Class 10 Science Chapter 1 Chemical Reactions and Equations

Class 10 Science Chapter 1 Chemical Reactions and Equations

Welcome to Class 10 Science Chapter 1 Chemical Reactions and Equations. This chapter is one of the most important topics in Science and forms the foundation for understanding various chemical processes that occur around us. It is essential for students preparing for board examinations, school tests, and other academic assessments.

In this chapter, students will learn about chemical reactions, how they occur, and how to represent them using chemical equations. Important topics include balancing chemical equations, types of chemical reactions, combination reactions, decomposition reactions, displacement reactions, double displacement reactions, oxidation, reduction, corrosion, and rancidity. These concepts help students understand the changes that substances undergo during chemical reactions.

The study material is prepared in a simple and easy-to-understand language so that students can grasp the concepts quickly. It is useful for learning basic principles, revising important topics, and strengthening scientific knowledge. Understanding this chapter is important because it serves as the foundation for many advanced topics in Chemistry.

Chemical Reactions and Equations

Selected Questions &Answers

Q: What is a combination reaction?

Ans: A reaction in which two or more reactants combine to form a single product is called a ‘combination reaction’.

Q: What will you observe if an iron nail is dipped in copper sulphate solution?

Ans: Iron will displace copper from copper sulphate, and the color of the nail will turn brown while the solution will become light blue.

Q: What is a redox reaction?

Ans: A chemical reaction in which both oxidation and reduction occur simultaneously is called a redox reaction.

Q: What is corrosion? HSLC ’23

Ans: The process of decay of metals is called corrosion.

Q: What is an exothermic reaction?

Ans: Reactions in which heat is evolved along with the formation of products are called exothermic reactions.

Q: Give one example of an exothermic reaction.

Ans: CH₄+ 2O₂→CO₂+ 2H₂O

Q: What is a decomposition reaction?

Ans: A reaction in which a single reactant breaks down to form two or more products is called a decomposition reaction.

Q: What is a balanced chemical equation? HSLC ’22

Ans: A chemical equation in which the number of atoms of each element is equal on both sides is called a balanced chemical equation.

Q: Why should a chemical equation be balanced? HSLC ’22

Ans: According to the law of conservation of mass, mass can neither be created nor destroyed in a reaction. Therefore, a chemical equation must be balanced.

Q: Give one example of a decomposition reaction.

Ans: CaCO₃→CaO + CO₂

Q: Why does white silver chloride turn grey when exposed to sunlight?

Ans: Sunlight decomposes silver chloride into silver and chlorine. Therefore, it turns grey.

Q: Write the reaction of calcium oxide with water.

Ans: CaO + H₃O → Ca(OH)₃

Q: What is a precipitation reaction?

Ans: A chemical reaction in which an insoluble substance (precipitate) is formed is called a precipitation reaction.

Q: What is a double displacement reaction?

Ans: A reaction in which exchange of ions takes place between reactants is called a double displacement reaction.

Q: Why do oily or fatty food items become rancid?

Ans: Oily or fatty food items become rancid due to oxidation.

Q: Write two methods to prevent corrosion.

Ans: Applying grease and painting.

Q: Write two methods to prevent rancidity of food.

Ans: Keeping food in a cool place and packing food in an environment of nitrogen gas.

Q: Why is respiration considered an exothermic reaction?

Ans: During respiration, heat is released along with carbon dioxide. Therefore, respiration is considered an exothermic reaction.

Q: Why should a magnesium ribbon be cleaned before burning in air?

Ans: The impurities on the magnesium ribbon must be removed before burning, because only pure magnesium burns properly.

Q: Write balanced chemical equations with state symbols for the following reactions:

(i) Aqueous barium chloride reacts with aqueous sodium sulphate to form insoluble barium sulphate and sodium chloride solution.

(ii) Aqueous sodium hydroxide reacts with aqueous hydrochloric acid to form sodium chloride solution and water.

Ans:

(i) BaCl₂ (aq) + Na₂SO₄ (aq)→BaSO₄ (s) + 2NaCl(aq)

(ii) NaOH(aq) + HCl(aq)→NaCl(aq) + H₂O(l)

Q: A substance ‘X’ is used for whitewashing. HSLC ’20

(i) Write the name and formula of ‘X’.

(ii) Write the reaction of ‘X’ with water.

Ans:

(i) ‘X’ is calcium oxide and its formula is CaO.

(ii) CaO + H₂O →Ca(OH)₂

Q: In Activity 1.7, why is the volume of gas collected in one test tube double that of the other? Name the gas.

Ans: In activity 1.7, electrolysis of water produces hydrogen gas and oxygen gas. The gas is hydrogen.

2H₂O→2H₂+ O₂

From two molecules of water, two molecules of hydrogen and one molecule of oxygen are produced. Therefore, the volume of hydrogen is double that of oxygen.

Q: Why does the color of copper sulphate solution change when an iron nail is dipped in it?

Ans: When an iron nail is dipped in copper sulphate solution, the color fades. This is because iron is more reactive than copper, so it displaces copper from copper sulphate and forms iron sulphate.

CuSO₄(aq) + Fe(s) → FeSO₄ (aq) + Cu(s)

Q: Give an example of a double displacement reaction.

Ans: Pb(NO₃)2(aq) + 2HCl(aq) → PbCl₂ (s) + 2HNO₃(aq). When hydrochloric acid reacts with lead nitrate solution, a white precipitate of lead chloride is formed.

Q: Identify the substances that are oxidized and reduced in the following reactions:

(i) 4Na(s) + O2 (g) →2Na₂O(s)

(ii) CuO(s) + H₂ (g) ? Cu(s) + H₂O(l)

Ans:

(i) In the first reaction, Na is oxidized, and oxygen is reduced.

(ii) In the second reaction, H2 is oxidized to form water, while CuO is reduced to copper.

Q: Which of the following statements is incorrect for the reaction:

2PbO(s) + C(s) → 2Pb(s) + CO₂ (g)

(a) Lead is reduced

(b) Carbon dioxide is oxidized

(c) Carbon is oxidized

(d) Lead oxide is reduced

(i) (a) and (b)

(ii) (a) and (c)

(iii) (a), (b) and (d)

(iv) All

Ans: (i) (a) and (b)

Q: Fe₂O₃ + 2Al → Al₂O₃ + 2Fe. The above reaction is an example of:

(a) Combination reaction

(b) Double displacement reaction

(c) Decomposition reaction

(d) Displacement reaction

Ans: (d) Displacement reaction

Q: What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer.

(a) Hydrogen gas and iron chloride are produced

(b) Chlorine gas and iron hydroxide are produced

(c) No reaction occurs

(d) Iron salt and water are produced

Ans: (a) Hydrogen gas and iron chloride are produced

Q: Write the following statements in the form of balanced chemical equations: HSLC ’16

(a) Hydrogen gas combines with nitrogen to form ammonia.

(b) Hydrogen sulphide burns in air to produce water and sulphur dioxide.

(c) Barium chloride reacts with aluminium sulphate to form aluminium chloride and a precipitate of barium sulphate.

(d) Potassium reacts with water to form potassium hydroxide and hydrogen gas.

Ans:

(a) 3H₂ (g) + N₂ (g)→2NH₃ (g)

(b) 2H₂S(g) + 3O₂ (g) →2H₂O(l) + 2SO₂ (g)

(c) 3BaCl₂ (aq) + Al₂ (SO₄)3(aq) →2AlCl₃ (aq) + 3BaSO₄ (s)

(d) 2K(s) + 2H₂O(l) →2KOH(aq) + H₂ (g)

Q: Balance the following equations:

(a) HNO₃ + Ca(OH)₂ → Ca(NO₃ )2 + H₂O

(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O

(c) NaCl + AgNO3 → AgCl + NaNO₃

(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl

Ans:

(a) 2HNO₃+ Ca(OH)₂ →Ca(NO₃ )2 + 2H₂O

(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

(c) NaCl + AgNO₃ → AgCl + NaNO₃

(d) BaCl₂ + H₂SO₄ ? BaSO₄ + 2HCl

Q: Write the balanced chemical equations for the following reactions.

(a) Calcium hydroxide + carbon dioxide ? calcium carbonate + water.

(b) Zinc + silver nitratesilver + zinc nitrate

(c) Aluminium + copper chloridealuminium chloride + copper

(d) Barium chloride + potassium sulphate ? barium sulphate + potassiumchloride

Ans:

(a) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

(b) Zn + 2AgNO₃→ Zn(NO₃ )2 + 2Ag

(c) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu

(d) BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl

Q: Write the balanced chemical equations for the following and identify the type of each reaction.

(a) Potassium bromide (aq) + barium iodide (aq) → potassium iodide(aq) + barium bromide (s)

(b) Zinc carbonate (s) → carbon dioxide (g) + zinc oxide (s). HSLC ’16 (c) Hydrogen (g) + chlorine (g)→ hydrogen chloride (g)

(d) Magnesium (s) + hydrochloric acid (aq) → magnesium chloride(aq) + hydrogen (g)

Ans:

(a) 2KBr(aq) + BaI₂ (aq) → 2KI(aq) + BaBr₂ (aq)

Type: Double displacement reaction.

(b) ZnCO₃ (s) → ZnO(s) + CO₂ (g)

Type: Decomposition reaction.

(c) H₂ (g) + Cl₂ (g)→ 2HCl(g)

Type: Combination reaction.

(d) Mg(s) + 2HCl(aq) → MgCl₂ (aq) + H₂ (g)

Type: Displacement reaction.

Q: What is meant by a balanced chemical equation? Why should chemical equations be balanced?

Ans: A balanced chemical equation represents that the number of atoms of each element is equal on both sides of the equation. Chemical equations must be balanced because:

(i) In a chemical reaction, mass is neither created nor destroyed.

(ii) The electrons lost by one atom are gained by another, so the total charge remains the same.

Q: What do you mean by exothermic and endothermic reactions? Giveexamples. HSLC ’16

Ans: Reactions in which heat is released along with the formation of products are called exothermic reactions. Example:

C + O₂ → CO₂+ 393.5 kJ

Reactions in which heat is absorbed are called endothermic reactions. Example:

H₂(g) + I2(g) → 2HI(g) – 52.2 kJ

Q: Why is respiration considered an exothermic reaction? Explain.

Ans: In respiration, oxidation of food occurs in the presence of oxygen inside cells, producing a considerable amount of heat energy. Therefore, respiration is considered an exothermic reaction.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

Q: Why is a decomposition reaction considered the opposite of a combination reaction? Write equations for these reactions. HSLC ’15

Ans: In a decomposition reaction, a single substance breaks down into two or more substances.

Example: 2HgO → 2Hg + O₂

On the other hand, in a combination reaction, two or more substances combine to form a single new substance.

Example: 2H₂+ O₂→ 2H₂O

Therefore, decomposition is considered the opposite of a combination reaction.

Q: Write one equation each for decomposition reactions caused by heat, light, and electricity.

Ans:

Thermal decomposition: CaCO₃ → CaO + CO₂

Photochemical decomposition: 2AgCl → 2Ag + Cl₂

Electrolytic decomposition: 2H₂O → 2H₂+ O₂

Q: What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Ans: In a displacement reaction, one element displaces another element from its compound. In a double displacement reaction, two compounds exchange their ions.

Equations:

Displacement reaction: Fe(s) + CuSO4(aq) → Cu(s) + FeSO4(aq)

Double displacement reaction: AgNO₃ (aq) + NaCl(aq) → AgCl(s) +NaNO₃ (aq)

Q: In silver refining, silver is obtained from silver nitrate solution by displacement using copper metal. Write the reaction involved.

Ans: 2AgNO₃ (aq) + Cu(s)→ Cu(NO₃ )2(aq) + 2Ag(s)

Q: Write the correct answer: HSLC ’15

(a) Which of the following metals can displace copper from copper sulphate solution?

(a) Silver

(b) Gold

(c) Zinc

(d) Mercury

Ans: (c) Zinc

(b) The reaction

Fe₂O₃ + 2Al→ Al₂O₃ + 2Fe is: HSLC ’16, ’18

(a) A combination reaction

(b) A decomposition reaction

(c) A displacement reaction

(d) A double displacement reaction

Ans: (c) A displacement reaction

(c) What happens when dilute hydrochloric acid is added to iron filings? HSLC ’17

(a) Hydrogen gas and iron chloride are produced

(b) Chlorine gas and iron hydroxide are produced

(c) No reaction occurs

(d) Iron salt and water are produced

Ans: (a) Hydrogen gas and iron chloride are produced

Q: Explain the following terms with two examples each based on the gain and loss of oxygen:

(a) Oxidation

(b) Reduction

Ans:

(a) Oxidation: A reaction in which a substance gains oxygen is called oxidation.

Examples:

2Cu + O₂ → 2CuO (Cu is oxidized to CuO)

2Mg + O₂→ 2MgO (Mg is oxidized to MgO)

(b) Reduction: A reaction in which a substance loses oxygen is called reduction.

Examples:

ZnO + C→ Zn + CO (ZnO is reduced to Zn)

CuO + H₂ → Cu + H₂O (CuO is reduced to Cu)

Q: What is a precipitation reaction? Explain with an example.

Ans: A reaction in which an insoluble substance (precipitate) is formed is called a precipitation reaction. For example, when sodium sulphate reacts with barium chloride, a white precipitate of barium sulphate is formed.

BaCl₂ + Na₂SO₄ → BaSO₄ → + 2NaCl

Q: A shiny brown element ‘X’ turns black when heated in air. Name‘X’ and the black compound formed. HSLC ’20

Ans: The element ‘X’ is copper (Cu), and the black compound formed is copper oxide (CuO).

2Cu + O₂ → 2CuO

Q: Why are iron objects painted? HSLC ’20

Ans: Iron objects are painted to prevent corrosion. Oxygen and moisture present in air react with iron to form rust. Painting prevents contact with air and moisture, thus protecting iron from rusting.

Q: Why are food items containing fats and oils packed in nitrogen?

Ans: They are packed in nitrogen to prevent oxidation. When fats and oils get oxidized, they become rancid, producing an unpleasant smell and taste.

Q: Explain the following with one example each:

(a) Corrosion

(b) Rancidity

Ans:

(a) Corrosion: When a metal is attacked by substances like moisture, acids, etc., it undergoes corrosion. Formation of black coating on silver and green coating on copper are examples.

(b) Rancidity: When oily or fatty food is kept for a long time, it reacts with oxygen, causing a change in taste and smell. This is called rancidity, making the food unfit for consumption.

Q: What changes occur during a chemical reaction?

Ans: The following changes occur during a chemical reaction:

(a) Change in colour

(b) Change in temperature

(c) Evolution of gas

(d) Change in state

Q: What is a skeletal chemical equation and a balanced equation?

Ans: A skeletal chemical equation represents a reaction using symbols and formulas of substances without balancing.

Example: Mg + HCl→ MgCl₂ + H₂

A balanced equation has an equal number of atoms of each element on both sides.

Example: Mg + 2HCl → MgCl₂ + H₂

Q: What is rust? What conditions are necessary for rusting?

Ans: When iron is exposed to air, a reddish-brown coating forms on its surface, called rust. Moisture and oxygen are necessary conditions for rusting.

Q: What is the law of conservation of mass?

Ans: In a chemical reaction, the total mass of reactants is equal to the total mass of products. Mass can neither be created nor destroyed.

Q: What is a double displacement reaction?

Ans: A reaction in which exchange of ions occurs between two reactants is called a double displacement reaction.

Q: What is a redox reaction? Give one example.

Ans: A reaction in which one substance is oxidized and another is reduced is called a redox reaction.

Q: When copper powder is heated in a china dish:

(a) What happens to its colour? HSLC ’16

(b) Write the balanced equation for the reaction.

Ans:

(a) The colour of copper powder turns black.

(b) 2Cu + O2 → 2CuO

Q: Take 3 mL sodium sulphate solution in one test tube and 3 mL barium chloride solution in another. Mix them and answer:

(a) What do you observe? HSLC ’19

Ans: A white precipitate of BaSO4 is observed.

(b) Write the chemical equation and mention the type of reaction.

Ans: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)

Type: Double displacement reaction

Q: Explain oxidation and reduction with one example each based on oxygen gain or loss. HSLC ’19

Ans:

(a) Oxidation: Gain of oxygen. Example: 2Cu + O₂ → 2CuO

(b) Reduction: Loss of oxygen. Example: CuO + H₂ → Cu + H₂O

Q: Write the chemical equation for the reaction between Plaster of Paris and water. HSLC ’17

Ans: CaSO₄·½H₂O + H₂O → CaSO₄·2H₂O

Q: What happens when mercuric oxide (HgO) is heated? Write the equation. HSLC ’17

Ans: On heating mercuric oxide, it decomposes into mercury and oxygen.

2HgO(s)→ 2Hg(l) + O₂(g)

Q: Take 2 g of calcium oxide in a beaker and add water slowly. Answer the following: HSLC ’17

(a) What change in temperature occurs?

(b) Write the balanced chemical equation.

Ans:

(a) The temperature increases.

(b) CaO(s) + H₂O(l) → Ca(OH)2(aq)

Q: Write the following statement as a chemical equation. HSLC ’18

Hydrogen gas combines with nitrogen to form ammonia.

Ans: 3H₂ + N₂ → 2NH₃

Multiple Choice Questions (MCQ)

Q: Which of the following is an example of a combination reaction?

(a) Burning of carbon in air

(b) Reaction between zinc and copper sulphate

(c) Electrolysis of water

(d) Reaction between calcium and carbon dioxide

Ans: (a) Burning of carbon in air

Q: Reactions in which heat is absorbed are called—

(a) Oxidation

(b) Reduction

(c) Endothermic

(d) Exothermic

Ans: (c) Endothermic

Q: Reactions in which oxygen is added are called—

(a) Oxidation

(b) Reduction

(c) Endothermic

(d) Exothermic

Ans: (a) Oxidation

Q: Reactions in which two or more reactants combine to form a single product are called—

(a) Displacement

(b) Decomposition

(c) Redox

(d) Combination

Ans: (d) Combination

Q: In a precipitation reaction, what is produced?

(a) Soluble salt

(b) Solvent

(c) Insoluble salt

(d) None of the above

Ans: (c) Insoluble salt

Q: Representing substances taking part in a chemical reaction using symbols and formulae is called—

(a) Chemical reaction

(b) Chemical equation

(c) Reactants

(d) None of the above

Ans: (b) Chemical equation

Q: Substances that take part in a chemical reaction are called—(a) Oxidizing agents

(b) Reducing agents

(c) Products

(d) Reactants

Ans: (d) Reactants

Q: Which of the following is not an example of an exothermic reaction?

(a) Combustion of natural gas

(b) Respiration

(c) Formation of quicklime from limestone

(d) Conversion of organic matter into compost

Ans: (c) Formation of quicklime from limestone

Q: Which of the following is not a redox reaction?

(a) 2Cu + O₂ → 2CuO

(b) ZnO + C → Zn + CO

(c) 2PbO + C → 2Pb + CO₂

(d) CuO + H₂ → Cu + H₂O

Ans: None of the above (All are redox reactions)

Q: What happens when dilute hydrochloric acid is added to iron filings?

(a) Hydrogen gas and iron chloride are produced

(b) Chlorine gas and iron hydroxide are produced

(c) No reaction occurs

(d) Iron salt and water are produced

Ans: (a) Hydrogen gas and iron chloride are produced

Q: When lead nitrate powder is heated, which gas is evolved?(a) Black fumes

(b) Brown fumes

(c) Blue fumes

(d) None of the above

Ans: (b) Brown fumes

Q: A reaction in which a compound breaks into two or more substances is called—

(a) Double displacement reaction

(b) Combination reaction

(c) Displacement reaction

(d) Decomposition reaction

Ans: (d) Decomposition reaction

Q: Fe₂O₃ + 2Al → Al₂O₃ + 2Fe is an example of—

(a) Combination reaction

(b) Decomposition reaction

(c) Displacement reaction

(d) Double displacement reaction

Ans: (c) Displacement reaction

Q: A reaction involving removal of oxygen or addition of hydrogen is called—

(a) Oxidation

(b) Reduction

(c) Endothermic

(d) Exothermic

Ans: (b) Reduction

Q: What is formed when hydrogen reacts with oxygen?

(a) Water

(b) Carbon dioxide

(c) Ammonia

(d) Nitric acid

Ans: (a) Water

Q: Which of the following is not a balanced equation?

(a) CaO + H₂O → Ca(OH)₂

(b) C + O2 → CO₂

(c) 2Mg + O₂ → 2MgO

(d) CH₄+ O₂ → CO₂ + H₂O

Ans: (d) CH₄+ O₂ → CO₂+ H₂O