NCERT Solutions for Class 10 Science Chapter 1: Chemical Reactions and Equations

These Class 10 Science Chapter 1 solutions cover Chemical Reactions and Equations from the latest NCERT textbook (session 2026–27). You will learn how to write and balance chemical equations, recognise the main types of chemical reactions — combination, decomposition, displacement, double displacement and redox — and understand everyday effects of oxidation such as corrosion and rancidity. Every in-text question and all 20 end-of-chapter Exercises are reproduced verbatim and solved step by step.

Class: 10 Subject: Science Chapter: 1 Title: Chemical Reactions and Equations Exercises: 20 + in-text questions Session: 2026–27
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Class 10 Science Chapter 1 Solutions – Overview

Chapter 1, Chemical Reactions and Equations, is the opening chapter of Class 10 Chemistry. A chemical reaction takes place whenever the nature and identity of substances change — shown by a change in state, change in colour, evolution of a gas, change in temperature, or formation of a precipitate. The chapter teaches you to express reactions as balanced chemical equations that obey the law of conservation of mass, and to add state symbols (s), (l), (g) and (aq). It then classifies reactions into five main types — combination, decomposition, displacement, double displacement and oxidation–reduction (redox) — and links them to exothermic and endothermic changes. Finally, it explains two harmful effects of oxidation in everyday life: corrosion (such as rusting of iron) and rancidity of oily food.

Key Concepts & Definitions

Chemical reaction: a change in which one or more substances (reactants) are converted into new substances (products) with different properties.

Balanced chemical equation: an equation in which the number of atoms of each element is equal on the reactant and product sides, obeying the law of conservation of mass.

Combination reaction: two or more substances combine to form a single product, e.g. CaO + H2O → Ca(OH)2.

Decomposition reaction: a single substance breaks down into two or more simpler substances using heat (thermal), light or electricity (electrolytic).

Displacement reaction: a more reactive element displaces a less reactive element from its compound, e.g. Fe + CuSO4 → FeSO4 + Cu.

Double displacement reaction: two compounds exchange ions; precipitation reactions are a common type, e.g. Na2SO4 + BaCl2 → BaSO4↓ + 2NaCl.

Oxidation: gain of oxygen or loss of hydrogen. Reduction: loss of oxygen or gain of hydrogen. A reaction where both occur together is a redox reaction.

Exothermic reaction: heat is released. Endothermic reaction: energy is absorbed.

Corrosion: slow attack of a metal by moisture, air and acids (e.g. rusting). Rancidity: oxidation of fats and oils that changes their smell and taste.

Reactions to remember: Combination: CaO(s) + H2O(l) → Ca(OH)2(aq) Decomposition: 2FeSO4(s) ⟶[heat] Fe2O3(s) + SO2(g) + SO3(g) Displacement: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s) Double displacement: Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq) Redox: CuO + H2 → Cu + H2O (CuO reduced, H2 oxidised)

In-text Questions — Answers

In-text Questions (Page 5)

1. Why should a magnesium ribbon be cleaned before burning in air?

ANSWER A magnesium ribbon kept in air gets coated with a layer of basic magnesium carbonate / magnesium oxide due to slow reaction with moisture and air. This coating prevents the metal from burning easily. So the ribbon is cleaned with sandpaper to remove this oxide layer, exposing the shiny metal so that it burns readily with a dazzling white flame.

2. Write the balanced equation for the following chemical reactions.

ANSWER (i) Hydrogen + Chlorine → Hydrogen chloride
H2 + Cl2 → 2HCl (ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
3BaCl2 + Al2(SO4)3 → 3BaSO4 + 2AlCl3 (iii) Sodium + Water → Sodium hydroxide + Hydrogen
2Na + 2H2O → 2NaOH + H2

3. Write a balanced chemical equation with state symbols for the following reactions.

ANSWER (i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq) (ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

In-text Questions (Page 10)

1. A solution of a substance ‘X’ is used for whitewashing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

ANSWER (i) The substance ‘X’ is calcium oxide (quick lime), formula CaO. (ii) CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat
Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime), whose solution is used for whitewashing.

2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.

ANSWER Activity 1.7 is the electrolysis of water: 2H2O → 2H2 + O2. Water contains hydrogen and oxygen in a 2 : 1 ratio by volume. Therefore the volume of hydrogen gas produced is double that of oxygen. The gas collected in double amount is hydrogen (formed at the cathode).

In-text Questions (Page 13)

1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

ANSWER Iron is more reactive than copper, so it displaces copper from copper sulphate solution in a displacement reaction:
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s) The blue colour of copper sulphate fades because Cu2+ ions are removed from the solution and replaced by light-green iron sulphate (FeSO4). A brownish layer of copper also deposits on the nail.

2. Give an example of a double displacement reaction other than the one given in Activity 1.10.

ANSWER Reaction of silver nitrate with sodium chloride:
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq) A white precipitate of silver chloride is formed by the exchange of ions, so it is a double displacement (precipitation) reaction.

3. Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) 4Na(s) + O2(g) → 2Na2O(s)
(ii) CuO(s) + H2(g) → Cu(s) + H2O(l)

ANSWER (i) Sodium (Na) gains oxygen, so sodium is oxidised. Oxygen is added to sodium; oxygen itself is reduced. (ii) Hydrogen gains oxygen to form water, so hydrogen is oxidised. Copper oxide (CuO) loses oxygen, so copper oxide is reduced to copper.

End-of-Chapter Exercises — Solutions

1. Which of the statements about the reaction below are incorrect?
2PbO(s) + C(s) → 2Pb(s) + CO2(g)
(a) Lead is getting reduced. (b) Carbon dioxide is getting oxidised. (c) Carbon is getting oxidised. (d) Lead oxide is getting reduced.
(i) (a) and (b)   (ii) (a) and (c)   (iii) (a), (b) and (c)   (iv) all

ANSWER (i) (a) and (b) are the incorrect statements. PbO loses oxygen, so lead oxide is reduced (statement d correct), and lead is part of this reduced compound. Carbon gains oxygen, so carbon is oxidised (statement c correct). Statement (a) is wrong because it is lead oxide, not free lead, that is reduced; statement (b) is wrong because CO2 is the product of oxidation, it is not itself being oxidised.

2. Fe2O3 + 2Al → Al2O3 + 2Fe
The above reaction is an example of a
(a) combination reaction. (b) double displacement reaction. (c) decomposition reaction. (d) displacement reaction.

ANSWER (d) displacement reaction. Aluminium is more reactive than iron, so it displaces iron from ferric oxide. (This particular reaction is the thermite reaction.)

3. What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
(a) Hydrogen gas and iron chloride are produced. (b) Chlorine gas and iron hydroxide are produced. (c) No reaction takes place. (d) Iron salt and water are produced.

ANSWER (a) Hydrogen gas and iron chloride are produced. Fe + 2HCl → FeCl2 + H2↑. Iron displaces hydrogen from dilute HCl, giving ferrous chloride and hydrogen gas.

4. What is a balanced chemical equation? Why should chemical equations be balanced?

ANSWER A balanced chemical equation is one in which the number of atoms of each element is equal on both the reactant and product sides of the equation. Equations must be balanced because of the law of conservation of mass — mass can neither be created nor destroyed in a chemical reaction. So the total mass and the number of atoms of each element must be the same before and after the reaction.

5. Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

ANSWER (a) 3H2 + N2 → 2NH3 (b) 2H2S + 3O2 → 2H2O + 2SO2 (c) 3BaCl2 + Al2(SO4)3 → 2AlCl3 + 3BaSO4 (d) 2K + 2H2O → 2KOH + H2

6. Balance the following chemical 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

ANSWER (a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O (b) 2NaOH + H2SO4 → Na2SO4 + 2H2O (c) NaCl + AgNO3 → AgCl + NaNO3  (already balanced) (d) BaCl2 + H2SO4 → BaSO4 + 2HCl

7. Write the balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

ANSWER (a) Ca(OH)2 + CO2 → CaCO3 + H2O (b) Zn + 2AgNO3 → Zn(NO3)2 + 2Ag (c) 2Al + 3CuCl2 → 2AlCl3 + 3Cu (d) BaCl2 + K2SO4 → BaSO4 + 2KCl

8. Write the balanced chemical equation for the following and identify the type of reaction in each case.
(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)

ANSWER (a) 2KBr(aq) + BaI2(aq) → 2KI(aq) + BaBr2(s) — Double displacement reaction (b) ZnCO3(s) → ZnO(s) + CO2(g) — Decomposition reaction (c) H2(g) + Cl2(g) → 2HCl(g) — Combination reaction (d) Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) — Displacement reaction

9. What does one mean by exothermic and endothermic reactions? Give examples.

ANSWER Exothermic reaction: a reaction in which heat is released along with the products. Examples: burning of natural gas, CH4 + 2O2 → CO2 + 2H2O + heat; respiration; reaction of quick lime with water. Endothermic reaction: a reaction in which energy (heat) is absorbed. Examples: thermal decomposition of calcium carbonate, CaCO3 ⟶[heat] CaO + CO2; reaction of barium hydroxide with ammonium chloride.

10. Why is respiration considered an exothermic reaction? Explain.

ANSWER During respiration, glucose obtained from food combines with oxygen in the cells of the body and breaks down to give carbon dioxide, water and energy:
C6H12O6(aq) + 6O2(aq) → 6CO2(aq) + 6H2O(l) + energy Since this reaction releases energy (which we need to stay alive), heat is given out. Therefore respiration is considered an exothermic reaction.

11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

ANSWER In a combination reaction, two or more substances combine to form a single product. In a decomposition reaction, a single substance breaks down into two or more simpler products — exactly the reverse process. That is why they are called opposites. Combination: CaO(s) + H2O(l) → Ca(OH)2(aq) Decomposition: CaCO3(s) ⟶[heat] CaO(s) + CO2(g)

12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

ANSWER Heat (thermal): CaCO3(s) ⟶[heat] CaO(s) + CO2(g) Light: 2AgCl(s) ⟶[sunlight] 2Ag(s) + Cl2(g) Electricity (electrolytic): 2H2O(l) ⟶[electricity] 2H2(g) + O2(g)

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

ANSWER In a displacement reaction, a more reactive element displaces a less reactive element from its compound (one element is displaced):
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s) In a double displacement reaction, two compounds exchange their ions to form two new compounds (ions are exchanged, often with a precipitate):
Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)

14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

ANSWER Copper is more reactive than silver, so it displaces silver from silver nitrate solution:
Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)

15. What do you mean by a precipitation reaction? Explain by giving examples.

ANSWER A precipitation reaction is a reaction in which two solutions react to form an insoluble solid (a precipitate) that separates out from the solution. It is a type of double displacement reaction. Examples: Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq) (white precipitate of BaSO4); and Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq) (yellow precipitate of PbI2).

16. Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
(b) Reduction

ANSWER (a) Oxidation is the gain of oxygen by a substance. Examples: 2Cu + O2 → 2CuO (copper is oxidised); 2Mg + O2 → 2MgO (magnesium is oxidised). (b) Reduction is the loss of oxygen by a substance. Examples: CuO + H2 → Cu + H2O (CuO is reduced); ZnO + C → Zn + CO (ZnO is reduced).

17. A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

ANSWER The shiny brown element ‘X’ is copper (Cu). On heating in air it reacts with oxygen and gets coated with a black layer of copper(II) oxide (CuO). 2Cu + O2 ⟶[heat] 2CuO (black)

18. Why do we apply paint on iron articles?

ANSWER Iron articles are painted to prevent rusting (corrosion). Paint forms a protective coating that stops moisture (water) and oxygen of the air from coming into direct contact with the iron surface. Since rusting needs both air and moisture, blocking this contact protects the iron from being corroded and increases the life of the article.

19. Oil and fat containing food items are flushed with nitrogen. Why?

ANSWER Oils and fats turn rancid (develop a bad smell and taste) when they are oxidised by the oxygen of the air. Nitrogen is an unreactive (inert) gas. Flushing the packet with nitrogen removes oxygen, so oxidation is prevented and the food does not become rancid — it stays fresh for longer.

20. Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity

ANSWER (a) Corrosion: the gradual eating away of a metal by the attack of moisture, air and acids around it. Example: rusting of iron, in which iron forms a reddish-brown coating of hydrated iron oxide. (b) Rancidity: the oxidation of fats and oils in food on exposure to air, which changes their smell and taste, making them unfit to eat. Example: deep-fried food or butter left exposed for a long time develops a foul smell.

Extra Practice Questions

Short Answer Type Questions

Q1. State the law of conservation of mass.

ANSWERIn a chemical reaction, mass can neither be created nor destroyed. The total mass of the reactants is equal to the total mass of the products, which is why chemical equations must be balanced.

Q2. What is observed when a few zinc granules are added to dilute hydrochloric acid? Write the equation.

ANSWERBubbles of hydrogen gas are seen around the zinc granules and the test tube becomes warm (exothermic). Zn + 2HCl → ZnCl2 + H2↑.

Q3. Why does silver chloride turn grey when left in sunlight? Write the reaction.

ANSWERWhite silver chloride decomposes in sunlight (a photochemical decomposition) into silver and chlorine: 2AgCl(s) → 2Ag(s) + Cl2(g). The grey colour is due to the silver metal formed; this is used in black-and-white photography.

Q4. Identify the oxidised and reduced substances: MnO2 + 4HCl → MnCl2 + 2H2O + Cl2.

ANSWERHCl loses hydrogen / gains nothing but is converted to Cl2, so HCl is oxidised to chlorine. MnO2 loses oxygen, so MnO2 is reduced to MnCl2. It is a redox reaction.

Q5. What type of reaction is the heating of lead nitrate? Write the balanced equation and the observation.

ANSWERIt is a thermal decomposition reaction. 2Pb(NO3)2(s) ⟶[heat] 2PbO(s) + 4NO2(g) + O2(g). Brown fumes of nitrogen dioxide are observed.

Long Answer Type Questions

Q1. Explain, with one example each, the five main types of chemical reactions.

ANSWERCombination: two or more reactants form one product, e.g. CaO + H2O → Ca(OH)2. Decomposition: one reactant breaks into two or more products, e.g. 2FeSO4 → Fe2O3 + SO2 + SO3. Displacement: a more reactive element displaces a less reactive one, e.g. Fe + CuSO4 → FeSO4 + Cu. Double displacement: two compounds exchange ions, e.g. Na2SO4 + BaCl2 → BaSO4 + 2NaCl. Oxidation–reduction (redox): oxidation and reduction occur together, e.g. CuO + H2 → Cu + H2O, where CuO is reduced and H2 is oxidised.

Q2. Describe the steps to balance the equation Fe + H2O → Fe3O4 + H2 by the hit-and-trial method.

ANSWERStart with the compound having the most atoms, Fe3O4. Balance oxygen: there are 4 O on the right and 1 in H2O, so put 4H2O on the left → Fe + 4H2O → Fe3O4 + H2. Next balance hydrogen: 4H2O gives 8 H, so put 4H2 on the right. Finally balance iron: 3 Fe in Fe3O4, so put 3Fe on the left. The balanced equation is 3Fe + 4H2O → Fe3O4 + 4H2. Checking, both sides have 3 Fe, 8 H and 4 O atoms. With state symbols: 3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g).

Q3. What is corrosion? Explain the conditions necessary for rusting of iron and two methods to prevent it.

ANSWERCorrosion is the slow attack of a metal by moisture, air and acids in its surroundings, e.g. rusting of iron, tarnishing of silver (black) and corrosion of copper (green). For rusting of iron, both air (oxygen) and moisture (water) must be present together; in their absence iron does not rust. Two methods of prevention are: (i) painting, oiling or greasing the surface to keep out air and water, and (ii) galvanisation — coating iron with a layer of zinc. Corrosion causes huge damage to bridges, ships, railings and car bodies, so prevention is important.

MCQs & Assertion–Reason

1. The reaction CaO + H2O → Ca(OH)2 is a:

(a) decomposition reaction    (b) displacement reaction    (c) combination reaction    (d) double displacement reaction

2. Which of the following is a decomposition reaction?

(a) 2H2 + O2 → 2H2O    (b) 2Pb(NO3)2 → 2PbO + 4NO2 + O2    (c) Fe + CuSO4 → FeSO4 + Cu    (d) NaCl + AgNO3 → AgCl + NaNO3

3. When an iron nail is dipped in copper sulphate solution, the blue colour fades because:

(a) iron is less reactive than copper    (b) iron displaces copper    (c) copper displaces iron    (d) no reaction occurs

4. In the reaction CuO + H2 → Cu + H2O, hydrogen is:

(a) reduced    (b) oxidised    (c) neither    (d) precipitated

5. The brown gas evolved on heating lead nitrate is:

(a) O2    (b) NO2    (c) SO2    (d) CO2

6. Which type of reaction is Na2SO4 + BaCl2 → BaSO4↓ + 2NaCl?

(a) combination    (b) decomposition    (c) double displacement    (d) redox only

7. Respiration is an example of a/an:

(a) endothermic reaction    (b) exothermic reaction    (c) decomposition only    (d) precipitation reaction

8. In electrolysis of water, the ratio of volumes of hydrogen to oxygen collected is:

(a) 1 : 2    (b) 2 : 1    (c) 1 : 1    (d) 1 : 8

9. The substance used for whitewashing walls is:

(a) CaCO3    (b) CaO    (c) Ca(OH)2    (d) CaCl2

10. Oils and fats are flushed with nitrogen to prevent:

(a) corrosion    (b) rancidity    (c) precipitation    (d) decomposition

Answer key: 1-(c), 2-(b), 3-(b), 4-(b), 5-(b), 6-(c), 7-(b), 8-(b), 9-(c), 10-(b).

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: Chemical equations should be balanced.

Reason: The law of conservation of mass requires the number of atoms of each element to be equal on both sides.

A-R 2. Assertion: A magnesium ribbon is cleaned with sandpaper before burning.

Reason: An oxide layer on its surface prevents it from burning readily.

A-R 3. Assertion: The reaction of zinc with dilute hydrochloric acid is endothermic.

Reason: The test tube becomes warm during the reaction.

A-R 4. Assertion: Decomposition reactions are the opposite of combination reactions.

Reason: In decomposition a single substance breaks into simpler substances, while in combination several substances form a single product.

A-R 5. Assertion: Iron articles are painted.

Reason: Paint keeps moisture and air away, preventing rusting.

Answer key: 1-(A), 2-(A), 3-(D), 4-(A), 5-(A).

Common Mistakes & Exam Tips

Watch out for these

  • Never change a subscript to balance an equation — only add coefficients in front of formulae (e.g. 4H2O, not H2O4).
  • Forgetting to write state symbols (s), (l), (g), (aq) when the question asks for them costs marks.
  • Confusing displacement (one element displaced) with double displacement (ions exchanged).
  • Mixing up oxidation (gain of oxygen / loss of hydrogen) and reduction (loss of oxygen / gain of hydrogen).
  • Writing the wrong precipitate colour — AgCl is white, PbI2 is yellow, BaSO4 is white.
  • Calling respiration a simple combustion — it is a slow, controlled exothermic reaction.

How to score full marks in this chapter

Always re-check a balanced equation by counting atoms of each element on both sides. Learn the five reaction types with one clean example each so you can identify any reaction quickly. For redox questions, state clearly which substance gains/loses oxygen or hydrogen. Memorise the standard examples — quick lime + water, ferrous sulphate heating, lead nitrate heating, silver chloride in sunlight, electrolysis of water, and the iron-nail-in-copper-sulphate experiment — as these are repeatedly asked in board exams.

Frequently Asked Questions

What is Class 10 Science Chapter 1 about?

Chapter 1, Chemical Reactions and Equations, teaches how to write and balance chemical equations, add state symbols, and identify the five main types of reactions — combination, decomposition, displacement, double displacement and redox — along with exothermic/endothermic changes and everyday effects of oxidation like corrosion and rancidity.

How do you balance a chemical equation?

Count the atoms of each element on both sides, then add the smallest whole-number coefficients in front of the formulae so that the number of atoms of every element is equal on both sides. Never change the subscripts inside a formula. Finally, add state symbols if required.

What is the difference between oxidation and reduction?

Oxidation is the gain of oxygen or loss of hydrogen by a substance, while reduction is the loss of oxygen or gain of hydrogen. When both occur in the same reaction it is called a redox reaction, e.g. CuO + H2 → Cu + H2O.

Are these Class 10 Science Chapter 1 solutions free?

Yes. All ClearStudy NCERT Solutions for Class 10 Science are free and follow the official NCERT textbook for session 2026–27.

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