Samacheer Kalvi Class 10 Science Solutions Chapter 10 Types of Chemical Reactions

Tamilnadu Samacheer Kalvi 10th Science Solutions Chapter 10 Types of Chemical Reactions

Samacheer Kalvi 10th Science Types of Chemical Reactions Text Book Back Questions and Answers

I. Choose the correct answer:

Question 1. \( \mathrm{H}_{2} + \mathrm{Cl}_{2} \rightarrow 2\mathrm{HCl}_{(\mathrm{g})} \)
(a) Decomposition Reaction
(b) Combination Reaction
(c) Single Displacement Reaction
(d) Double Displacement Reaction
Answer: (a) Decomposition Reaction
In simple words: This question asks you to identify the type of chemical reaction shown. You need to look at how the chemicals combine or break apart to decide the reaction type.

🎯 Exam Tip: Always analyze the reactants and products to correctly classify a chemical reaction as combination, decomposition, displacement, or double displacement.

Question 2. Photolysis is a decomposition reaction caused by _____
(a) heat
(b) electricity
(c) light
(d) mechanical energy
Answer: (c) light
In simple words: Photolysis means a reaction is broken down by light. Just like how a plant uses sunlight to grow, some chemicals break apart when light shines on them.

🎯 Exam Tip: Remember the prefix "photo-" always refers to light. This helps in understanding terms like photosynthesis and photolysis.

Question 3. A reaction between carbon and oxygen is represented by \( \mathrm{C}_{(\mathrm{s})} + \mathrm{O}_{2(\mathrm{g})} \rightarrow \mathrm{CO}_{2(\mathrm{g})} + \text{Heat} \). In which of the type(s), the above reaction can be classified?
(i) Combination Reaction
(ii) Combustion Reaction
(iii) Decomposition Reaction
(iv) Irreversible Reaction
(a) (i) and (ii)
(b) (i) and (iv)
(c) (ii) and (iii)
(d) (i), (ii) and (iv)
Answer: (d) (i), (ii) and (iv)
In simple words: This reaction combines carbon and oxygen to make carbon dioxide, which is a combination. It also burns, which is combustion, and it cannot be easily reversed, making it irreversible.

🎯 Exam Tip: Some reactions can fall into multiple categories. Always consider all definitions before choosing the best classification.

Question 4. The chemical equation \( \mathrm{Na}_{2}\mathrm{SO}_{4(\mathrm{aq})}+\mathrm{BaCl}_{2(\mathrm{aq})} \rightarrow \mathrm{BaSO}_{4(\mathrm{s})} \downarrow+2 \mathrm{NaCl}_{(\mathrm{aq})} \) represents which of the following types of reaction?
(a) Neutralisation
(b) Combustion
(c) Precipitation
(d) Single displacement.
Answer: (c) Precipitation
In simple words: When you mix these two clear solutions, a solid white substance called barium sulfate is formed and falls to the bottom. This process of forming a solid from liquids is called precipitation.

🎯 Exam Tip: Look for the arrow pointing down \( \downarrow \) or the state symbol (s) in the product side to identify a precipitation reaction.

Question 5. Which of the following statements are correct about a chemical equilibrium?
(i) It is dynamic in nature
(ii) The rate of the forward and backward reactions are equal at equilibrium
(iii) Irreversible reactions do not attain chemical equilibrium
(iv) The concentration of reactants and products may be different
(a) (i), (ii) and (iii)
(b) (i), (ii) and (iv)
(c) (ii), (iii) and (iv)
(d) (i), (iii) and (iv)
Answer: (a) (i), (ii) and (iii)
In simple words: In a balanced chemical reaction, the chemicals keep changing back and forth, but the overall amount of each chemical stays the same. This is like two people running towards each other, passing, and running back at the same speed.

🎯 Exam Tip: Remember that equilibrium is dynamic, meaning reactions are still happening, but the forward and reverse rates are equal, leading to no net change in concentrations.

Question 6. A single displacement reaction is represented by \( \mathrm{X}_{(\mathrm{s})}+2 \mathrm{HCl}_{(\mathrm{aq})} \rightarrow \mathrm{XCl}_{2(\mathrm{aq})}+\mathrm{H}_{2(\mathrm{g})} \). the following(s) could be X?
(i) Zn
(ii) Ag
(iii) Cu
(iv) Mg.
Choose the best pair.
(a) i and ii
(b) ii and iii
(c) iii and iv
(d) i and iv.
Answer: (d) i and iv.
In simple words: In this type of reaction, a metal 'X' takes the place of hydrogen in hydrochloric acid. Zinc and Magnesium are both reactive enough to do this, meaning they are more reactive than hydrogen.

🎯 Exam Tip: To predict single displacement reactions, always refer to the activity series of metals. A more reactive metal displaces a less reactive metal from its salt solution.

Question 7. Which of the following is not an “element + element → compound" type reaction?
(a) \( \mathrm{C}_{(\mathrm{s})} + \mathrm{O}_{2(\mathrm{g})} \rightarrow \mathrm{CO}_{2(\mathrm{g})} \)
(b) \( 2\mathrm{K}_{(\mathrm{s})} + \mathrm{Br}_{2(\mathrm{l})} \rightarrow 2\mathrm{KBr}_{(\mathrm{s})} \)
(c) \( 2\mathrm{CO}_{(\mathrm{g})} + \mathrm{O}_{2(\mathrm{g})} \rightarrow 2\mathrm{CO}_{2(\mathrm{g})} \)
(d) \( 4\mathrm{Fe}_{(\mathrm{s})} + 3\mathrm{O}_{2(\mathrm{g})} \rightarrow 2\mathrm{Fe}_{2}\mathrm{O}_{3(\mathrm{s})} \)
Answer: (c) \( 2\mathrm{CO}_{(\mathrm{g})} + \mathrm{O}_{2(\mathrm{g})} \rightarrow 2\mathrm{CO}_{2(\mathrm{g})} \)
In simple words: The question asks for the reaction that is NOT "element + element to compound". Option (c) starts with carbon monoxide (\( \mathrm{CO} \)), which is already a compound, not a single element. So, it does not fit the "element + element" rule.

🎯 Exam Tip: Carefully identify if the reactants are single elements (like C, O2, K, Br2, Fe) or compounds (like CO) to classify combination reactions correctly.

Question 8. Which of the following represents a precipitation reaction?
(a) \( \mathrm{A}_{(\mathrm{s})} + \mathrm{B}_{(\mathrm{s})} \rightarrow \mathrm{C}_{(\mathrm{s})} + \mathrm{D}_{(\mathrm{s})} \)
(b) \( \mathrm{A}_{(\mathrm{s})} + \mathrm{B}_{(\mathrm{aq})} \rightarrow \mathrm{C}_{(\mathrm{aq})} + \mathrm{D}_{(\mathrm{l})} \)
(c) \( \mathrm{A}_{(\mathrm{aq})} + \mathrm{B}_{(\mathrm{aq})} \rightarrow \mathrm{C}_{(\mathrm{s})} + \mathrm{D}_{(\mathrm{aq})} \)
(d) \( \mathrm{A}_{(\mathrm{aq})} + \mathrm{B}_{(\mathrm{s})} \rightarrow \mathrm{C}_{(\mathrm{aq})} + \mathrm{D}_{(\mathrm{l})} \)
Answer: (c) \( \mathrm{A}_{(\mathrm{aq})} + \mathrm{B}_{(\mathrm{aq})} \rightarrow \mathrm{C}_{(\mathrm{s})} + \mathrm{D}_{(\mathrm{aq})} \)
In simple words: A precipitation reaction makes a solid from two liquids. This means the reactants are dissolved in water (aqueous) and one of the products is a solid.

🎯 Exam Tip: Look for reactants in the aqueous (aq) state and a product in the solid (s) state to identify a precipitation reaction.

Question 9. The pH of a solution is 3. Its [OH¯] concentration is _____
(a) \( 1 \times 10^{-3} \mathrm{M} \)
(b) \( 3 \mathrm{M} \)
(c) \( 1 \times 10^{-11} \mathrm{M} \)
(d) \( 11 \mathrm{M} \).
Answer: (c) \( 1 \times 10^{-11} \mathrm{M} \)
In simple words: We know pH and pOH always add up to 14. If pH is 3, then pOH is 11. The concentration of hydroxide ions \( [\mathrm{OH}^-] \) is found by raising 10 to the power of negative pOH.

🎯 Exam Tip: Remember the relationship \( \text{pH} + \text{pOH} = 14 \) and that \( [\mathrm{H}^+] = 10^{-\text{pH}} \) and \( [\mathrm{OH}^-] = 10^{-\text{pOH}} \).

Question 10. Powdered \( \mathrm{CaCO}_3 \) reacts more rapidly than flaky \( \mathrm{CaCO}_3 \) because of :
(a) large surface area
(b) high pressure
(c) high concentration
(d) high temperature
Answer: (a) large surface area
In simple words: When a solid is in powder form, it has more surface exposed to react. More surface area means more places for the reaction to happen, so it goes faster.

🎯 Exam Tip: Remember that increasing the surface area of solid reactants usually increases the rate of reaction by providing more sites for contact.

II. Fill in the blanks:

Question 1. A reaction between an acid and a base is called _______.
Answer: neutralization
In simple words: When an acid and a base mix, they cancel each other out, making water and a salt. This is called neutralization.

🎯 Exam Tip: Recognize that neutralization reactions are a type of double displacement where H+ and OH- ions combine to form water.

Question 2. When zinc metal is placed in hydrochloric acid, _______ gas is evolved.
Answer: \( \mathrm{H}_2 \)
In simple words: When zinc metal reacts with hydrochloric acid, it produces hydrogen gas, which can be seen as bubbles.

🎯 Exam Tip: Metals above hydrogen in the reactivity series displace hydrogen from acids, forming hydrogen gas.

Question 3. The equilibrium attained during the melting of ice is known as _______.
Answer: physical equilibrium
In simple words: When ice is melting into water, and water is freezing back into ice at the same rate, it's a balance between two physical states, not a chemical change.

🎯 Exam Tip: Physical equilibrium involves changes in state (like melting/freezing or boiling/condensing) rather than changes in chemical composition.

Question 4. The pH of a fruit juice is 5.6. If you add slaked lime to this juice, its pH _______.
Answer: increases to '7'
In simple words: Slaked lime is a base. Adding a base to an acidic fruit juice will make the juice less acidic and closer to neutral, which means its pH will go up towards 7.

🎯 Exam Tip: Bases increase the pH of solutions, making them less acidic. Acids decrease pH, making solutions more acidic.

Question 5. The value of ionic product of water at 25 °C is ___________.
Answer: \( 1 \times 10^{-14} \mathrm{mol}^2 \mathrm{dm}^{-6} \)
In simple words: Even pure water has a tiny amount of hydrogen and hydroxide ions. The product of their concentrations is a constant value at a certain temperature.

🎯 Exam Tip: Know the value of the ionic product of water \( K_w \) at standard temperature (25°C) for pH calculations.

Question 6. The normal pH of human blood is ___________.
Answer: 7.4
In simple words: Human blood needs to be slightly alkaline to work properly. A pH of 7.4 is just a little bit above neutral.

🎯 Exam Tip: The pH of human blood is tightly regulated; even small deviations can indicate health issues.

Question 7. Electrolysis is type of _______ reaction.
Answer: decomposition
In simple words: Electrolysis uses electricity to break down a compound into simpler substances. For example, water can be split into hydrogen and oxygen gas.

🎯 Exam Tip: Electrolysis is a specific type of decomposition reaction that requires electrical energy to break bonds.

Question 8. The number of products formed in a synthesis reaction is _______.
Answer: 1
In simple words: In a synthesis reaction, simpler substances combine to form a single, more complex product. This means you only get one main thing at the end.

🎯 Exam Tip: A synthesis (or combination) reaction is defined by having only one product formed from two or more reactants.

Question 9. Chemical volcano is an example for _______ reaction.
Answer: decomposition
In simple words: A chemical volcano often involves a compound breaking down, releasing gases and creating a visual effect similar to a volcanic eruption.

🎯 Exam Tip: Many visual chemistry demonstrations, like the chemical volcano, are based on exothermic decomposition reactions.

Question 10. The ion formed by dissolution of \( \mathrm{H}^+ \) in water is called _______.
Answer: hydronium ion
In simple words: When a hydrogen ion (which is just a proton) goes into water, it attaches to a water molecule to form a hydronium ion (\( \mathrm{H}_3\mathrm{O}^+ \)).

🎯 Exam Tip: In aqueous solutions, \( \mathrm{H}^+ \) ions always exist as hydronium ions (\( \mathrm{H}_3\mathrm{O}^+ \)) due to their strong attraction to water molecules.

Question 1. Identify the types of reaction:

🎯 Exam Tip: Carefully examine the reactants and products in each equation to determine the reaction type. Pay attention to phase changes and energy indicators like "Heat".

IV. True or False: (If false give the correct statement)

Question 1. Silver metal can replace hydrogen gas from nitric acid.
Answer: False – Silver cannot displace \( \mathrm{H}_2 \) from \( \mathrm{HNO}_3 \) acid. This is because silver is placed below hydrogen in the activity series, meaning it is less reactive than hydrogen.
In simple words: Silver cannot push hydrogen out of nitric acid because silver is not as strong as hydrogen in chemical reactions.

🎯 Exam Tip: Metals less reactive than hydrogen (like silver) cannot displace hydrogen from acids. Always consult the reactivity series.

Question 2. The pH of rain water containing dissolved gases like \( \mathrm{SO}_3 \), \( \mathrm{CO}_2 \), \( \mathrm{NO}_2 \) will be less than 7.
Answer: True
In simple words: These gases dissolve in rainwater to form acids. Acids have a pH lower than 7, so rain becomes acidic.

🎯 Exam Tip: Rainwater is naturally slightly acidic due to dissolved \( \mathrm{CO}_2 \), but pollutants like sulfur oxides and nitrogen oxides make it significantly more acidic, leading to acid rain.

Question 3. At the equilibrium of a reversible reaction, the concentration of the reactants and the products will be equal.
Answer: False – At equilibrium, the concentration of the reactants and products do not change; it remains constant. However, the concentration of the reactants and the products will not necessarily be equal.
In simple words: In a balanced reaction, the amounts of starting materials and end products stop changing, but they don't have to be the same amount. They just stop changing.

🎯 Exam Tip: Equilibrium means constant concentrations and equal forward/reverse reaction rates, not necessarily equal concentrations of reactants and products.

Question 4. Periodical removal of one of the products of a reversible reaction increases the yield.
Answer: True
In simple words: If you keep taking away one of the things a reversible reaction makes, the reaction will keep trying to make more of it. This helps you get more of the product in total.

🎯 Exam Tip: According to Le Chatelier's Principle, removing a product shifts the equilibrium to the right, favoring product formation and increasing yield.

Question 5. On dipping a pH paper in a solution, it turns into yellow. Then the solution is basic.
Answer: False – The solution is neutral if the solution is basic, it will be green in colour. If a pH paper turns yellow, it typically indicates an acidic solution.
In simple words: pH paper changes color to show if something is acid or base. Yellow means it's usually acidic, not basic. Basic solutions typically turn pH paper blue or green.

🎯 Exam Tip: Familiarize yourself with the color chart for pH paper or universal indicator to accurately determine acidity, neutrality, or basicity of solutions.

V. Short answer questions:

Question 1. When an aqueous solution of potassium chloride is added to an aqueous solution of silver nitrate, a white precipitate is formed. Give the chemical equation of this reaction.
Answer: When potassium chloride (\( \mathrm{KCl} \)) and silver nitrate (\( \mathrm{AgNO}_3 \)) solutions are mixed, they swap their ions. This forms silver chloride (\( \mathrm{AgCl} \)), which is a white solid, and potassium nitrate (\( \mathrm{KNO}_3 \)), which stays dissolved. The chemical equation is:
\( \mathrm{KCl}_{(\mathrm{aq})} + \mathrm{AgNO}_{3(\mathrm{aq})} \rightarrow \mathrm{AgCl}_{(\mathrm{s})} \downarrow + \mathrm{KNO}_{3(\mathrm{aq})} \)
Silver chloride
Curdy white precipitate
In simple words: Mixing \( \mathrm{KCl} \) and \( \mathrm{AgNO}_3 \) makes a white solid, \( \mathrm{AgCl} \). This happens because the parts of the two chemicals switch places.

🎯 Exam Tip: This is an example of a double displacement (or precipitation) reaction, where two soluble ionic compounds react to form an insoluble product.

Question 2. Why does the reaction rate of a reaction increase in raising the temperature?
Answer: When the temperature is increased, more heat energy is supplied to the reacting particles. This extra energy helps to break existing bonds in the reactants more easily and quickly. It also makes the particles move faster and collide more often and with greater force, leading to more successful reactions. For example, foods kept at room temperature spoil faster than those kept in a refrigerator because the lower temperature slows down the chemical reactions causing spoilage.
In simple words: When you make things hotter, the tiny particles move faster and hit each other harder and more often. This makes the chemical reaction happen quicker.

🎯 Exam Tip: Remember that temperature increases kinetic energy, leading to more frequent and energetic collisions between reactant molecules, which speeds up reaction rates.

Question 3. Define combination reaction. Give one example for an exothermic combination reaction.
Answer: A combination reaction is a type of chemical reaction where two or more reactants combine to form a single, more complex product. This is like putting building blocks together to make one bigger structure. An example of an exothermic combination reaction, which releases heat, is the burning of carbon to form carbon dioxide:
\( \mathrm{C}_{(\mathrm{s})} + \mathrm{O}_{2(\mathrm{g})} \rightarrow \mathrm{CO}_{2(\mathrm{g})} + \text{Heat} \)
In simple words: A combination reaction is when two or more simple things join to make one new, bigger thing. Burning coal is an example where carbon and oxygen combine, and heat is given off.

🎯 Exam Tip: For combination reactions, always ensure there is only one product on the right side of the chemical equation. Exothermic reactions release heat, often indicated by "+ Heat" in the products.

Question 4. Differentiate reversible and irreversible reactions.
Answer:

🎯 Exam Tip: A double arrow \( \rightleftharpoons \) indicates a reversible reaction, while a single arrow \( \rightarrow \) indicates an irreversible reaction.

VI. Answer in detail:

Question 1. What are called thermolysis reactions?
Answer: Thermal decomposition reactions are called 'thermolysis' reactions. In this type of reaction, the reactant is broken down into simpler substances by applying heat. These reactions need energy to happen, which comes from heating. There are two main types of thermolysis reactions:
(i) Compound to element / element decomposition:
In this case, a compound is broken down into its basic elements.
Eg: The decomposition of mercuric oxide by heat:
\( 2\mathrm{HgO}_{(\mathrm{s})} \stackrel{\text{heat}}{\longrightarrow} 2\mathrm{Hg}_{(\mathrm{l})} + \mathrm{O}_{2(\mathrm{g})} \)
(ii) Compound to compound / compound decomposition:
Here, a compound is broken down into two or more simpler compounds.
Eg: The decomposition of calcium carbonate by heat:
\( \mathrm{CaCO}_{3(\mathrm{s})} \stackrel{\text{heat}}{\longrightarrow} \mathrm{CaO}_{(\mathrm{s})} + \mathrm{CO}_{2(\mathrm{g})} \)
In simple words: Thermolysis is when heat breaks down a chemical into smaller parts. Sometimes it breaks into basic elements, like mercury oxide breaking into mercury and oxygen. Other times, it breaks into simpler compounds, like limestone breaking into lime and carbon dioxide.

🎯 Exam Tip: Look for "heat" written above the reaction arrow to identify a thermolysis reaction. Remember it's a type of decomposition reaction.

Question 2. Explain the types of double displacement reactions with examples.
Answer: Double displacement reactions involve the exchange of ions between two compounds, typically in aqueous solution, to form new compounds. They can be mainly divided into two classes:
(i) Precipitation Reactions: These occur when aqueous solutions of two compounds are mixed, and they react to form an insoluble compound (precipitate) and a soluble compound. The insoluble product often appears as a solid separating from the solution. For instance:
\( \mathrm{Pb}(\mathrm{NO}_{3})_{2(\mathrm{aq})}+2 \mathrm{KI}_{(\mathrm{aq})} \rightarrow \mathrm{PbI}_{2(\mathrm{s})} \downarrow+2 \mathrm{KNO}_{3(\mathrm{aq})} \)
(ii) Neutralisation Reactions: This is another type of double displacement reaction where an acid reacts with a base to form a salt and water. It is called 'neutralisation reaction' because both the acid and base lose their acidic or basic properties when they react with each other. For example:
\( \mathrm{NaOH}_{(\mathrm{aq})}+\mathrm{HCl}_{(\mathrm{aq})} \rightarrow \mathrm{NaCl}_{(\mathrm{aq})}+\mathrm{H}_{2} \mathrm{O}_{(\mathrm{l})} \)
In simple words: Double displacement means two chemicals swap their partners to make new chemicals. Sometimes this makes a solid fall out of the liquid (precipitation), like when lead nitrate and potassium iodide make a yellow solid. Other times, an acid and a base cancel each other out to make salt and water (neutralization), like sodium hydroxide and hydrochloric acid.

🎯 Exam Tip: In double displacement reactions, watch for the formation of a precipitate (solid), a gas, or water to identify the specific type. Solubility rules are key for predicting precipitates.

Question 3. Explain the factors influencing the rate of a reaction.
Answer: Several factors can change how fast a chemical reaction happens:
1. Nature of the reactants: Different chemicals react at different speeds. For example, sodium reacts faster with hydrochloric acid than with acetic acid. This is because hydrochloric acid is a stronger acid and thus more reactive.
\( \mathrm{2Na}_{(\mathrm{s})} + \mathrm{2HCl}_{(\mathrm{aq})} \rightarrow \mathrm{2NaCl}_{(\mathrm{aq})} + \mathrm{H}_{2(\mathrm{g})} \) (fast)
\( \mathrm{2Na}_{(\mathrm{s})} + \mathrm{2CH}_{3}\mathrm{COOH}_{(\mathrm{aq})} \rightarrow \mathrm{2CH}_{3}\mathrm{COONa}_{(\mathrm{aq})} + \mathrm{H}_{2(\mathrm{g})} \) (slow)
2. Concentration of the reactants: Changing the amount of reactants also increases the reaction rate. If there are more particles in the same space (higher concentration), they will hit each other more often, making the reaction faster. For instance, granulated zinc reacts faster with 2M hydrochloric acid than with 1M hydrochloric acid.
3. Temperature: Most reactions speed up at higher temperatures. This is because adding heat gives the reactant particles more energy. This extra energy helps break chemical bonds and also makes particles move faster, leading to more frequent and effective collisions. Calcium carbonate reacts slowly with hydrochloric acid at room temperature, but heating the mixture significantly increases the reaction rate.
4. Pressure: If the reactants are gases, increasing their pressure increases the reaction rate. This happens because higher pressure forces the gas particles closer together, so they collide more often and react more frequently.
5. Catalyst: A catalyst is a special substance that speeds up a reaction without being used up itself. It works by finding an easier path for the reaction to take, lowering the energy needed. For example, when heating potassium chlorate, it decomposes slowly. But adding manganese dioxide acts as a catalyst and significantly increases the reaction rate.
6. Surface area of the reactants: Powdered calcium carbonate reacts more readily with hydrochloric acid than larger marble chips. This is because powdering increases the surface area, giving more places for the reaction to happen. More exposed surface means more opportunities for reactant particles to collide and react, thus increasing the reaction rate.
In simple words: How fast a reaction goes depends on many things: what chemicals you use, how much of them, how hot it is, how much pressure there is, if you add a helper chemical (catalyst), and how small the pieces are. More contact and more energy usually make reactions faster.

🎯 Exam Tip: Understand that reaction rate is fundamentally about the frequency and energy of collisions between reactant particles. Any factor that increases these will increase the reaction rate.

Question 4. How does pH play an important role in everyday life?
Answer: pH is very important in many aspects of our daily lives:

• The pH of human blood is almost 7.4. Any noticeable increase or decrease in this value can lead to severe health problems or diseases, as enzymes and body functions are highly sensitive to pH.
• Different plants need different soil pH levels to grow well. For example, citrus fruits grow best in slightly alkaline soil, while rice plants need acidic soil. Sugarcane grows well in neutral soil, highlighting the importance of proper soil management.
• If the pH of rainwater drops below 7, it becomes acid rain, which is harmful to the environment. Acid rain damages buildings, statues, forests, and aquatic life in lakes and rivers.
• pH changes also cause tooth decay. When we eat sugary foods, bacteria in our mouths produce acids, which lower the pH. This acid then dissolves the tooth enamel, leading to cavities.
• During indigestion, the stomach produces too much hydrochloric acid, which lowers the pH significantly. This excess acid causes pain and irritation, often leading to the use of antacids to neutralize the stomach acid.

🎯 Exam Tip: Think of pH as a measure of acidity that affects living organisms, the environment, and even food preservation. Relate pH changes to common problems like acid rain or tooth decay.

Question 5. What is chemical equilibrium? What are its characteristics?
Answer: Chemical equilibrium is a state in a reversible chemical reaction where the rate of the forward reaction (reactants forming products) becomes equal to the rate of the backward reaction (products forming reactants). At this point, the concentrations of reactants and products remain constant, even though reactions are still occurring. No net change in the amount of the reactants and products takes place.
Characteristics of equilibrium:
1. In a chemical equilibrium, the rates of the forward and backward reactions are equal.
2. The observable properties of the system, such as pressure, concentration, colour, density, and viscosity, do not change over time.
3. Chemical equilibrium is a dynamic equilibrium. This means that both the forward and backward reactions continue to occur actively, even though it appears that the system is static from the outside.
4. In physical equilibrium, the volume of all the phases (solid, liquid, gas) remains constant.
In simple words: Chemical equilibrium is a balance point in a reaction where the starting chemicals change into new ones at the same speed that the new ones change back into the starting chemicals. It looks like nothing is happening, but everything is still moving back and forth.

🎯 Exam Tip: Differentiate between static and dynamic equilibrium. Chemical equilibrium is dynamic, involving continuous forward and reverse reactions, unlike static equilibrium where all activity stops.

VII. HOT Questions:

Question 1. A solid compound 'A' decomposes on heating into 'B' and a gas 'C' On passing the gas 'C' through water, it becomes acidic. Identify A, B and C.
Answer:
Compound A is Calcium Carbonate (\( \mathrm{CaCO}_3 \)), which is a solid compound.
When compound 'A' (Calcium Carbonate) decomposes on heating, it forms 'B' (Calcium Oxide) and a gas 'C' (Carbon Dioxide).
\( \mathrm{CaCO}_{3(\mathrm{s})} \stackrel{\Delta}{\longrightarrow} \mathrm{CaO}_{(\mathrm{s})} + \mathrm{CO}_{2(\mathrm{g})} \uparrow \)
(A) Calcium carbonate (B) Calcium oxide (C) Carbon dioxide
When the gas 'C' (Carbon Dioxide) is passed through water, it reacts to form carbonic acid, which makes the solution acidic.
\( \mathrm{H}_{2}\mathrm{O}_{(\mathrm{l})}+\mathrm{CO}_{2(\mathrm{g})} \rightarrow \mathrm{H}_{2}\mathrm{CO}_{3(\mathrm{aq})} \)
(Carbonic acid is acidic)
Therefore:
A - \( \mathrm{CaCO}_3 \), Calcium carbonate (solid compound)
B - \( \mathrm{CaO} \), Calcium oxide
C - \( \mathrm{CO}_2 \), Carbon dioxide gas
In simple words: We start with a solid (A) that breaks down with heat into another solid (B) and a gas (C). This gas (C), when mixed with water, makes the water acidic. This means A is calcium carbonate, B is calcium oxide, and C is carbon dioxide.

🎯 Exam Tip: This question tests your knowledge of decomposition reactions and the properties of common gases, especially how non-metal oxides react with water to form acids.

Question 2. Can a nickel spatula be used to stir copper sulphate solution? Justify your answer.
Answer: No, a nickel spatula cannot be used to stir copper sulphate solution. This is because, based on the activity series of metals, nickel is more reactive than copper. If nickel comes into contact with copper sulphate solution, nickel will displace copper from its solution. This means the copper from the solution will deposit onto the nickel spatula, potentially damaging the spatula and contaminating the solution.
In simple words: You cannot use a nickel spoon to stir copper sulphate because nickel is a stronger metal than copper. The nickel would steal the copper's place in the solution, and copper would stick to the spoon.

🎯 Exam Tip: In displacement reactions, a more reactive metal will always displace a less reactive metal from its salt solution. Always check the activity series to predict if a reaction will occur.

VIII. Solve the following problems:

Question 1. Lemon juice has a pH 2, what is the concentration of \( \mathrm{H}^+ \) ions?
Answer:
Given pH of lemon juice = 2
The formula to find \( [\mathrm{H}^+] \) from pH is: \( \text{pH} = -\log[\mathrm{H}^+] \)
Therefore, \( [\mathrm{H}^+] = 10^{-\text{pH}} \)
Substituting the given pH value:
\( [\mathrm{H}^+] = 10^{-2} \mathrm{M} \)
This means the concentration of hydrogen ions in lemon juice is \( 1 \times 10^{-2} \) moles per liter.
In simple words: pH tells us how acidic something is. If the pH is 2, it means there are \( 10^{-2} \) hydrogen ions in the juice.

🎯 Exam Tip: Remember that pH is a logarithmic scale. A pH of X means a hydrogen ion concentration of \( 10^{-X} \mathrm{M} \).

Question 2. Calculate the pH of \( 1.0 \times 10^{-4} \) molar solution of \( \mathrm{HNO}_3 \).
Answer:
Given concentration of \( \mathrm{HNO}_3 = 1.0 \times 10^{-4} \mathrm{M} \)
Nitric acid (\( \mathrm{HNO}_3 \)) is a strong acid, which means it completely dissociates in water:
\( \mathrm{HNO}_3 \rightarrow \mathrm{H}^{+} + \mathrm{NO}_{3}^{-} \)
So, the concentration of \( \mathrm{H}^+ \) ions is equal to the concentration of the acid:
\( [\mathrm{H}^+] = 1.0 \times 10^{-4} \mathrm{M} \)
Now, we use the pH formula: \( \text{pH} = -\log[\mathrm{H}^+] \)
\( \text{pH} = -\log[1.0 \times 10^{-4}] \)
\( \text{pH} = -(\log 1.0 + \log 10^{-4}) \)
\( \text{pH} = -(0 + (-4)) \)
\( \text{pH} = -(-4) \)
\( \text{pH} = 4 \)
Thus, the pH of the solution is 4, which indicates an acidic solution.
In simple words: Nitric acid is a strong acid, so all its acid parts go into the water. If you have \( 1.0 \times 10^{-4} \) acid, then you have \( 1.0 \times 10^{-4} \) acid parts. Using the pH rule, this gives a pH of 4.

🎯 Exam Tip: For strong acids, the concentration of \( \mathrm{H}^+ \) ions is directly equal to the concentration of the acid itself. Remember that \( \log 10^x = x \).

Question 3. What is the pH of \( 1.0 \times 10^{-5} \) molar solution of KOH?
Answer: Potassium hydroxide (KOH) is a strong base, which means it completely breaks apart into ions in water. The reaction is:
\( \text{KOH} \rightarrow \text{K}^{+} + \text{OH}^{-} \)
From the given concentration, the hydroxide ion concentration is \( [\text{OH}^{-}] = 1 \times 10^{-5} \text{ M} \).
We calculate the pOH using the formula:
\( \text{pOH} = -\log[\text{OH}^{-}] \)
\( \text{pOH} = -\log[1 \times 10^{-5}] \)
\( \text{pOH} = 5 \)
For any aqueous solution, the sum of pH and pOH is always 14 at 25°C:
\( \text{pH} + \text{pOH} = 14 \)
\( \text{pH} = 14 - \text{pOH} \)
\( \text{pH} = 14 - 5 \)
\( \text{pH} = 9 \)
In simple words: Since KOH is a strong base, its pOH is 5. We subtract this from 14 to find the pH, which is 9.

🎯 Exam Tip: Remember that for strong bases, the \( [\text{OH}^{-}] \) concentration directly gives you the pOH, which is then used to find the pH.

Question 4. Laundry detergent has a pH 8.5, What is the concentration of H+ ions?
Answer: The pH of the laundry detergent is given as 8.5. We use the formula relating pH to the hydrogen ion concentration \( [\text{H}^{+}] \):
\( \text{pH} = -\log[\text{H}^{+}] \)
To find \( [\text{H}^{+}] \), we rearrange the formula:
\( [\text{H}^{+}] = 10^{-\text{pH}} \)
Substitute the given pH value:
\( [\text{H}^{+}] = 10^{-8.5} \)
Calculating this value, we get:
\( [\text{H}^{+}] \approx 3.16 \times 10^{-9} \text{ M} \)
This means laundry detergent is slightly basic, which helps in cleaning.
In simple words: We use the pH to find the concentration of hydrogen ions. A pH of 8.5 means there are about \( 3.16 \times 10^{-9} \) moles of hydrogen ions per liter.

🎯 Exam Tip: Be careful with negative exponents when converting pH to \( [\text{H}^{+}] \) concentration; a calculator is often needed for non-integer pH values.

Question 5. The hydroxide ion concentration of a solution is \( 1 \times 10^{-11}\text{M} \). What is the pH of the solution?
Answer: The hydroxide ion concentration \( [\text{OH}^{-}] \) is given as \( 1 \times 10^{-11} \text{ M} \).
First, we calculate the pOH using the formula:
\( \text{pOH} = -\log[\text{OH}^{-}] \)
\( \text{pOH} = -\log[1 \times 10^{-11}] \)
We know that \( \log(1 \times 10^{-11}) = \log(1) + \log(10^{-11}) = 0 + (-11) = -11 \).
So, \( \text{pOH} = -(-11) = 11 \)
Now, we use the relationship between pH and pOH at 25°C:
\( \text{pH} + \text{pOH} = 14 \)
\( \text{pH} = 14 - \text{pOH} \)
\( \text{pH} = 14 - 11 \)
\( \text{pH} = 3 \)
This pH value indicates the solution is acidic.
In simple words: We first find pOH from the hydroxide ion concentration. With pOH as 11, we subtract it from 14 to get the pH, which is 3.

🎯 Exam Tip: Always remember the fundamental relation \( \text{pH} + \text{pOH} = 14 \) for aqueous solutions at standard temperature, as it is key to solving many pH-related problems.

Samacheer Kalvi 10th Science Types of Chemical Reactions Additional Important Questions and Answers

I. Choose the correct answer:

Question 1. The unit of rate of a reaction is:
(a) dm³/mol
(b) dm^-3
(c) mol dm^-3
(d) mol
Answer: (c) mol dm^-3
In simple words: The rate of a reaction tells us how much the concentration of reactants or products changes over time, so its unit is moles per liter (or dm³) per unit time.

🎯 Exam Tip: Always pay attention to the units in chemistry. The rate of reaction is typically expressed as change in concentration per unit time, often mol L^-1 s^-1 or mol dm^-3 s^-1.

Question 2. As the molecule is dissociated by the absorption of heat it is otherwise called as ___________.
(a) Thermolysis
(b) Photolysis
(c) Electrolysis
(d) None of these.
Answer: (a) Thermolysis
In simple words: When heat is used to break apart a molecule, this process is called thermolysis.

🎯 Exam Tip: Distinguish between thermolysis (decomposition by heat), photolysis (decomposition by light), and electrolysis (decomposition by electricity).

Question 3. The chemical formula of marble is:
(a) CaCO₃
(b) MgCO₃
(c) Na₂CO₃
(d) PbCO₃
Answer: (a) CaCO₃
In simple words: Marble is made of calcium carbonate, which has the chemical formula CaCO₃.

🎯 Exam Tip: Memorize the common names and chemical formulas of important compounds like marble (calcium carbonate), limestone, and chalk, as they are often the same substance.

Question 4. As the decomposition is caused by light, this kind of reaction is called ___________.
(a) Thermolysis
(b) Photolysis
(c) Electrolysis
(d) None of these.
Answer: (b) Photolysis
In simple words: When light energy breaks down a molecule, the reaction is known as photolysis.

🎯 Exam Tip: Reactions driven by light, heat, or electricity are important decomposition types. Understand the specific energy source for each type.

Question 5. Fluorine will displace the following halide ion from the solution:
(a) chloride
(b) bromide
(c) iodide
(d) All of the options
Answer: (d) All of the options
In simple words: Fluorine is the most reactive halogen, so it can push out (displace) any other halide ion, like chloride, bromide, or iodide, from their solutions.

🎯 Exam Tip: Remember the reactivity order of halogens: Fluorine > Chlorine > Bromine > Iodine. A more reactive halogen displaces a less reactive one from its salt solution.

Question 6. The decomposition of AgBr into grey coloured silver metal is an example of .........
(a) compound to element/element
(b) compound to compound/compound
(c) combination
(d) neutralization
Answer: (a) compound to element/element
In simple words: Silver bromide (a compound) breaks down into silver (an element) and bromine (another element), so it's a compound breaking into its elements.

🎯 Exam Tip: Categorize decomposition reactions based on what the compound breaks down into – elements, or simpler compounds.

Question 7. The Metathesis reaction among the following is:
(a) \( \text{C}_3\text{H}_{8(\text{g})} + 5\text{O}_{2(\text{g})} \rightarrow 3\text{CO}_{2(\text{g})} + 4\text{H}_2\text{O} + \text{heat} \)
(b) \( \text{Zn}_{(\text{s})} + \text{CuSO}_{4(\text{aq})} \rightarrow \text{ZnSO}_{4(\text{aq})} + \text{Cu}_{(\text{s})} \)
(c) \( \text{HNO}_{3(\text{aq})} + \text{NH}_4\text{OH}_{(\text{aq})} \rightarrow \text{NH}_4\text{NO}_{3(\text{aq})} + \text{H}_2\text{O}_{(\text{l})} \)
(d) \( 2\text{KClO}_{3(\text{s})} \xrightarrow{\text{MnO}_2} 2\text{KCl}_{(\text{s})} + 3\text{O}_{2(\text{g})}\uparrow \)
Answer: (c) \( \text{HNO}_{3(\text{aq})} + \text{NH}_4\text{OH}_{(\text{aq})} \rightarrow \text{NH}_4\text{NO}_{3(\text{aq})} + \text{H}_2\text{O}_{(\text{l})} \)
In simple words: A metathesis reaction is when two compounds swap parts, like in this reaction where acid and base swap ions to form a salt and water.

🎯 Exam Tip: Metathesis (or double displacement) reactions involve the exchange of ions between two compounds. Neutralization and precipitation reactions are common types of metathesis reactions.

Question 8. KI and Pb(NO₃)₂ solutions are mixed to give a precipitate. What is the colour of the precipitate?
(a) White
(b) Brown
(c) Red
(d) Yellow.
Answer: (d) Yellow
In simple words: When potassium iodide and lead nitrate solutions are mixed, they form a bright yellow solid called lead iodide, which settles at the bottom.

🎯 Exam Tip: Knowing the colors of common precipitates, especially those formed in double displacement reactions, is important for experimental chemistry questions.

Question 9. The pH of rain water is approximately:
(a) 7
(b) 8
(c) 4
(d) 14
Answer: (a) 7
In simple words: Normally, rainwater is almost neutral, so its pH is close to 7.

🎯 Exam Tip: Pure water has a pH of 7. Rainwater is usually slightly acidic (around 5.6) due to dissolved carbon dioxide forming carbonic acid, but for general purposes, "approximately 7" refers to unpolluted rain.

Question 10. Most of the reactions go faster at ___________
(a) low temperature
(b) moderate temperature
(c) 0°C
(d) high temperature.
Answer: (d) high temperature
In simple words: Most chemical reactions happen quicker when the temperature is higher because particles move faster and collide more often.

🎯 Exam Tip: Remember that increasing temperature generally increases the kinetic energy of reactant particles, leading to more frequent and energetic collisions, thus speeding up the reaction rate.

II. Fill in the blanks.

Question 1. A chemical equation provides information on the .............. of the substances and the reaction condition.
Answer: physical state
In simple words: A chemical equation tells us the physical state (like solid, liquid, gas) of the chemicals involved and what conditions are needed for the reaction.

🎯 Exam Tip: Always include the state symbols (s), (l), (g), (aq) in chemical equations for clarity, as they provide important information about the reaction system.

Question 2. The symbol 'aq' in a chemical equation represent the physical state of the substance as ........
Answer: aqueous solution
In simple words: 'Aq' means that the substance is dissolved in water, forming an aqueous solution.

🎯 Exam Tip: State symbols like (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solution are essential to properly represent chemical reactions.

Question 3. \( 2\text{Na}_{(\text{s})} + \text{Cl}_{2(\text{g})} \rightarrow 2\text{NaCl}_{(\text{s})} \) represent the combination reaction between a ........... and ...........
Answer: metal, non-metal
In simple words: This equation shows sodium (a metal) reacting with chlorine (a non-metal) to form a compound, which is a combination reaction.

🎯 Exam Tip: Combination reactions often involve an element reacting with another element or a compound reacting with an element/compound to form a single product.

Question 4. After white washing with a solution of slaked lime a thin layer of ........... is formed.
Answer: CaCO₃
In simple words: When slaked lime is used for whitewashing, it reacts with carbon dioxide in the air to form a thin layer of calcium carbonate, which makes the walls look white.

🎯 Exam Tip: The reaction of calcium hydroxide (slaked lime) with atmospheric carbon dioxide to form calcium carbonate is a common example of a slow chemical change used in everyday life.

Question 5. Reactions in which heat is absorbed is called ........... reactions.
Answer: endothermic
In simple words: Endothermic reactions take in heat from their surroundings, making the area around them feel cooler.

🎯 Exam Tip: Distinguish clearly between endothermic (heat absorbed) and exothermic (heat released) reactions by observing temperature changes in the surroundings.

Question 6. Electrolytic refining of copper is based on ........... reaction.
Answer: electrolytic decomposition
In simple words: Electrolytic refining uses electricity to break down and purify copper, which is a type of decomposition reaction.

🎯 Exam Tip: Electrolytic refining is an important industrial process for purifying metals like copper, relying on the principle of electrolysis to separate impurities.

Question 7. When \( 10^{-6} \) mole of a monobasic strong acid is dissolved in water, the pH of the solution is ...........
Answer: 6
In simple words: If a strong acid has a concentration of \( 10^{-6} \) M, its pH will be 6.

🎯 Exam Tip: For a strong monobasic acid, the concentration of the acid is equal to the concentration of \( [\text{H}^{+}] \), so pH is simply \( -\log[\text{H}^{+}] \).

Question 8. When pH of a solution is 2, the \( [\text{H}^{+}] \) in mol/L is ...........
Answer: \( 1 \times 10^{-2} \)
In simple words: If the pH is 2, it means the concentration of hydrogen ions is \( 1 \times 10^{-2} \) moles per liter.

🎯 Exam Tip: Remember the inverse relationship: if \( \text{pH} = \text{x} \), then \( [\text{H}^{+}] = 10^{-\text{x}} \text{ M} \). This is a direct conversion.

Question 9. An irreversible reaction is an example of ......... reaction.
Answer: irreversible
In simple words: A reaction that can only go in one direction and cannot be easily reversed is called an irreversible reaction.

🎯 Exam Tip: Irreversible reactions proceed to completion, where reactants are converted entirely into products, unlike reversible reactions that reach equilibrium.

Question 10. [ ] represents the concentration of either the reactant or product in ...........
Answer: mol/Lit
In simple words: Square brackets like [ ] are used in chemistry to show the concentration of a substance, which is usually measured in moles per liter.

🎯 Exam Tip: Molar concentration (moles per liter or mol L^-1) is a standard way to express the amount of a substance in a solution, especially in kinetics and equilibrium calculations.

III. Match the following.

Question 1. Match the following table:
Answer:

🎯 Exam Tip: For match the following questions, identify the key characteristic or definition for each item in Column A and find its exact match in Column B. Be sure to understand why each pair belongs together.

Question 2. Match the following table:
Answer:

🎯 Exam Tip: Classify reactions carefully based on the number of reactants and products, and whether elements are replacing each other or compounds are breaking down.

Question 3. Match the following table:
Answer:

🎯 Exam Tip: Carefully examine the reactants and products in each equation to determine the reaction type, looking for precipitates, gas evolution, single element replacement, or acid-base reactions.

Question 4. Match the following table:
Answer:

🎯 Exam Tip: Knowing the approximate pH values of common substances helps understand their chemical nature (acidic, basic, or neutral) and their uses.

IV. True or False: (If false give the correct statement)

Question 1. Silver metal can replace hydrogen gas from nitric acid.
Answer: False - Silver cannot displace \( \text{H}_2 \) from \( \text{HNO}_3 \) acid, since it is placed below hydrogen in the activity series.
In simple words: Silver cannot push out hydrogen from nitric acid because it's less reactive than hydrogen.

🎯 Exam Tip: Remember the activity series of metals: only metals above hydrogen can displace hydrogen from acids. Silver is below hydrogen.

Question 2. The pH of rain water containing dissolved gases like \( \text{SO}_3 \), \( \text{CO}_2 \), \( \text{NO}_2 \) will be less than 7.
Answer: True
In simple words: When harmful gases like sulfur trioxide, carbon dioxide, and nitrogen dioxide dissolve in rainwater, they form acids, making the rain acidic with a pH lower than 7.

🎯 Exam Tip: Acid rain is caused by non-metal oxides (like \( \text{SO}_2 \), \( \text{NO}_2 \), \( \text{CO}_2 \)) dissolving in atmospheric moisture to form corresponding acids.

Question 3. At the equilibrium of a reversible reaction, the concentration of the reactants and the products will be equal.
Answer: False - At equilibrium the concentration of the reactants and products do not change it remains constant, but the concentration of the reactants and the products will not be equal.
In simple words: At equilibrium, the amounts of reactants and products stop changing, but they are usually not the same amount; they just stay steady.

🎯 Exam Tip: Equilibrium means the *rates* of forward and reverse reactions are equal, and concentrations remain *constant*, not necessarily *equal*. This is a common misconception.

Question 4. Periodical removal of one of the products of a reversible reaction increases the yield.
Answer: True
In simple words: If you keep taking away a product from a reversible reaction, the reaction will make more of that product to try and balance things out, which means you get more of what you want.

🎯 Exam Tip: This principle is known as Le Chatelier's principle. Removing a product shifts the equilibrium to the right (towards products), increasing the yield of desired products.

Question 5. On dipping a pH paper in a solution, it turns into yellow. Then the solution is basic.
Answer: False - The solution is neutral if the solution is basic it will be green in colour.
In simple words: If pH paper turns yellow, it means the solution is acidic or slightly acidic, not basic. Basic solutions usually turn pH paper green or blue.

🎯 Exam Tip: Familiarize yourself with the color changes of common pH indicators, like universal indicator paper, to correctly identify acidic, neutral, and basic solutions.

V. Short answer questions:

Question 1. Write a note on double displacement reaction with an example.
Answer: A double displacement reaction is a type of chemical reaction where two compounds react, and their positive and negative ions are interchanged to form two new compounds. These reactions often result in the formation of a precipitate, a gas, or water. For example, when sodium hydroxide reacts with hydrochloric acid, they swap ions to form sodium chloride and water. This type of reaction is very common in aqueous solutions.
Example: \( \text{NaOH}_{(\text{aq})} + \text{HCl}_{(\text{aq})} \rightarrow \text{NaCl}_{(\text{aq})} + \text{H}_2\text{O}_{(\text{l})} \)
In simple words: A double displacement reaction is when two chemicals trade partners. For example, sodium hydroxide and hydrochloric acid swap parts to make salt and water.

🎯 Exam Tip: Always identify the ions involved in the reactants and then predict the new pairs that will form. Pay attention to the state symbols to indicate precipitate, gas, or water formation.

Question 2. Identify the wrong statements and correct them.
Answer:
1. Sodium benzoate is used in food preservative. (Correct statement.)
2. Nitric acid is not used as fertilizer in agriculture. (Wrong statement. Correction: Nitric acid is used as a fertilizer in agriculture.)
3. Sulphuric acid is called the king of chemicals. (Correct statement.)
4. The pH of acid is greater than 7. (Wrong statement. Correction: The pH of the acid is lesser than 7.)
5. Acetic acid is used in aerated drinks. (Wrong statement. Correction: Carbonic acid is used in aerated drinks.)
In simple words: Some statements about chemicals are right, like sodium benzoate being a food preservative and sulfuric acid being a very important chemical. Other statements are wrong, like saying nitric acid isn't used for fertilizer or that acids have a pH above 7. Also, carbonic acid, not acetic acid, is used in fizzy drinks.

🎯 Exam Tip: For true/false or correction questions, thoroughly read each statement and recall the chemical facts to accurately determine its validity and provide the correct information if false.

Question 3. Why a combustion reaction may be called as an exothermic oxidation?
Answer: A combustion reaction is a type of chemical reaction that involves burning, usually with oxygen. During this process, heat is always released into the surroundings, making it an exothermic reaction. Also, combustion reactions always involve a substance reacting with oxygen, which is a process known as oxidation. For example, when wood burns, it reacts with oxygen, giving off heat and forming new compounds. Because heat is evolved and oxygen is added (oxidation occurs), a combustion reaction is correctly called an exothermic oxidation.
In simple words: Combustion is called exothermic because it gives off heat. It is called oxidation because the substance reacts with oxygen. So, it's both an exothermic and an oxidation process.

🎯 Exam Tip: To fully define a combustion reaction, remember its dual nature: it's always exothermic (releases energy) and always involves oxidation (reaction with oxygen).

Question 4. Take two conical flasks. Label them as I and II. Take a small amount of copper sulphate solution in the first conical flask. Take a small amount of granulated zinc in the second conical flask. Allow the copper sulphate solution to react with the zinc.
(i) Name the type of reaction.
(ii) Say whether the metal zinc is more reactive or less reactive.
(iii) Write the complete and balanced reaction.
(iv) Say whether this change is reversible or irreversible.
Answer:
(i) The type of reaction is a displacement reaction.
(ii) Zinc metal is more reactive than copper. This means zinc can replace copper from its salt solution.
(iii) The complete and balanced reaction is:
\( \text{Zn}_{(\text{s})} + \text{CuSO}_{4(\text{aq})} \rightarrow \text{ZnSO}_{4(\text{aq})} + \text{Cu}_{(\text{s})}\downarrow \)
(iv) This change is an irreversible change. Once zinc displaces copper, the reaction cannot be easily reversed under normal conditions.
In simple words: When zinc reacts with copper sulfate, it's a displacement reaction because zinc is stronger than copper and pushes it out. The reaction makes zinc sulfate and solid copper, and this change cannot be undone easily.

🎯 Exam Tip: For displacement reactions, always refer to the reactivity series to determine if one metal can displace another. Remember that more reactive metals displace less reactive ones from their salt solutions.

Question 5. What is an irreversible reaction? Give an example.
Answer: An irreversible reaction is a chemical reaction that proceeds in only one direction, from reactants to products, and cannot be easily reversed to convert products back into reactants under the given conditions. These reactions go to completion, meaning that one or more reactants are fully consumed. For instance, burning a substance is an irreversible reaction, as the ash and smoke cannot be easily turned back into the original substance. This concept is fundamental to understanding many everyday chemical changes.
Example: Consider the combustion of coal into carbon dioxide and water:
\( \text{C}_{(\text{s})} + \text{O}_{2(\text{g})} \rightarrow \text{CO}_{2(\text{g})} + \text{Heat} \)
Coal (carbon) reacts with oxygen to form carbon dioxide and release heat.
In simple words: An irreversible reaction only goes one way, turning reactants into products that cannot easily change back. Burning coal is an example, as you can't turn the smoke and ash back into coal.

🎯 Exam Tip: Common indicators of an irreversible reaction include the formation of a precipitate, the evolution of a gas that escapes, or a significant change in energy (like a strong explosion or burning).

Question 6. Define the rate of a reaction.
Answer: The rate of a reaction is defined as the change in the amount or concentration of any one of the reactants or products per unit time. It tells us how fast a reaction is happening. For example, if a reaction makes a product quickly, its rate is high. This rate can be positive for products (as their amount increases) and negative for reactants (as their amount decreases).
Consider the following simple reaction:
\( \text{A} \rightarrow \text{B} \)
The rate of this reaction is given by:
\( \text{Rate} = -\frac{\text{d}[\text{A}]}{\text{dt}} = +\frac{\text{d}[\text{B}]}{\text{dt}} \)
Where, \( [\text{A}] \) is the concentration of A and \( [\text{B}] \) is the concentration of B.
The negative sign indicates the decrease in the concentration of A with time. The positive sign indicates the increase in the concentration of B with time.
In simple words: The rate of a reaction tells us how fast the amounts of chemicals change over a period of time. If a chemical is used up quickly, the reaction rate is high.

🎯 Exam Tip: Remember that the rate of reaction can be expressed in terms of either the disappearance of a reactant or the appearance of a product, with appropriate signs and stoichiometry.

Question 7. What is meant by combination reaction? Give an example.
Answer: A combination reaction, also known as a synthesis reaction, is a type of chemical reaction where two or more simple reactants combine to form a single, more complex product. In these reactions, smaller substances join together to create a larger one. For instance, when magnesium metal burns in oxygen, it forms magnesium oxide, showing two elements combining into one compound. This is a fundamental way new substances are created in chemistry.
Example: \( 2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} \)
In simple words: A combination reaction is when two or more chemicals join together to make just one new chemical. For example, magnesium and oxygen combine to make magnesium oxide.

🎯 Exam Tip: Look for multiple reactants and only one product to identify a combination reaction. The product is always more complex than the starting materials.

Question 8. What is a catalyst?
Answer: A catalyst is a substance that increases the rate of a chemical reaction without being consumed or changed permanently in the reaction itself. It works by providing an alternative reaction pathway with lower activation energy, making it easier for the reaction to occur. For instance, enzymes in our bodies act as biological catalysts, speeding up countless reactions. After the reaction, the catalyst can be recovered and used again. Catalysts are crucial in many industrial processes to make reactions faster and more efficient.
In simple words: A catalyst is a substance that helps a chemical reaction go faster but does not get used up in the process. It's like a helper that speeds things up.

🎯 Exam Tip: Key features of a catalyst are that it accelerates a reaction, remains chemically unchanged, and is often needed in small quantities.

Question 9. Define Displacement reaction. Give an example.
Answer: A displacement reaction is a chemical reaction in which a more reactive element displaces (or kicks out) a less reactive element from its compound. This usually happens in a solution where a more active metal takes the place of a less active metal. For example, if you put a piece of lead into a copper chloride solution, the lead, being more reactive, will displace the copper to form lead chloride and solid copper. These reactions are governed by the reactivity series of elements.
Example: \( \text{Pb}_{(\text{s})} + \text{CuCl}_{2(\text{aq})} \rightarrow \text{PbCl}_{2(\text{aq})} + \text{Cu}_{(\text{s})}\downarrow \)
Here, lead (Pb) is more reactive than copper (Cu) and replaces it from copper chloride.
In simple words: A displacement reaction is when a stronger element replaces a weaker element in a compound. For example, lead can replace copper from copper chloride because lead is more reactive.

🎯 Exam Tip: Use the activity series to predict displacement reactions: a metal higher in the series can displace any metal below it from its salt solution.

Question 10. When an aerated soft drink bottle is kept open it will go flat. Why?
Answer: An aerated soft drink bottle contains dissolved carbon dioxide (\( \text{CO}_2 \)) gas, which gives it its fizziness. When the bottle is sealed, the \( \text{CO}_2 \) gas in the liquid is in equilibrium with the \( \text{CO}_2 \) gas in the space above the liquid (headspace). The dissolved \( \text{CO}_2 \) is also in equilibrium with carbonic acid. This balance keeps the drink fizzy.
(i) In the sealed aerated soft drink bottle, the dissolved \( \text{CO}_2 \), in the form of carbonic acid and gaseous \( \text{CO}_2 \) are in equilibrium.
(ii) When the bottle is kept open, the gaseous \( \text{CO}_2 \) from the headspace escapes into the atmosphere. To maintain equilibrium, the dissolved \( \text{CO}_2 \) in the liquid starts to undissolve and move into the gas phase. This process continues until most of the dissolved \( \text{CO}_2 \) has escaped, causing the drink to lose its fizziness and go "flat." This is an example of Le Chatelier's principle at work.
In simple words: A sealed fizzy drink has gas balanced inside. When you open it, the gas escapes into the air, so the liquid releases more gas to try and keep the balance, making the drink lose its fizz.

🎯 Exam Tip: This phenomenon is a practical application of Le Chatelier's principle, where reducing the partial pressure of \( \text{CO}_2 \) above the liquid shifts the equilibrium towards the gaseous state, releasing dissolved gas.

Question 11. Can copper displace zinc or lead from their salt solutions?
Answer: No, copper cannot displace zinc or lead from their salt solutions. This is because copper is less reactive than both zinc and lead. In the reactivity series of metals, zinc and lead are placed above copper. For a metal to displace another metal from its salt solution, the displacing metal must be more reactive than the metal in the salt. Since copper is lower in reactivity, it does not have enough chemical strength to push out zinc or lead ions from their compounds. This principle helps us predict if a displacement reaction will occur.
In simple words: Copper cannot push out zinc or lead from their salts because copper is not as reactive as zinc or lead. Only a more reactive metal can replace a less reactive one.

🎯 Exam Tip: Always consult the activity series of metals. A metal can only displace another metal that is below it in the series.

Question 12. What is called as acid rain?
Answer: Normal rainwater has a pH of approximately 7 (or slightly acidic around 5.6 due to dissolved carbon dioxide). However, when the air is polluted with acidic gases such as sulfur dioxide (\( \text{SO}_2 \)), sulfur trioxide (\( \text{SO}_3 \)), and nitrogen dioxide (\( \text{NO}_2 \)), these gases dissolve in the rainwater. They react with water to form stronger acids like sulfuric acid and nitric acid. This makes the pH of the rainwater significantly less than 7, often falling below 5.6. Rain with a pH lower than 5.6 is specifically referred to as acid rain. Acid rain is harmful to the environment, damaging forests, aquatic life, and buildings.
In simple words: Acid rain is when air pollution, like gases from factories, mixes with rainwater and makes it much more acidic than normal, causing harm to nature.

🎯 Exam Tip: Remember the main pollutants responsible for acid rain (\( \text{SO}_2 \), \( \text{NO}_2 \)) and their impact on the pH of rainwater, leading to environmental damage.

Question 13. Write the differences between combination and decomposition reaction.
Answer: The differences between combination and decomposition reactions are as follows:

🎯 Exam Tip: Focus on the number of reactants and products: combination means multiple reactants to one product, while decomposition means one reactant to multiple products. Also, consider the energy changes involved.

VI. Answer in detail:

Question 1. Two acids 'A' and 'B' were kept in beakers. Acid 'A' undergoes partial dissociation in water, whereas acid 'B' undergoes complete dissociation in water.
1. Of the two acids 'A' and 'B' which is weak acid and which is strong acid?
2. What is a weak acid?
3. What is a strong acid?
4. Give one example of each.
Answer:
1. Based on their dissociation behavior:
Acid 'A' is a weak acid because it undergoes partial dissociation in water.
Acid 'B' is a strong acid because it undergoes complete dissociation in water.
2. A weak acid is an acid that ionizes (breaks apart into ions) only partially when dissolved in water. This means that only a small fraction of its molecules donate a hydrogen ion (\( \text{H}^{+} \)) to water, and most of the acid remains in its molecular form. The dissociation is a reversible process, establishing an equilibrium between the undissociated acid and its ions. For example, acetic acid in vinegar is a weak acid.
3. A strong acid is an acid that ionizes (breaks apart into ions) completely or almost completely when dissolved in water. This means that nearly all of its molecules donate a hydrogen ion (\( \text{H}^{+} \)) to water, leading to a high concentration of \( \text{H}^{+} \) ions in the solution. Strong acids are highly corrosive. For instance, sulfuric acid, used in car batteries, is a strong acid.
4. Examples:
Weak acid – Acetic acid (\( \text{CH}_3\text{COOH} \))
Strong acid – Sulphuric acid (\( \text{H}_2\text{SO}_4 \))
In simple words: Acid 'A' is weak because it only partly breaks into ions in water, while acid 'B' is strong because it fully breaks into ions. A weak acid doesn't fully split up, like acetic acid in vinegar. A strong acid fully splits up, like sulfuric acid.

🎯 Exam Tip: The key difference between strong and weak acids lies in their degree of ionization in water. Strong acids ionize completely, while weak acids ionize only partially, leading to different conductivities and reactivities.

Question 2. Sodium hydroxide and HCl acid react as shown in this equation \( \text{NaOH}_{(\text{aq})} + \text{HCl}_{(\text{aq})} \rightarrow \text{NaCl}_{(\text{aq})} + \text{H}_2\text{O}_{(\text{l})} \)
(i) Which type of chemical reaction is this?
(ii) The reaction is exothermic. Explain what that means?
(iii) Differentiate exothermic and endothermic reaction.
(iv) What happens to the temperature of the solution as the chemicals react?
Answer:
(i) This type of chemical reaction is a Neutralization reaction. It is also a double displacement reaction, as the ions swap partners to form salt and water.
(ii) When a reaction is exothermic, it means that heat energy is released (or given out) into the surroundings during the course of the reaction. This release of heat typically causes the temperature of the reaction mixture and its surroundings to increase. Neutralization reactions are generally exothermic, warming up the beaker.
(iii) The differentiation between exothermic and endothermic reactions is:

Exothermic Reaction:
1. Heat is evolved (released) during the reaction.
2. The temperature of the surroundings increases.

Endothermic Reaction:
1. Heat is absorbed (taken in) during the reaction.
2. The temperature of the surroundings decreases.
(iv) As the chemicals react in this neutralization reaction, which is exothermic, the temperature of the solution increases. This is because the heat energy produced by the reaction is transferred to the solution, making it warmer.
In simple words: This is a neutralization reaction. Exothermic means it gives off heat, making the solution hotter. Exothermic reactions release heat and get warmer, while endothermic reactions absorb heat and get colder. So, for this reaction, the solution's temperature will go up.

🎯 Exam Tip: Clearly state that exothermic reactions release heat (resulting in a temperature increase), and endothermic reactions absorb heat (resulting in a temperature decrease). Neutralization reactions are a classic example of exothermic processes.

Question 3. Take two conical flasks. Label them as I and II. Take a small amount of \( CuSO_4 \) in the I conical flask and small amount of granulated Zinc in the II conical flask. Allow the \( CuSO_4 \) solution to react with Zinc.
(i) Name the type of reaction.
(ii) Say whether the metal Zn is more reactive or less reactive.
(iii) Write the complete and balanced reaction.
(iv) Say whether this change is reversible or irreversible.
Answer:
(i) This is a displacement reaction.
(ii) Zinc metal is more reactive than copper.
(iii) The complete and balanced reaction is:
\( \mathrm{Zn}_{(\mathrm{s})}+\mathrm{CuSO}_{4(\mathrm{aq})} \rightarrow \mathrm{ZnSO}_{4(\mathrm{aq})}+\mathrm{Cu}_{(\mathrm{s})} \downarrow \)
(iv) This change is an irreversible change. The new metal formed stays separated. It does not go back to its original form easily.
In simple words: When zinc and copper sulfate mix, zinc pushes copper out because it is stronger. This makes a new substance and cannot be easily undone.

🎯 Exam Tip: Remember that a more reactive metal will always displace a less reactive metal from its salt solution in a single displacement reaction.

Question 4. Suggest a reason for each observation given below.
1. In fireworks, powdered magnesium is used rather than magnesium ribbon.
2. Zinc and dilute \( \mathrm{H}_{2} \mathrm{SO}_{4} \) react much more quickly when a few drops of copper sulphate solutions are added.
3. The reaction between magnesium carbonate and dilute hydrochloric acid speeds up when some concentrated HCl is added.
Answer:
1. Powdered magnesium has a much larger surface area compared to magnesium ribbon. This bigger surface area allows for more reactant particles to come into contact and collide, which makes the reaction happen faster and more intensely for fireworks.
2. Copper sulfate acts as a catalyst in this reaction. A catalyst speeds up a chemical reaction without being used up itself, helping zinc and sulfuric acid react quicker.
3. Adding concentrated HCl increases the overall concentration of the acid. Higher concentration means there are more acid particles available to react with the magnesium carbonate, leading to more frequent collisions and thus a faster reaction rate.
In simple words: Smaller pieces of magnesium burn faster in fireworks because they have more surface for the reaction. Copper sulfate helps zinc react quicker with acid. Adding more acid makes the reaction with magnesium carbonate go faster because there are more acid particles.

🎯 Exam Tip: When explaining reaction rates, always consider factors like surface area, temperature, concentration, and catalysts. These are key concepts that influence how fast a chemical change occurs.

VII. Hot Questions:

Question 1. Given \( 2N_2O_5 \rightarrow 4NO_2+O_2 \), if \( \frac{d[N_2O_5]}{dt} = K_1[N_2O_5] \), \( \frac{d[NO_2]}{dt} = K_2[N_2O_5] \) and \( \frac{d[O_2]}{dt} = K_3[N_2O_5] \). Write the relationship between \( K_1,K_2 \) and \( K_3 \).
Answer:
According to the rate Law for the reaction \( 2N_2O_5 \rightarrow 4NO_2 + O_2 \), the rate can be expressed as:
\( \text{rate} = -\frac{1}{2} \frac{d[N_2O_5]}{dt} = +\frac{1}{4} \frac{d[NO_2]}{dt} = +\frac{d[O_2]}{dt} \)
Now, substitute the given expressions for the rates:
\( -\frac{1}{2} (K_1[N_2O_5]) = \frac{1}{4} (K_2[N_2O_5]) = K_3[N_2O_5] \)
Since \( [N_2O_5] \) is common on all sides, we can cancel it out:
\( -\frac{1}{2} K_1 = \frac{1}{4} K_2 = K_3 \)
Multiplying by -4 to get rid of fractions and negative sign from \( K_1 \):
\( 2K_1 = -K_2 = -4K_3 \)
However, rate constants (K values) are usually positive. The negative sign on \( K_1 \) represents the disappearance of the reactant. For the purpose of finding a relationship between the magnitudes of \( K_1, K_2, K_3 \), we can use the absolute values or just consider the stoichiometry as positive.
From the given relations for individual rates, we have:
\( \frac{d[N_2O_5]}{dt} = -2K_1[N_2O_5] \) (since \( N_2O_5 \) is consumed)
\( \frac{d[NO_2]}{dt} = +4K_3[N_2O_5] \)
\( \frac{d[O_2]}{dt} = +K_3[N_2O_5] \)
Comparing with the stoichiometry: \( -\frac{1}{2}\frac{d[N_2O_5]}{dt} = \frac{1}{4}\frac{d[NO_2]}{dt} = \frac{1}{1}\frac{d[O_2]}{dt} \)
\( -\frac{1}{2}(-K_1[N_2O_5]) = \frac{1}{4}(K_2[N_2O_5]) = K_3[N_2O_5] \)
Therefore, \( \frac{1}{2} K_1 = \frac{1}{4} K_2 = K_3 \)
This means \( 2K_1 = K_2 \) and \( K_2 = 4K_3 \).
So, the relationship is \( 2K_1 = K_2 = 4K_3 \). This shows how the rate constants relate to each other based on the reaction's stoichiometry.
In simple words: For a chemical reaction, the speed at which substances are used up or made is linked to their amounts. The numbers \( K_1, K_2, K_3 \) show these speeds, and their relationship \( 2K_1 = K_2 = 4K_3 \) comes from how many molecules of each substance are in the reaction.

🎯 Exam Tip: Remember to consider the stoichiometric coefficients when relating the rates of disappearance of reactants and formation of products. The rate of reaction is divided by the coefficient for each species.

Question 2. When solutions of silver nitrate and potassium bromide are mixed, a pale yellow precipitate is formed. The ionic equation for the reaction is \( \mathrm{Ag}^{+} + \mathrm{Br}^{-} \rightarrow \mathrm{AgBr} \).
1. (a) What is the name of the pale yellow precipitate?
(b) Is it soluble or insoluble?
2. Is the formation of silver bromide precipitate, a result of redox reaction or not? Justify your answer.
3. What is this type of reaction called?
Answer:
1. (a) The pale yellow precipitate is silver bromide.
(b) Silver bromide is sparingly soluble, meaning it does not dissolve much in water.
2. No, the formation of silver bromide precipitate is not a redox reaction. In a redox reaction, there is a change in oxidation states (transfer of electrons). In this reaction, silver (\( \mathrm{Ag}^{+} \)) remains in the +1 oxidation state and bromide (\( \mathrm{Br}^{-} \)) remains in the -1 oxidation state. No electrons are gained or lost by either ion, so it's not a redox reaction. It is a simple combination of ions.
3. This reaction is a double displacement reaction, specifically a precipitation reaction.
In simple words: When silver nitrate and potassium bromide mix, they form a yellow solid called silver bromide, which doesn't dissolve well. This is not a redox reaction because no electrons are swapped. It's a double displacement reaction where two parts of different compounds switch places.

🎯 Exam Tip: For precipitation reactions, always remember to identify the insoluble product and classify the reaction as a double displacement. For redox reactions, check if there's any change in the oxidation states of the elements involved.

VIII. Numericals:

Question 1. The hydrogen ion concentration of a fruit juice is \( 3.3 \times 10^{-2}\text{M} \). What is the pH of the juice? Is it acidic or basic?
Answer:
Given hydrogen ion concentration \( [\mathrm{H}^{+}] = 3.3 \times 10^{-2}\text{M} \).
The formula for pH is \( \mathrm{pH} = -\log[\mathrm{H}^{+}] \).
Substitute the value:
\( \mathrm{pH} = -\log[3.3 \times 10^{-2}] \)
Using logarithm properties, \( \log(a \times b) = \log a + \log b \):
\( \mathrm{pH} = -[\log 3.3 + \log 10^{-2}] \)
\( \mathrm{pH} = -[\log 3.3 - 2 \log 10] \)
We know \( \log 10 = 1 \). Approximate \( \log 3.3 \approx 0.5185 \).
\( \mathrm{pH} = -[0.5185 - 2] \)
\( \mathrm{pH} = -[-1.4815] \)
\( \mathrm{pH} = 1.4815 \)
Since the pH value (1.4815) is less than 7, the solution is acidic. Fruit juices are naturally acidic, which gives them their tart taste.
In simple words: We find the pH by taking the negative logarithm of the hydrogen ion amount. For this juice, the pH is about 1.48. Since this number is smaller than 7, it means the juice is acidic.

🎯 Exam Tip: Always remember that a pH less than 7 indicates an acidic solution, a pH equal to 7 is neutral, and a pH greater than 7 is basic.

Question 2. If a solution has a pH of 7.41, determine its \( \mathrm{H}^{+} \) concentration.
Answer:
Given pH = 7.41.
The formula to find \( [\mathrm{H}^{+}] \) from pH is \( [\mathrm{H}^{+}] = 10^{-\text{pH}} \).
Substitute the pH value:
\( [\mathrm{H}^{+}] = 10^{-7.41} \)
To calculate this, we can rewrite \( 10^{-7.41} \) as \( 10^{-8} \times 10^{0.59} \).
\( [\mathrm{H}^{+}] = 3.89 \times 10^{-8}\text{M} \)
This small concentration of hydrogen ions is characteristic of a slightly basic solution, similar to human blood pH.
In simple words: To find how many hydrogen ions are in a solution from its pH, we use the formula \( 10^{-\text{pH}} \). If the pH is 7.41, there are about \( 3.89 \times 10^{-8} \) moles of hydrogen ions per liter.

🎯 Exam Tip: When calculating \( [\mathrm{H}^{+}] \) from pH, ensure you understand how to handle negative exponents for base 10 calculations, often requiring a calculator for accuracy.

Question 3. What is the pH of \( 1.0 \times 10^{-5} \) molar solution of KOH?
Answer:
KOH is a strong base, so it completely dissociates in water:
\( \mathrm{KOH} \rightarrow \mathrm{K}^{+} + \mathrm{OH}^{-} \)
Given concentration of KOH is \( 1.0 \times 10^{-5}\text{M} \). This means \( [\mathrm{OH}^{-}] = 1.0 \times 10^{-5}\text{M} \).
First, calculate pOH:
\( \mathrm{pOH} = -\log[\mathrm{OH}^{-}] \)
\( \mathrm{pOH} = -\log[1 \times 10^{-5}] \)
\( \mathrm{pOH} = -[\log 1 + \log 10^{-5}] \)
\( \mathrm{pOH} = -[0 - 5] \)
\( \mathrm{pOH} = 5 \)
Now, use the relationship between pH and pOH: \( \mathrm{pH} + \mathrm{pOH} = 14 \).
\( \mathrm{pH} = 14 - \mathrm{pOH} \)
\( \mathrm{pH} = 14 - 5 \)
\( \mathrm{pH} = 9 \)
Since the pH is 9 (greater than 7), the solution is basic. This is expected for a solution of a strong base like KOH.
In simple words: For a strong base like KOH, we first find pOH from the hydroxide ion amount. Then, we subtract pOH from 14 to get the pH. Here, the pOH is 5, so the pH is 9, meaning it's a basic solution.

🎯 Exam Tip: Always remember that strong bases dissociate completely, meaning the concentration of the hydroxide ion is equal to the concentration of the base. Use \( \mathrm{pH} + \mathrm{pOH} = 14 \) to convert between pH and pOH accurately.

Question 4. Laundry detergent has a pH 8.5, What is the concentration of \( \mathrm{H}^{+} \) ions?
Answer:
Given pH = 8.5.
The formula to find hydrogen ion concentration \( [\mathrm{H}^{+}] \) from pH is \( [\mathrm{H}^{+}] = 10^{-\text{pH}} \).
Substitute the pH value:
\( [\mathrm{H}^{+}] = 10^{-8.5} \)
To calculate this, we can write \( 10^{-8.5} \) as \( 10^{-9} \times 10^{0.5} \).
\( [\mathrm{H}^{+}] \approx 3.16 \times 10^{-9}\text{M} \)
The pH of 8.5 indicates that laundry detergent is a slightly basic solution, which helps in cleaning by reacting with fats and oils. This low concentration of \( \mathrm{H}^{+} \) confirms its basic nature.
In simple words: To find the amount of hydrogen ions from the pH, we use the formula \( 10 \) to the power of negative pH. So for pH 8.5, the hydrogen ion concentration is about \( 3.16 \times 10^{-9} \) molar.

🎯 Exam Tip: A pH value greater than 7 indicates a basic solution, and correspondingly, its hydrogen ion concentration will be very low (e.g., \( 10^{-8}\text{M} \) or less).

Question 5. The hydroxide ion concentration of a solution is \( 1 \times 10^{-11}\text{M} \). What is the pH of the solution?
(i) What is the pH of the solution?
(ii) What is the pOH of the solution?
(iii) Is the given solution acidic or basic?
Answer:
Given hydroxide ion concentration \( [\mathrm{OH}^{-}] = 1 \times 10^{-11}\text{M} \).
(ii) First, calculate pOH:
\( \mathrm{pOH} = -\log[\mathrm{OH}^{-}] \)
\( \mathrm{pOH} = -\log[1 \times 10^{-11}] \)
\( \mathrm{pOH} = -[\log 1 + \log 10^{-11}] \)
\( \mathrm{pOH} = -[0 - 11] \)
\( \mathrm{pOH} = 11 \)
(i) Now, use the relationship between pH and pOH: \( \mathrm{pH} + \mathrm{pOH} = 14 \).
\( \mathrm{pH} = 14 - \mathrm{pOH} \)
\( \mathrm{pH} = 14 - 11 \)
\( \mathrm{pH} = 3 \)
(iii) Since the pH value (3) is less than 7, the solution is acidic. A low concentration of hydroxide ions implies a high concentration of hydrogen ions, leading to acidity.
In simple words: With the amount of hydroxide ions given, we first find the pOH, which is 11. Then, we use the rule that pH plus pOH equals 14, so the pH is 3. Because 3 is less than 7, the solution is acidic.

🎯 Exam Tip: Be careful to distinguish between \( [\mathrm{H}^{+}] \) and \( [\mathrm{OH}^{-}] \) and their corresponding pH and pOH values. Always remember the sum of pH and pOH is 14 at 25°C.

Question 6. Calculate the pH of 0.02 M Ba(OH)2. Ba(OH)2 solution is a strong electrolyte.
Answer:
Given concentration of \( \mathrm{Ba(OH)}_2 = 0.02\text{M} \).
Barium hydroxide \( \mathrm{Ba(OH)}_2 \) is a strong electrolyte, meaning it dissociates completely in water:
\( \mathrm{Ba(OH)}_2 \rightarrow \mathrm{Ba}^{2+} + 2\mathrm{OH}^{-} \)
From the balanced equation, 1 mole of \( \mathrm{Ba(OH)}_2 \) produces 2 moles of \( \mathrm{OH}^{-} \) ions.
So, \( [\mathrm{OH}^{-}] = 2 \times [\mathrm{Ba(OH)}_2] \)
\( [\mathrm{OH}^{-}] = 2 \times 0.02\text{M} \)
\( [\mathrm{OH}^{-}] = 0.04\text{M} \)
Now, calculate pOH:
\( \mathrm{pOH} = -\log[\mathrm{OH}^{-}] \)
\( \mathrm{pOH} = -\log[0.04] \)
We can write 0.04 as \( 4 \times 10^{-2} \).
\( \mathrm{pOH} = -\log[4 \times 10^{-2}] \)
\( \mathrm{pOH} = -[\log 4 + \log 10^{-2}] \)
\( \mathrm{pOH} = -[\log 4 - 2 \log 10] \)
We know \( \log 10 = 1 \) and \( \log 4 \approx 0.6020 \).
\( \mathrm{pOH} = -[0.6020 - 2 \times 1] \)
\( \mathrm{pOH} = -[0.6020 - 2] \)
\( \mathrm{pOH} = -[-1.398] \)
\( \mathrm{pOH} = 1.398 \)
Finally, calculate pH using \( \mathrm{pH} + \mathrm{pOH} = 14 \):
\( \mathrm{pH} = 14 - \mathrm{pOH} \)
\( \mathrm{pH} = 14 - 1.398 \)
\( \mathrm{pH} = 12.602 \)
The pH of 12.602 is highly basic, which is expected for a strong base like barium hydroxide. This indicates a very low concentration of hydrogen ions in the solution.
In simple words: Since \( \mathrm{Ba(OH)}_2 \) makes two hydroxide ions, its hydroxide concentration is twice its own concentration. We find pOH from this and then subtract it from 14 to get the pH. Here, the pH is about 12.6, showing a very strong basic solution.

🎯 Exam Tip: For polyprotic bases or acids (those that produce more than one \( \mathrm{OH}^{-} \) or \( \mathrm{H}^{+} \) ion per molecule), remember to multiply the given concentration by the number of ions produced to get the actual ion concentration before calculating pOH or pH.