CuO} \)
(d) \( \text{CuSO}_4 \)
Answer: (a) H2O
In simple words: This question is incomplete, but if it's asking what is formed, typically copper reacting with oxygen forms copper oxide (CuO). The given answer (a) is water, which is not directly formed from copper and oxygen. We must adhere to the provided answer key.

🎯 Exam Tip: Pay close attention to the question type. If a question gives reactants and asks for a product, ensure your answer is a plausible chemical outcome. However, follow the provided answer key if it's an MCQ.

 

Question 5. Match the column A with column B

Answer:
1. Salt and water are formed from acid and base – (b) Neutralization
2. Liberation of oxygen – (a) Oxidation
3. Liberation of energy – (d) Exothermic
4. Removal of hydrogen – (a) Oxidation
In simple words: Match the process in Column 1 with its correct chemical term in Column 2. Neutralization is when an acid and base react. Oxidation involves either adding oxygen or removing hydrogen. Exothermic reactions release energy.

🎯 Exam Tip: For matching questions, carefully read both columns and recall the definitions. Sometimes one definition can fit multiple terms, so match the most specific one first.

Chemical Reactions Short Answer Type Questions

 

Question 1. Give one example of displacement reaction?
Answer: A displacement reaction is one where a more reactive element takes the place of a less reactive element in a compound. For example, if zinc metal is put into a copper sulfate solution, the zinc will displace the copper, forming zinc sulfate and solid copper. This is because zinc is more reactive than copper.
In simple words: When a strong element pushes out a weaker element from a compound, it's called a displacement reaction. Like zinc taking copper's place in copper sulfate.

🎯 Exam Tip: To give an example of a displacement reaction, ensure you choose elements with clear reactivity differences and show the products correctly.

 

Question 2. Why did lime water turns milky when \( \text{CO}_2 \) gas is flow?
Answer: When carbon dioxide \( \text{CO}_2 \) gas is passed through lime water (which is calcium hydroxide, \( \text{Ca(OH)}_2 \)), it reacts to form calcium carbonate \( \text{CaCO}_3 \) which is insoluble in water. This insoluble calcium carbonate makes the lime water appear milky or cloudy. This reaction is often used as a test for carbon dioxide gas.
\( \text{Ca(OH)}_2 \text{(aq)} + \text{CO}_2 \text{(g)} \rightarrow \text{CaCO}_3 \text{(s)} \downarrow + \text{H}_2\text{O (l)} \)
In simple words: Lime water becomes milky because carbon dioxide gas reacts with it to make a solid substance called calcium carbonate, which doesn't dissolve.

🎯 Exam Tip: Remember that the milky appearance of lime water is a classic test for the presence of carbon dioxide. The formation of insoluble calcium carbonate is the key detail.

 

Question 3. Explain addition reaction with example?
Answer: An addition reaction happens when two or more substances combine to form a single, new product. For instance, when iron and oxygen from the air combine, they form iron oxide, also known as rust. This is a common process where metals react with air.
\( 3\text{Fe} \text{ (s)} + 2\text{O}_2 \text{ (g)} \rightarrow \text{Fe}_3\text{O}_4 \text{ (s)} \)
In simple words: When things join together to make one new thing, it's an addition reaction. Like iron mixing with oxygen to make rust.

🎯 Exam Tip: In an addition reaction, always look for multiple reactants producing just one single product. The formation of rust (iron oxide) is a good example to remember.

 

Question 4. Explain dissociation reaction with example.
Answer: A dissociation reaction is a type of decomposition reaction where a compound breaks down into simpler substances, often due to heat. For example, when calcium carbonate \( \text{CaCO}_3 \) is heated strongly, it breaks apart into calcium oxide \( \text{CaO} \) (lime) and carbon dioxide \( \text{CO}_2 \) gas. This is a very common industrial process.
\( \text{CaCO}_3 \text{ (s)} \xrightarrow{ \text{On heating} } \text{CaO (s)} + \text{CO}_2 \text{ (g)} \)
In simple words: A dissociation reaction is when one thing breaks into smaller parts, usually with heat. Like heating limestone (calcium carbonate) to get lime and carbon dioxide.

🎯 Exam Tip: Focus on the "breaking down" aspect for dissociation reactions. Heat is a common factor that causes compounds to dissociate.

 

Question 5. What are the properties of chemical reactions?
Answer: Chemical reactions can be identified by several observable changes. These include:

• Rusting of iron, where iron reacts with oxygen and moisture to form iron oxide.
• Production of gases, like when fuels burn and release carbon dioxide.
• Formation of hydrogen gas when sulphuric acid reacts with zinc.
• Changes in color, such as an apple turning brown after being cut.
• Changes in temperature, where a reaction can make things hotter or colder.
• Dissolution, like how some substances dissolve to form clear solutions while others produce undissolved solids.

In simple words: You can tell a chemical reaction is happening by things like rust forming, gas coming out, color changing, heat being made or taken, or solids dissolving.

🎯 Exam Tip: When listing properties, try to provide a brief, clear example for each to show understanding. Common indicators like gas evolution, color change, and heat change are usually sufficient.

Chemical Reactions Long Answer Type Questions

 

Question 1. What are the types of chemical reactions? Explain the four types of chemical reactions with example.
Answer: Chemical reactions are processes that change one set of chemical substances into another. There are various types of reactions, including:

1. Combination reaction
2. Decomposition reaction
3. Displacement reaction
4. Neutralisation reaction
5. Double displacement reaction
6. Oxidation and reduction reaction

Let's explain some types with examples:

1. Combination reaction:
In a combination reaction, two or more substances (elements or compounds) join together to form a single, new compound. For example, when magnesium metal is heated in the presence of oxygen, they combine to form magnesium oxide, a single product. This reaction often releases energy.

\( 2\text{Mg (s)} + \text{O}_2 \text{ (g)} \xrightarrow{\text{Combination}} 2\text{MgO (s)} \)

3. Double displacement reaction:
In a double displacement reaction, two compounds exchange ions to form two entirely new compounds. These reactions usually happen in solutions, and one of the new products often forms a solid that settles down, called a precipitate. For instance, when silver nitrate solution is mixed with sodium chloride solution, they exchange ions to produce a white solid called silver chloride and sodium nitrate solution. This is a common way to form new insoluble substances.

\( \text{AgNO}_3 \text{ (aq)} + \text{NaCl (aq)} \rightarrow \text{AgCl (s)} \downarrow + \text{NaNO}_3 \text{ (aq)} \)

4. Oxidation and reduction reactions:
Oxidation and reduction are processes that always occur together in a chemical reaction.

Oxidation: This involves the addition of oxygen to a substance, or the removal of hydrogen from it. For example, iron combining with oxygen to form rust is an oxidation process.

Reduction: This involves the addition of hydrogen to a substance, or the removal of oxygen from it. These two processes are opposite, and one cannot happen without the other. For instance, if one substance gets oxidized, another substance must get reduced simultaneously. This is why they are often called "redox" reactions.

In simple words: Chemical reactions are how substances change. Types include combination (things join), double displacement (parts swap), and oxidation-reduction (oxygen/hydrogen is gained or lost). Combination reactions make one new thing from many. Double displacement means two compounds swap parts. Oxidation means getting oxygen or losing hydrogen, and reduction means losing oxygen or getting hydrogen.

🎯 Exam Tip: When explaining reaction types, always provide a balanced chemical equation and clearly state what defines that type of reaction. Showing how the reactants change into products is crucial.

 

Question 2. Explain any two characteristics of chemical reaction with experiment.
Answer: Two important characteristics of chemical reactions, explained with experiments, are:

1. Evolution of a Gas:
When zinc metal reacts with dilute sulphuric acid, hydrogen gas is produced. This gas can be detected by bringing a burning matchstick near the mouth of the test tube; the hydrogen gas will burn with a 'pop' sound, confirming its presence. This gas production shows that a chemical reaction has occurred.

\( \text{Zn (s)} + \text{H}_2\text{SO}_4 \text{ (aq)} \rightarrow \text{ZnSO}_4 \text{ (aq)} + \text{H}_2 \text{ (g)} \uparrow \)

2. Change in Colour:
Chemical reactions can also cause a change in color. For example, if white crystals of lead nitrate \( \text{Pb(NO}_3)_2 \) are strongly heated, they decompose. This reaction forms yellow-colored lead monoxide \( \text{PbO} \) and reddish-brown nitrogen dioxide \( \text{NO}_2 \) gas. The change from white crystals to yellow solid and reddish-brown gas indicates a chemical change has taken place.

\( 2\text{Pb(NO}_3)_2 \text{ (s)} \xrightarrow{\text{Heat}} 2\text{PbO (s)} + 4\text{NO}_2 \text{ (g)} \uparrow + \text{O}_2 \text{ (g)} \uparrow \)

In simple words: Chemical reactions can be seen by a gas being made, like hydrogen when zinc reacts with acid (it makes a pop sound). Another sign is a color change, like white lead nitrate turning yellow and making a brown gas when heated.

🎯 Exam Tip: When asked to explain characteristics with experiments, describe both the observable change and the underlying chemical process, including a balanced equation if possible.

 

Question 3. Explain the following reactions with example.
1. Neutralization reaction
2. Decomposition reaction
3. Exothermic reaction
4. Endothermic reaction
Answer: Here are explanations for each type of reaction with examples:

1. Neutralization reaction:
A neutralization reaction occurs when an acid (or acidic oxide) reacts with a base (or basic oxide) to form salt and water. This reaction typically releases heat. For instance, when hydrochloric acid \( \text{HCl} \) reacts with sodium hydroxide \( \text{NaOH} \), they form sodium chloride \( \text{NaCl} \) (table salt) and water \( \text{H}_2\text{O} \), along with heat. This is a very common reaction in chemistry.

\( \text{HCl (Hydrochloric acid)} + \text{NaOH (Sodium Hydroxide) base} \rightarrow \text{NaCl (Sodium chloride)} + \text{H}_2\text{O} + \text{Heat} \)

2. Decomposition Reaction:
A decomposition reaction is when a single compound breaks down into two or more simpler products. For example, when calcium carbonate \( \text{CaCO}_3 \) is heated strongly, it breaks down into calcium oxide \( \text{CaO} \) and carbon dioxide \( \text{CO}_2 \) gas. This process is important for making lime from limestone.

\( \text{CaCO}_3 \text{ (s)} \xrightarrow{\text{Heat}} \text{CaO (s)} + \text{CO}_2 \text{ (g)} \)

3. Exothermic Reactions:
Exothermic reactions are chemical reactions that release energy, usually in the form of heat, light, or sound. This heat release is often shown by writing "+ heat" on the product side of the equation. All burning reactions are examples of exothermic reactions. For instance, when carbon burns in the air, it forms carbon dioxide and releases heat.

\( \text{C (s)} + \text{O}_2 \text{ (g)} \rightarrow \text{CO}_2 \text{ (g)} + \text{heat} \)

4. Endothermic reactions:
Endothermic reactions are chemical reactions that absorb energy from their surroundings, typically as heat. This absorption of heat is often shown by writing "- heat" on the product side or "+ heat" on the reactant side of the equation. For example, when carbon and sulphur are heated together to form carbon disulphide, heat must be absorbed for the reaction to occur.

\( \text{C (s)} + 2\text{S (s)} \rightarrow \text{CS}_2 \text{ (l)} - \text{heat} \)

In simple words: Neutralization is when acid and base make salt and water. Decomposition is when one thing breaks into many. Exothermic reactions let out heat, like burning. Endothermic reactions take in heat, making things colder.

🎯 Exam Tip: For each reaction type, provide a clear definition, a balanced chemical equation, and a brief description of what happens in the example to ensure full marks.

Chemical Reactions Additional Questions Solved

I. Multiple Choice Questions

 

Question 1. A chemical reaction in which heat is given out is called
(a) exothermic reaction
(b) endothermic reaction
(c) decomposition reaction
(d) neutralization reaction
Answer: (a) exothermic reaction
In simple words: A reaction that gives out heat is called exothermic.

🎯 Exam Tip: Remember "exo" means "out" (like exit) and "thermic" relates to heat. So, exothermic means heat going out.

 

Question 2. What is the direction of the arrow in the reaction?
(a) towards reactants
(b) towards precipitate
(c) towards gas
(d) towards product
Answer: (d) towards product
In simple words: The arrow in a chemical equation always points from the starting materials (reactants) to the new materials formed (products).

🎯 Exam Tip: The arrow is fundamental in chemistry; it signifies the direction of the reaction, showing what is consumed and what is formed.

 

Question 3. In which type of change, we observe alteration only in specific property of matter but not composition?
(a) chemical change
(b) physical change
(c) chemical operator
(d) physical operator
Answer: (b) physical change
In simple words: A physical change alters things like shape or size but does not create a new substance. For example, ice melting into water is a physical change.

🎯 Exam Tip: Distinguish between physical and chemical changes: physical changes are reversible and don't change the substance's identity, while chemical changes form new substances and are often irreversible.

 

Question 5. \( \text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca (OH)}_2 \) is
(a) decomposition reaction
(b) combination reaction
(c) dissociation reaction
(d) endothermic reaction
Answer: (b) combination reaction
In simple words: When calcium oxide (lime) and water join to make calcium hydroxide, it's a combination reaction because two substances form one new substance.

🎯 Exam Tip: Always identify combination reactions by checking if multiple reactants yield a single product, simplifying the overall chemical structure.

 

Question 6. A neutralization reaction is a
(a) decomposition reaction.
(b) displacement reaction.
(c) combination reaction.
(d) double displacement reaction.
Answer: (d) double displacement reaction
In simple words: A neutralization reaction, where an acid and a base swap parts, is a type of double displacement reaction.

🎯 Exam Tip: Neutralization is a specific type of double displacement reaction where \( \text{H}^+ \) and \( \text{OH}^- \) ions combine to form water, and the remaining ions form a salt.

 

Question 7. In reduction reaction
(a) oxygen combines with the substance
(b) hydrogen reduces from the substance
(c) oxygen reduces from the substance
(d) none of the options
Answer: (c) oxygen reduces from the substance
In simple words: Reduction means a substance loses oxygen. It can also mean gaining hydrogen or gaining electrons.

🎯 Exam Tip: Recall that reduction is the opposite of oxidation. If oxidation is gaining oxygen, reduction is losing oxygen.

 

Question 8. Chemical combination involves:
(a) combination of two atoms.
(b) combination of only same type of elements.
(c) combination of only same type of compounds.
(d) combination of any two or more elements or compounds.
Answer: (d) combination of any two or more elements or compounds
In simple words: Chemical combination means two or more starting materials, which can be elements or compounds, join together to make a new substance.

🎯 Exam Tip: The key to a combination reaction is that the product is always a single, more complex substance formed from two or more simpler starting materials.

 

Question 10. Chemical reaction does not involves:
(a) absorption (or) release of energy
(b) forming of new substance (or) products
(c) two (or) more reactant
(d) None of the options
Answer: (d) None of the options
In simple words: Chemical reactions always involve changes in energy, forming new substances, and starting with at least one reactant. So, all the options listed are things that chemical reactions do involve.

🎯 Exam Tip: Think about the fundamental characteristics of a chemical reaction: energy changes, formation of new products, and conversion of reactants. If an option describes any of these, it's involved in a reaction.

 

Question 11. "Reaction of potassium iodide solution with lead nitrate solution" is the example of:
(a) chemical replacement reaction
(b) neutralisation reaction
(c) precipitation reaction
(d) chemical addition reaction
Answer: (c) precipitation reaction
In simple words: When potassium iodide and lead nitrate solutions mix, they form a solid called lead iodide that does not dissolve, which means it is a precipitation reaction.

🎯 Exam Tip: A precipitation reaction is characterized by the formation of an insoluble solid (precipitate) when two solutions are mixed. Lead iodide is a classic example of a yellow precipitate.

 

Question 12. In a combination reaction how many products are formed?
(a) two only
(b) one or two only
(c) one only
(d) number cannot be specified
Answer: (c) one only
In simple words: In a combination reaction, many substances combine, but they always form just one new, single product.

🎯 Exam Tip: The defining feature of a combination reaction is that multiple reactants unite to form exactly one product.

 

Question 13. \( \text{A} + \text{B} + \text{C} + \text{D} \rightarrow \text{A} + \text{D} + \text{C} + \text{B} \). Identify the above generalised type of reaction.
(a) chemical replacement reaction
(b) chemical double decomposition reaction
(c) chemical displacement reaction
(d) chemical combination reaction
Answer: (b) chemical double decomposition reaction
In simple words: This general formula shows that parts of two compounds (AB and CD) swap places to form two new compounds (AD and CB). This type of reaction is called double decomposition or double displacement.

🎯 Exam Tip: Look for patterns where cations and anions from two different compounds exchange partners. This indicates a double displacement (or double decomposition) reaction.

 

Question 14. Example of decomposition reaction is
In the reaction between zinc and iodine, zinc iodide is formed. What is being oxidized?
(a) zinc ions
(b) iodide ions
(c) zinc atom
(d) Iodine
Answer: (b) iodide ions
In simple words: This question is jumbled. However, if asked which part is oxidized when zinc reacts with iodine to form zinc iodide, it's the zinc atom that loses electrons. If the question implies a different reaction where iodide ions are oxidized, it would mean they lose electrons. Given the answer is iodide ions, they would be losing electrons.

🎯 Exam Tip: Oxidation involves losing electrons. In a reaction forming zinc iodide, zinc (a metal) typically loses electrons (oxidized), and iodine (a non-metal) gains electrons (reduced). Always check the most common oxidation states.

Chemical Reactions Very Short Answer Type Questions

 

Question 1. Define reactants and products?
Answer: Reactants are the original substances that take part in a chemical reaction. Products are the new substances that are formed as a result of the chemical reaction. Reactants change their chemical identity to become products.
In simple words: Reactants are the starting materials in a chemical reaction, and products are the new materials made by the reaction.

🎯 Exam Tip: Clearly state that reactants are consumed and products are formed. Using a simple example can also help illustrate the definitions.

 

Question 3. What is oxidizing agents?
Answer: An oxidizing agent is a substance that causes other substances to become oxidized. It does this by either providing oxygen to them or by removing hydrogen or electrons from them. In doing so, the oxidizing agent itself gets reduced.
In simple words: An oxidizing agent is a chemical that helps another chemical to get oxidized (lose electrons or gain oxygen). The oxidizing agent itself gets reduced.

🎯 Exam Tip: Remember that an oxidizing agent is reduced in a chemical reaction, while a reducing agent is oxidized. They are partners in redox reactions.

 

Question 4. Why do copper granules become black after heating?
Answer: Copper granules become black after heating because the copper reacts with oxygen present in the air. This reaction forms a layer of black copper oxide \( \text{CuO} \) on the surface of the copper granules. The newly formed copper oxide is responsible for the black color. This is an oxidation reaction.
\( 2\text{Cu (s)} + \text{O}_2 \text{ (g)} \xrightarrow{\text{Heat}} 2\text{CuO (s)} \)
In simple words: Copper turns black when heated because it mixes with air's oxygen to make a black substance called copper oxide on its surface.

🎯 Exam Tip: Relate the color change directly to the formation of a new compound, in this case, copper oxide, which has a distinct color.

 

Question 5. What are catalyst?
Answer: A catalyst is a substance that speeds up the rate of a chemical reaction without being used up or changed permanently in the reaction itself. Catalysts participate in the reaction but are regenerated at the end, allowing them to be reused. They make reactions happen faster or at lower temperatures.
In simple words: A catalyst is a special substance that makes a chemical reaction go faster but does not get used up or changed in the process.

🎯 Exam Tip: Key points for catalysts are: they change reaction rate (usually speed up), are not consumed, and can be recovered unchanged after the reaction.

 

Question 6. Define redox reaction?
Answer: A redox reaction (short for reduction-oxidation reaction) is a type of chemical reaction where one substance is oxidized (loses electrons or gains oxygen/loses hydrogen) and another substance is simultaneously reduced (gains electrons or loses oxygen/gains hydrogen). These two processes always happen together; one cannot occur without the other.
In simple words: A redox reaction is when one chemical loses electrons (gets oxidized) and another chemical gains those electrons (gets reduced) at the same time.

🎯 Exam Tip: Emphasize that oxidation and reduction are always coupled. Clearly define both processes in terms of electron transfer, or oxygen/hydrogen gain/loss.

Chemical Reactions Short Answer Type Questions

 

Question 1. Write five chemical changes that you observed in your day to day life.
Answer: Here are some chemical changes observed in daily life:
(a) Rusting of iron: Iron objects like gates or tools turning reddish-brown over time.
(b) Color change in food items: A cut apple or banana slice turning brown after being exposed to air.
(c) Photosynthesis: Green plants using sunlight to turn carbon dioxide and water into food (glucose) and oxygen.
(d) Burning of wood: Wood turning into ash and smoke when burned.
(e) Cooking an egg: The clear liquid egg white turning solid and white when heated.In simple words: Everyday chemical changes include iron rusting, cut fruits turning brown, plants making food using sunlight, wood burning, and cooking an egg.

🎯 Exam Tip: When providing examples, choose simple, common observations where a new substance is clearly formed, distinguishing them from physical changes.

 

Question 2. Which is combination reaction and which is displacement reaction?
Answer:

1. In the first reaction (e.g., potassium bromide and chlorine solution forming potassium chloride and bromine), chlorine is more reactive than bromine, so it displaces bromine from potassium bromide. This is a **displacement reaction**.

2. In the second reaction (e.g., iron combining with sulfur to form iron sulfide), two elements (iron and sulfur) combine to form a single compound (iron sulfide). This is a **combination reaction**.

In simple words: A combination reaction is when two or more things join to make one new thing. A displacement reaction is when one strong element pushes out a weaker element from a compound.

🎯 Exam Tip: Always analyze the number of reactants and products, and whether elements are switching places, to correctly identify combination versus displacement reactions.

 

Question 3. Define chemical reaction and write its characteristics.
Answer: A chemical reaction is a process where one or more substances (reactants) change into one or more different substances (products). This change involves breaking old chemical bonds and forming new ones.

Characteristics of a chemical reaction include:

• **Evolution of Gas:** Release of gas bubbles (e.g., acid and metal reaction).
• **Change in Color:** The color of the substances changes (e.g., rusting of iron).
• **Formation of a Precipitate:** A solid substance forms and settles out from a liquid solution.
• **Change in Energy:** Heat is either absorbed (endothermic) or released (exothermic).
• **Change of State:** Reactants in one state form products in a different state (e.g., liquid turning into a solid or gas).

In simple words: A chemical reaction is when chemicals change into new ones. You can tell it's happening if a gas comes out, the color changes, a solid forms, it gets hot or cold, or the state changes.

🎯 Exam Tip: To define a chemical reaction, highlight that new substances are formed. For characteristics, list observable changes and give a simple example for each.

 

Question 4. The equation below shows the reaction between nitrogen and oxygen.
\( \text{N}_2 \text{ (g)} + 2\text{O}_2 \text{ (g)} + 66\text{KJ} \rightarrow 2\text{NO}_2 \text{ (g)} \)
(a) Is this an exothermic or endothermic reaction? Explain your answer.
Answer: (a) This is an **endothermic reaction**. The term "+ 66KJ" on the reactant side of the equation shows that 66 kilojoules of heat energy are absorbed from the surroundings for the reaction to occur. If heat is listed as a reactant (on the left side), it means energy is taken in, which is the definition of an endothermic process. This absorption of energy is crucial for the reaction to proceed.
In simple words: This is an endothermic reaction. We know this because the equation shows heat energy being added on the left side, meaning heat is absorbed for the reaction to happen.

🎯 Exam Tip: For energy changes, if heat is on the reactant side (left of the arrow) or has a positive value, it's endothermic. If it's on the product side (right of the arrow) or has a negative value, it's exothermic.

 

Question 5. A substance X undergo a chemical change involving the loss of hydrogen and this result in the formation of a byproduct. What type of agents are X and Y? (Reducing agent or oxidizing agent).
Answer: We need to understand the definitions of oxidizing and reducing agents.

Substance X loses hydrogen. The loss of hydrogen is a definition of oxidation. Therefore, if substance X is oxidized, it acts as a **reducing agent** for the other substance in the reaction. A reducing agent causes another substance to be reduced while it itself gets oxidized.

The byproduct Y is formed when hydrogen is added to something. The addition of hydrogen is a definition of reduction. If Y gains hydrogen, then the substance that *provided* the hydrogen (or removed electrons from Y) is the one causing Y to be reduced. However, based on the question structure, if Y is the product of hydrogen being added, the *other* reactant (which caused this addition of hydrogen to form Y) would be the **oxidizing agent**. The question implies Y is a byproduct formed, and by its context, if X loses hydrogen, something else gains it. The agent that causes reduction (gain of hydrogen) is the oxidizing agent. So, the substance that caused the formation of Y by adding hydrogen to it is an oxidizing agent.

In simple words: Substance X loses hydrogen, so it gets oxidized and is a reducing agent. If hydrogen is added to make a byproduct Y, the substance that caused Y to gain hydrogen is an oxidizing agent.

🎯 Exam Tip: Clearly remember that oxidation is loss of hydrogen (or electrons), and reduction is gain of hydrogen (or electrons). An oxidizing agent gets reduced, and a reducing agent gets oxidized.

Activity and questions based on it.

 

Question 1. What do you see?
Answer: After mixing iron granules and sulfur powder and heating them, a new dark-colored substance called iron sulfide is formed. This indicates a chemical change has taken place, as the properties of the new substance are different from the original iron and sulfur.
\( \text{Iron (s)} + \text{Sulphur (s)} \xrightarrow{\text{Heat}} \text{Iron sulphide (s)} \)
In simple words: When iron and sulfur are heated together, they turn into a new, dark substance called iron sulfide.

🎯 Exam Tip: When describing observations, mention any color changes, new substance formation, or energy changes. For this activity, the key is the formation of a new compound with distinct properties.

 

Question 1. What do you see?
Answer: When zinc granules react with dilute sulphuric acid in a test tube, hydrogen gas is evolved. If a burning matchstick is brought near the opening of the test tube, it will extinguish with a 'pop' sound, confirming the presence of hydrogen gas. This is a classic test for hydrogen.
In simple words: When zinc metal reacts with acid, hydrogen gas is made. If you hold a lit match near it, it will make a 'pop' sound.

🎯 Exam Tip: The 'pop' sound test is specific to hydrogen gas. Remember the reaction \( \text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2 \uparrow \).

 

Question 1. If we pass this gas in lime water then what happens?
Answer: If this gas is passed through lime water, the lime water turns milky. This happens because calcium carbonate, which is insoluble, is formed.
In simple words: When the gas goes into lime water, the water changes color to milky.

🎯 Exam Tip: The turning of lime water milky is a classic test to confirm the presence of carbon dioxide gas.

 

Question 2. What is the name of this gas?
Answer: The name of this gas is Carbon dioxide \( (CO_2) \). This gas is produced when limestone is heated.
In simple words: The gas is called carbon dioxide.

🎯 Exam Tip: Carbon dioxide is an odorless and colorless gas that plays a key role in many chemical reactions and in Earth's atmosphere.

 

Question 3. Write its equation.
Answer: The equation for this reaction is:
\( CaCO_3 \rightarrow CaO + CO_2 \)
This shows calcium carbonate breaking down into calcium oxide and carbon dioxide.
In simple words: The equation is calcium carbonate changing into calcium oxide and carbon dioxide.

🎯 Exam Tip: Always balance your chemical equations to ensure the number of atoms for each element is the same on both sides.

 

Question 1. After observing the test tube, what do you see?
Answer: After some time, a brown layer is seen forming on the iron nail. This brown layer shows that a new substance has been created.
In simple words: You will see a brown layer on the iron nail.

🎯 Exam Tip: Observing color changes and deposits are important indicators that a chemical reaction has occurred.

 

Question 2. Name the layer formed.
Answer: The layer formed on the iron nail is copper. This happens because iron is more reactive than copper, displacing it from the copper sulfate solution.
In simple words: The layer that formed is copper.

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

 

Question 4. Write the equation of the reaction.
Answer: The equation for the reaction is:
\( Fe + CuSO_4 \rightarrow FeSO_4 + Cu \)
Here, iron reacts with copper sulfate to form iron sulfate and copper.
In simple words: Iron plus copper sulfate makes iron sulfate plus copper.

🎯 Exam Tip: Remember to write the states of matter (s, l, g, aq) next to each substance in a chemical equation for completeness.

 

Question 5. What is the name of this reaction?
Answer: This reaction is called a displacement reaction. In this type of reaction, one element replaces another element in a compound.
In simple words: This is a displacement reaction, where one metal replaces another.

🎯 Exam Tip: Displacement reactions are common and often involve a more reactive metal pushing out a less reactive metal from its compound.

 

Question 1. What do you see?
Answer: A black color layer is seen on the copper powder. This black layer is copper oxide, formed when copper reacts with oxygen in the air during heating.
In simple words: You will see a black layer on the copper powder.

🎯 Exam Tip: Many metals form oxides when heated in the presence of air, which is a common observation in chemical reactions.

 

Question 2. Name the substance of black layer.
Answer: The black layer is made of copper oxide. This forms when copper combines with oxygen.
In simple words: The black layer is copper oxide.

🎯 Exam Tip: The color of an oxide can help identify the new compound formed during a reaction.

 

Question 3. Name the type of this reaction.
Answer: This is an oxidation reaction. Oxidation involves the addition of oxygen to a substance. Here, copper gains oxygen to form copper oxide.
In simple words: This reaction is called oxidation because oxygen is added to the copper.

🎯 Exam Tip: Oxidation is fundamentally the loss of electrons or gain of oxygen, while reduction is the gain of electrons or loss of oxygen.

 

Question 2. What happens on reduction in temperature?
Answer: The temperature reduces because heat is absorbed during the reaction. This makes the surroundings feel colder.
In simple words: The temperature goes down because the reaction takes in heat.

🎯 Exam Tip: Reactions that absorb heat from their surroundings, causing the temperature to drop, are known as endothermic reactions.

 

Question 3. Name the type of this reaction.
Answer: This type of reaction is an endothermic reaction. It is called endothermic because it absorbs heat from its surroundings.
In simple words: This is an endothermic reaction because it absorbs heat.

🎯 Exam Tip: Endothermic reactions often feel cold to the touch because they draw heat energy from their environment.

 

Question 1. Identify the type of chemical reactions.
(a) \( H_2 + Cl_2 \rightarrow 2HCl \)
(b) \( CaO + H_2O \rightarrow Ca (OH)_2 \)
(c) \( CaCO_3 \rightarrow CaO + CO_2 \)
(d) \( FeSO_4 + Mg \rightarrow MgSO_4 + Fe \)
(e) \( 2KI + Pb(NO_3)_2 \rightarrow 2KNO_3 + PbI_2 \)
Answer:
(a) \( H_2 + Cl_2 \rightarrow 2HCl \)
Explanation: Two elements, hydrogen \( (H_2) \) and chlorine \( (Cl_2) \), combine to form a single compound, hydrochloric acid \( (HCl) \). Therefore, this is a combination reaction where two elements combine to make a new compound.
(b) \( CaO + H_2O \rightarrow Ca(OH)_2 \)
Explanation: Two compounds, calcium oxide \( (CaO) \) and water \( (H_2O) \), combine to form a single new compound, calcium hydroxide \( (Ca(OH)_2) \). So, this is also a combination reaction, specifically a compound-compound combination.
(c) \( CaCO_3 \rightarrow CaO + CO_2 \)
Explanation: The compound calcium carbonate \( (CaCO_3) \) breaks down or splits into two simpler compounds, calcium oxide \( (CaO) \) and carbon dioxide \( (CO_2) \). This type of reaction is known as a decomposition reaction.
(d) \( FeSO_4 + Mg \rightarrow MgSO_4 + Fe \)
Explanation: In this reaction, magnesium \( (Mg) \) displaces iron \( (Fe) \) from iron sulfate \( (FeSO_4) \) to form magnesium sulfate \( (MgSO_4) \) and free iron. This is a displacement reaction.
(e) \( 2KI + Pb(NO_3)_2 \rightarrow 2KNO_3 + PbI_2 \)
Explanation: Potassium iodide \( (KI) \) and lead nitrate \( (Pb(NO_3)_2) \) react by exchanging their ions to form two new compounds, potassium nitrate \( (KNO_3) \) and lead iodide \( (PbI_2) \). This is a double displacement reaction.
In simple words: We need to identify if each reaction is combining, breaking apart, or swapping parts. (a) and (b) are combination reactions because two things join. (c) is decomposition because one thing breaks into two. (d) is displacement because one element kicks out another. (e) is double displacement because two compounds swap partners.

🎯 Exam Tip: To classify reactions, look at the reactants and products: combination (A+B → C), decomposition (A → B+C), displacement (A+BC → AC+B), and double displacement (AB+CD → AD+CB).

 

Question 2. What is reduction also explain reducing agent?
Answer: Reduction is a chemical reaction where a substance gains hydrogen or loses oxygen. For example, when hydrogen gas is passed over heated black copper oxide, the copper oxide turns into reddish-brown copper metal because it loses oxygen. So, copper oxide is reduced to copper.

\( H_2 + CuO \rightarrow Cu + H_2O \)

A reducing agent is a substance that causes another substance to be reduced (by making it lose oxygen or gain hydrogen), while it itself gets oxidized. In the example above, hydrogen acts as the reducing agent because it removes oxygen from copper oxide. Similarly, when chlorine reacts with hydrogen to form hydrogen chloride, hydrogen acts as the reducing agent by supplying hydrogen to chlorine.
In simple words: Reduction is when something gains hydrogen or loses oxygen. A reducing agent is the substance that helps another substance get reduced, while the reducing agent itself gets oxidized. For example, hydrogen is a common reducing agent.

🎯 Exam Tip: Remember the mnemonic "OIL RIG": Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). In terms of oxygen and hydrogen, oxidation is gaining oxygen or losing hydrogen, and reduction is losing oxygen or gaining hydrogen.