Class 10 Science 1 Chapter 3 Chemical Reactions and Equations Exercise Questions and Answers | Maharashtra Board | Rukmini Classes

 

Class 10 Science 1 Chapter 3 – Chemical Reactions and Equations

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In this post, you will find complete exercise questions and answers for Class 10 Science 1 Chapter 3 – Chemical Reactions and Equations (Maharashtra Board). All solutions are explained in a simple, step-by-step manner according to the latest SSC syllabus to help students understand the concepts easily and score better in exams.

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3. Chemical Reactions and Equations

Exercise

1. Choose the correct option from the bracket and explain the statement giving reason.

(Oxidation, displacement, electrolysis, reduction, zinc, copper, double displacement, decomposition)

a. To prevent rusting, a layer of Zinc metal is applied on iron sheets.

Reason: Iron sheets are coated with zinc by galvanization. Zinc prevents iron from coming in contact with air and moisture, so rusting is prevented.

b. The conversion of ferrous sulphate to ferric sulphate is Oxidation reaction.

Reason: In this reaction, iron changes from Fe²⁺ to Fe³⁺. Increase in oxidation state is called oxidation.

c. When electric current is passed through acidulated water Electrolysis of water takes place.

Reason: Electric current decomposes acidulated water into hydrogen and oxygen gases. This process is called electrolysis.

d. Addition of an aqueous solution of ZnSO₄ to an aqueous solution of BaCl₂ forms a white precipitate. It is an example of Double displacement reaction.

Reason: The ions exchange places to form BaSO₄ (white precipitate) and ZnCl₂. Therefore, it is a double displacement reaction.

 2. Write answers to the following.

a.) What is the reaction called when oxidation and reduction take place simultaneously? Explain with one example.

Answer:

When oxidation and reduction take place simultaneously in the same reaction, it is called a redox reaction.

• Oxidation: Addition of oxygen or removal of hydrogen (loss of electrons).
• Reduction: Removal of oxygen or addition of hydrogen (gain of electrons).

Redox Reaction = Oxidation + Reduction

Example:

CuO + H₂ → Cu + H₂O

Explanation:

1. Copper oxide (CuO) loses oxygen and is converted into copper (Cu). Therefore, CuO undergoes reduction.

2. Hydrogen (H₂) gains oxygen and forms water (H₂O). Therefore, H₂ undergoes oxidation.

3. Since oxidation and reduction occur at the same time, the reaction is called a redox reaction.

Q.2.b). How can the rate of the chemical reaction, namely, decomposition of hydrogen peroxide be increased?

Answer:

The decomposition of hydrogen peroxide is a slow reaction.

Reaction:

2H₂O₂ ーMnO₂→ 2H₂O + O₂↑

• Hydrogen peroxide decomposes into water and oxygen gas. The rate of this reaction can be increased by adding manganese dioxide (MnO₂) as a catalyst.

• A catalyst is a substance that increases the rate of a chemical reaction without undergoing any permanent chemical change itself.

• Therefore, the rate of decomposition of hydrogen peroxide is increased by using manganese dioxide (MnO₂) as a catalyst.

Q2. c). Explain the terms reactant and product giving examples.

Answer:

Reactants:

The substances that take part in a chemical reaction and undergo chemical change are called reactants.

Products:

The new substances formed after a chemical reaction are called products.

Example:

Chemical equation:

2H₂ + O₂ → 2H₂O

Reactants: Hydrogen (H₂) and Oxygen (O₂)

Product: Water (H₂O)

Thus, hydrogen and oxygen are the reactants, while water is the product.

Q.2. d) . Explain the types of reaction with reference to oxygen and hydrogen. Illustrate with examples.

Answer:

With reference to oxygen and hydrogen, chemical reactions are of two types:

1. Oxidation Reaction:

A reaction in which oxygen is added or hydrogen is removed from a substance is called an oxidation reaction.

Example:

2Cu + O₂ → 2CuO

In this reaction, copper combines with oxygen to form copper oxide. Hence, it is an oxidation reaction.

2. Reduction Reaction:

A reaction in which oxygen is removed or hydrogen is added to a substance is called a reduction reaction.

Example:

CuO + H₂ → Cu + H₂O

In this reaction, copper oxide loses oxygen and is converted into copper. Hence, it is a reduction reaction. 

 

Q2.e). Explain the similarity and difference in two events, namely adding NaOH to water and adding CaO to water.

Answer:

Reaction between NaOH and water:

NaOH + H₂O → Na⁺ + OH⁻ + Heat

Reaction between CaO and water:

CaO + H₂O → Ca(OH)₂ + Heat

Similarities

In both cases, heat is released. Hence, both are exothermic processes.

Differences

1. NaOH dissolves in water and dissociates into Na⁺ and OH⁻ ions. No new substance is formed.
2. CaO reacts with water to form a new substance, calcium hydroxide [Ca(OH)₂]. This is a chemical reaction.

3.Explain the following terms with  examples.

a). Endothermic reaction

Answer:

An endothermic reaction is a chemical reaction in which heat is absorbed from the surroundings or heat has to be supplied from outside. Such reactions require heat to proceed.

Example:

NH₄Cl(s) + H₂O → NH₄Cl(aq) − Heat

When solid ammonium chloride (NH₄Cl) is dissolved in water, the test tube becomes cold because heat is absorbed from the surroundings. Hence, it is an endothermic reaction.

b). Combination reaction

A combination reaction is a chemical reaction in which two or more reactants combine to form a single product.

General equation:

A + B → AB

Examples:

2Mg + O₂ → 2MgO

In this reaction, magnesium combines with oxygen to form magnesium oxide (MgO). Magnesium oxide is the single product formed.

CaO + H₂O → Ca(OH)₂ + Heat

In this reaction, calcium oxide combines with water to form calcium hydroxide [Ca(OH)₂]. Calcium hydroxide is the single product formed, and heat is released.

c). Balanced equation

A balanced chemical equation is a chemical equation in which the number of atoms of each element is the same on both the reactant side and the product side. This follows the law of conservation of mass.

Examples of Balanced Chemical Equations

2Mg + O₂ → 2MgO

2H₂ + O₂ → 2H₂O

2H₂O₂ → 2H₂O + O₂

CaO + H₂O → Ca(OH)₂

Zn + CuSO₄ → ZnSO₄ + Cu

CuO + H₂ → Cu + H₂O

d) Displacement reaction

A displacement  reaction is a chemical reaction in which a more reactive element displaces a less reactive element from its compound and forms a new compound.

General equation:

A + BC → AC + B

Example:

CuSO₄ + Zn → ZnSO₄ + Cu

Explanation:

In this reaction, zinc is more reactive than copper. Therefore, zinc displaces copper from copper sulphate solution to form zinc sulphate and copper.

4. Give scientific reasons. 

a).When the gas formed on heating 

limestone is passed through freshly 

prepared lime water, the lime water 

turns milky.

Answer:

When limestone (CaCO₃) is heated, it decomposes to form calcium oxide (CaO) and carbon dioxide (CO₂).

Reaction:

CaCO₃ ---∆----≻ CaO + CO₂

The carbon dioxide (CO₂) gas produced is passed through freshly prepared lime water [Ca(OH)₂]. It reacts with lime water to form calcium carbonate (CaCO₃), which is a white insoluble precipitate. Therefore, the lime water turns milky.

Reaction:

Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O

Hence, lime water turns milky due to the formation of white calcium carbonate precipitate (CaCO₃).

b). It takes time for pieces of Shahabad tile to disappear in HCl, but its powder disappears rapidly.

Answer:

Shahabad tile is mainly made of calcium carbonate (CaCO₃).

The rate of a chemical reaction depends on the surface area of the reactants.

Powdered Shahabad tile has a larger surface area than pieces of Shahabad tile.

Therefore, more particles of the powder come in contact with dilute HCl, so the reaction occurs faster.

Hence, the powder disappears rapidly, whereas the pieces take more time to disappear.

Reaction:

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑

C).   While preparing dilute sulphuric acid from concentrated sulphuric acid in the laboratory, the concentrated sulphuric acid is added slowly to water with constant stirring.

Answer:

When concentrated sulphuric acid is mixed with water, a large amount of heat is released. This is a highly exothermic process.

If water is added to concentrated sulphuric acid, the mixture may boil suddenly and splash, causing serious burns.

Therefore, concentrated sulphuric acid is added slowly to water with constant stirring.

Constant stirring helps to distribute the heat evenly and prevents overheating and splashing, making the process safe.

d). It is recommended to use an air-tight container for storing oil for a long time.

Answer:

When oil comes in contact with oxygen present in air, it undergoes oxidation.

Oxidation causes the oil to become rancid, producing an unpleasant smell and taste.

An air-tight container prevents the oil from coming in contact with air, thereby slowing down oxidation.

Hence, it is recommended to store oil in an air-tight container for a long time.

Answer:

• The given picture shows the corrosion of iron.
• The chemical formula of rust is Fe₂O₃·H₂O (hydrated iron(III) oxide).
• Rust does not form by the direct reaction of oxygen with iron. It is formed by an electrochemical reaction.
• Different regions on the surface of iron act as the anode and cathode.

At the anode:

Fe(s) → Fe²⁺(aq) + 2e⁻

Iron is oxidised to Fe²⁺ ions.

At the cathode:

O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O

Oxygen is reduced to form water.

• The Fe²⁺ ions migrate from the anode region, react with water, and are further oxidised to form Fe³⁺ ions.
• A reddish-brown hydrated oxide is formed from Fe³⁺ ions. This is called rust, which gets deposited on the surface of iron.
• Due to the action of oxygen, moisture, and other components of the atmosphere, metals undergo oxidation, resulting in their damage. This process is called corrosion.

Thus, the reddish-brown layer formed on iron is rust, and this process is known as the corrosion of iron.

6. Identify from the following reactions the reactants that undergo oxidation and reduction.

a. Fe + S → FeS

Oxidation: Fe (Iron)

Reduction: S (Sulphur)

Reason: Iron loses electrons (oxidation) and sulphur gains electrons (reduction).

b. 2Ag₂O → 4Ag + O₂

Oxidation: Oxygen (O²⁻ in Ag₂O)

Reduction: Ag₂O (Silver ions, Ag⁺)

Reason: Oxygen is released as O₂ (oxidation), while Ag⁺ is converted into Ag (reduction).

c. 2Mg + O₂ → 2MgO

Oxidation: Mg (Magnesium)

Reduction: O₂ (Oxygen)

Reason: Magnesium combines with oxygen (oxidation), and oxygen gains electrons (reduction).

d. NiO + H₂ → Ni + H₂O

Oxidation: H₂ (Hydrogen)

Reduction: NiO (Nickel oxide)

Reason: Hydrogen gains oxygen to form water (oxidation), while NiO loses oxygen to form nickel (reduction).

7.Balance the following equations stepwise.

a.H₂S₂O₇(l) + H₂O(l) → H₂SO₄(l)

Answer:

Given equation:

H₂S₂O₇(l) + H₂O(l) → H₂SO₄(l)

Step 1: Count atoms.

LHS: H = 4, S = 2, O = 8

RHS: H = 2, S = 1, O = 4

Step 2: Put coefficient 2 before H₂SO₄.

Balanced equation:

H₂S₂O₇(l) + H₂O(l) → 2H₂SO₄(l)

b.  SO₂(g) + H₂S(aq) → S(s) + H₂O(l)

Answer 

Given equation:

SO₂(g) + H₂S(aq) → S(s) + H₂O(l)

Step 1: Put coefficient 2 before H₂S.

SO₂ + 2H₂S → S + H₂O

Step 2: Put coefficient 3 before S.

SO₂ + 2H₂S → 3S + H₂O

Step 3: Put coefficient 2 before H₂O.

Balanced equation:

SO₂(g) + 2H₂S(aq) → 3S(s) + 2H₂O(l)

c. Ag(s) + HCl(aq) → AgCl(s) + H₂(g)

Answer

Given equation:

Ag(s) + HCl(aq) → AgCl(s) + H₂(g)

Step 1: Put coefficient 2 before Ag, HCl and AgCl.

Balanced equation:

2Ag(s) + 2HCl(aq) → 2AgCl(s) + H₂(g)

Note: This equation is balanced mathematically, but silver (Ag) does not react with dilute HCl under normal conditions.

d. NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + H₂O(l)

Answer

Given equation:

NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + H₂O(l)

Step 1: Put coefficient 2 before NaOH.

2NaOH + H₂SO₄ → Na₂SO₄ + H₂O

Step 2: Put coefficient 2 before H₂O.

Balanced equation:

2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)

8. Identify the Endothermic and Exothermic Reactions

a. HCl + NaOH → NaCl + H₂O + Heat

Answer: 

Exothermic reaction (Heat is released.)

b. 2KClO₃(s) —Δ→ 2KCl(s) + 3O₂↑

Answer:  

Endothermic reaction (Heat is absorbed for decomposition.)

c. CaO + H₂O → Ca(OH)₂ + Heat

Answer: 

Exothermic reaction (Heat is released.)

d. CaCO₃(s) —Δ→ CaO(s) + CO₂↑

Answer: 

Endothermic reaction (Heat is absorbed for decomposition.)

9. Match the columns in the following table 

Answer