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NCERT Solutions of Science for Class – 10 Lesson – 1 CHEMICAL REACTIONS AND EQUATIONS


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

Ans- Magnesium ribbon should be cleaned before burning to remove the protective layer of basic magnesium oxide from its surface.

Q.2- Write the balanced equation for the following chemical reactions :

(i) Hydrogen + Chlorine → Hydrogen chloride

(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride

(iii) Sodium + Water → Sodium hydroxide + Hydrogen

(i) H2 + Cl2 → 2HCl

(ii) 3 BaCl2 + Al2(SO4)3 → BaSO4 + 2 AlCl3

(iii) 2Na + 2H2O → 2NaOH + H2

Q.3- Write the balanced chemical equation with state symbols for the following reactions :

i) Solutions of Barium Chloride and Sodium Sulphate in water react to give insoluble Sulphate and the solution of Sodium Chloride.

ii) Sodium Hydroxide solution ( in water ) to produce Sodium Chloride and Water.


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

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


Q.1- A solution of substance ‘X’ is used for white washing.

i) Name the substance ‘X’ and write its formula.

ii) Write the reaction of the substance X named in (i) above with water.

Ans- i) X is Calcium Oxide and its formula is CaO.

ii) CaO (s) + H2O (l) → Ca(OH)2 (aq)

Q.2- Why is the amount of gas collected in one of the test tubes in text book Activity 1.7 (i.e., electrolysis of water) double of the amount collected in the other? Name this gas.

Ans- When electric current is passed through acidulated water, the reaction taking place is –

H2O (l) → 2 H2 (g) + O2 (g)

Therefore hydrogen and oxygen produced are in the ratio 2 : 1 by volume. Hence volume of gas collected in one test tube is double the amount of gas in the other tube.

This gas is hydrogen.


Q.1- Why does the colour of Copper Sulphate solution change, when an iron nail is dipped in it?

Ans- This is because iron displaces Copper from Copper Sulphate solution. The concentration of the Copper Sulphate solution decreases and blue colour of solution gradually fades away and turns light green.

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

Q.2- Give an example of a double displacement reaction other than the one given in the activity 1.10 given in textbook.

Ans- NaCl (aq) + AgNO3(aq) →AgCl (s)+ NaNO3 (aq)

Q.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)

Ans- (i) In first reaction, substance oxidised is Na as it gains oxygen and oxygen is reduced.

(ii) In second reaction, substance reduced is Cu as hydrogen gains oxygen and is oxidised.


Q.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 being reduced.

(i) (a) and (b)

(ii) (a) and (c)

(iii) (a), (b) and (c)

(iv) all

Ans- (i) (a) and (b)

Q.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

Ans- (d) Displacement Reaction.

Q.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.

Ans- (a) Hydrogen gas and iron chloride are produced.

Q.4- What is the balanced chemical equation? Why should chemical equation be balanced?

Ans- It is a chemical equation in which number of atoms of each element are equal on both sides of the equation.

The chemical equation should be balanced because law of conservation of mass said that the total mass of the reactants must be equal to the total mass of the products.

Q.5- Translate the following statement 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.

Ans- (a) 3H2 (g) + N2 (g) → 2NH3 (g)

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

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

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

Q.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


(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.7- Write the balanced chemical equation 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


(a) Ca (OH)2 + CO2 → CaCO3 + H2O

(b) Zn + 2AgNO3 → Zn(NO3)2 + 2 Ag

(c) 2Al + 3 CuCl2 → 2AlCl3 + 3 Cu

(d) BaCl2 + K2SO4 → BaSO4 + 2KCl

Q.8- Write the balanced chemical equations for the following reactions and identify the type of reaction in each case :

(a) Potassium bromide (aq) + Barium iodide (aq) → Potassium iodide (aq) + Barium

(b) Zinc carbonate(s) → Zinc oxide (s) + Carbon dioxide (g) bromide(s)

(c) Hydrogen (g) + Chloride (g) → Hydrogen chloride (g)

(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)


(a) 2KBr (aq) + Bal2(aq) → 2Kl(aq) + BaBr2(s)
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.9- What does one mean by exothermic and endothermic reactions? Give examples.

Ans- Exothermic reaction : It is a chemical reaction in which heat energy is given out.

Eg. N2 (g) + 3H2 (g) → 2NH3 (g) + Heat

Endothermic reaction : It is a chemical reaction in which heat energy is absorbed.

Eg. N2 (g) + O2 (g) → 2NO(g) – Heat

Q.10- Why is respiration considered as an exothermic reaction? Explain.

Ans- This is because during respiration, oxidation of glucose occurs which produces heat energy.

C6H12O+ 6O→ 6CO+ 6H2O + Energy

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

Ans- During decomposition reaction a single substance breaks down into two or more substances which is just the reverse of combination reaction. Examples –

i) 2NaCl —> 2Na + Cl2

ii) CaCO3 → CaO + CO2

(iii) 2H2O→ 2H2 + O2

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

Ans- (i) Decomposition reaction where energy is supplied in the form of heat :

CaCO3 → CaO + CO2

(ii) Decomposition reaction where energy is supplied in the form of electricity :

2H2O→ 2H2 + O2

(iii) Decomposition reaction where energy is supplied in the form of light :

2AgCl → 2Ag + Cl2

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

Ans- Displacement Reaction- In this reaction a most active element displaces the less active element from solution. Eg.

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

Double Displacement Reaction- The reaction in which two reactants exchange their ions is called double displacement reaction. Eg.

NaCl + AgNO3 → AgCl + NaNO3

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


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

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

Ans- Precipitation reaction- A reaction in which an insoluble product or precipitate is produced is called precipitation reaction. Eg.

NaCl + AgNO3 → AgCl + NaNO3


Q.16- Explain the following terms of gain or loss of oxygen with two examples each :

(a) Oxidation

(b) Reduction

Ans- (a) Oxidation- A chemical reaction in which a substance gains oxygen or loses hydrogen is called oxidation. Eg.

2Mg (s) + O2 (g) → 2MgO (s)

2Cu + O2 → 2CuO

(b) Reduction– A chemical reaction in which a substance loses oxygen or gains hydrogen is called reduction. Eg.

ZnO + C → Zn + CO

2HgO → 2Hg + O2

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

Ans- The element ‘X’ is copper.

2Cu(s) + O2(g) → 2CuO(s)

The black colored compound formed is Copper Oxide.

Q.18- Why do we apply paint on iron articles?

Ans- We apply paint on iron articles to protect them from rusting. By applying paint the iron surface does not come in contact with air or oxygen and moisture which cause rusting.

Q.19- Oil and fat containing food items are flushed with Nitrogen. Why?

Ans- Oil and fat containing food items are flushed with Nitrogen to prevent them from oxidised.

Q.20- Explain the following terms with one example each :

(a) Corrosion

(b) Rancidity

Ans- (a) Corrosion – The process in which metals are eaten up slowly by the action of air and moisture on their surface is called corrosion. Eg. Rusting of iron.

(b) Rancidity – When fats and oils or food containing oils or fats get oxidised with air or oxygen, their smell and taste change. This process is called rancidity.