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

## NCERT Intext Questions

Page Number 6
1. Why should a magnesium ribbon be cleaned before burning in air ?
Ans. Magnesium is a reactive metal. It reacts with oxygen present in air to form magnesium oxide on its surface, which slows down or hinders the burning of magnesium in air. Hence, magnesium ribbon must be cleaned before burning it in air.
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
Ans. (i) H2 + Cl2 → 2HCl
(ii) 3 BaCl2 + Al2(SO4)3 → 3BaSO4 + 2AlCl3
(iii) 2Na + 2H2O → 2NaOH + H2
3. Write a balanced chemical equation with state symbols for the following reactions:
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Ans. (i) BaCl2(aq) + Na2SO4 (aq) → BaSO4(s) + 2NaCl (aq)
(ii) NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l)
Page Number 10
1. A solution of a 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) The substance, used for white washing is calcium oxide and its formula is CaO. (ii) When CaO reacts with H2O, it produces calcium hydroxide, also known as lime water. $$\underset{\text{Calcium oxide}}{CaO(s)} + \underset{\text{Water}}{H2O(l)} → \underset{\text{Calcium hydroxide}}{Ca(OH)2 (aq)} + Heat↑$$
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. Electrolysis of water:
2H2O(l) → 2H2(g) + O2
Water contains 2 part of hydrogen and one part of oxygen. Therefore during the electrolysis of water the amount of hydrogen gas collected in one test tube is double than the amount of oxygen collected in other test tube.
Hence, the name of gas is hydrogen.
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Page Number 13
1. Why does the colour 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 displacement reaction takes place. The colour of copper sulphate solution fades due to the formation of light green solution of iron sulphate. This is because iron is more reactive than copper. $$Fe(s) + \underset{\text{(Blue solution)}}{CuSO4(aq)} → \underset{\text{(Greenish solution)}}{FeSO4(aq)} + Cu(s)$$
2. Give an example of a double displacement reaction other than the one given in Activity 1.10
Ans. A double displacement reaction involves the exchange of ions between the reactants. Example: When sodium carbonate reacts with calcium chloride it forms a precipitate of calcium carbonate and sodium chloride. In this, exchange of carbonate ion and chloride ion takes place and two new compounds are formed.
Na2CO3(aq) + CaCl2(aq) → CaCO3(aq) + 2NaCl(aq)
3. Identify the substances that are oxidised and the substances which 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 this reaction sodium gains oxygen to form 2Na2O. Hence Na is being oxidised and oxygen is getting reduced.
(ii) In this reaction Hydrogen gains oxygen to form water. Hence H2 is being oxidised and CuO losses oxygen, so reduced.

## NCERT Exercise Questions

1. Which of the statements about the reaction below are incorrect ?
2 PbO(s) + C(s) → 2Pb(s) + CO2(g)
(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
Ans. (i) (a) and (b)
Explanation: In the reaction C (carbon) is gaining oxygen thus it is getting oxidised and PbO (lead oxide) is losing oxygen thus it is reduced.
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
Explanation: In this reaction Al is more reactive than Fe, and it displaces Fe to form Al2O3. So it is an example of displacement reaction.
3. 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 takes place.
(d) Iron salt and water are produced.
Ans. (a) Hydrogen gas and iron chloride are produced. Explanation: When HCl is added to iron filing H2 gas and FeCl2 is produced.
Fe + 2HCl → FeCl2 + H2
4. What is a balanced chemical equation? Why should chemical equations be balanced?

Ans. A balanced chemical equation is the one which has number of atoms of different elements in the reactants equals to the total number of atoms of different elements in the product.
Example: 2H2O → 2H2 + O2
On reactant side — H-atom = 2 × 2 = 4, O-atom = 2
On product side — H-atom = 2 × 2 = 4, O-atom = 2
Hence it is a balanced chemical reaction.
The chemical equations should be
balanced to satisfy the law of conservation of mass.

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.
Ans.  (a) H2(g) + N2(g) → NH3(g)
By balancing the above equation we can get
3H2(g) + N2(g) → 2NH3(g)
(b) H2S(g) + O2(g) → H2O(l) + SO2(g)
On balancing the equation: 2H2S(g) + 3O2(g) → 2H2O(l) + 2SO2(g)
(c) BaCl2(aq) + Al2(SO4)3(aq) → AlCl3(aq) + BaSO4(s)
On balancing the equation:
3BaCl2(aq) + Al2(SO4)3(aq) → 21AlCl3(aq) + 3BaSO4(s)
(d) K(s) + H2O(l) → KOH(aq) + H2(g)
On balancing the equation:
2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
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

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

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

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

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

Ans. (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

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

Ans. Exothermic reaction: When in a chemical reaction energy is released in the form of heat, the reaction is called exothermic reaction.
Example: C (g) + O2 (g) → CO2 (g)
Endothermic reaction: When in a chemical reaction energy is absorbed in the form of heat, the reaction is called endothermic reaction.

$$\text{Example: } CaCO_3\space \xrightarrow{Heat} \text{CaO +} CO_2↑$$

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

Ans. Respiration is an exothermic process because during respiration glucose combines with oxygen in the cells of our body to form carbon dioxide and water along with the production of energy.

$$\underset{\text{ Glucose}}{C_6H_{12}O_6 (aq)} + 6O_2 (g) + 6CO_2 (g) → 6H_2O (l) + Energy$$

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

Ans. In a decomposition reaction, a single compound breaks down to produce two or more substances whereas in combination reaction two or more substances combine to form new compound. Hence both are opposite to each other.

$$\text{Example: } 2H_2O(l) \xrightleftarrows[Combination]{Decomposition} 2H_2(g) + O_2(g)$$

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

Ans. $$\underset{\text{Calcium carbonate}}{CaCO_3(s)}\xrightarrow{Heat}CaO(s) + CO_2(g) \\ \underset{\text{Silver bromide}}{2AgBr(s)} \xrightarrow{Heat} 2Ag(s) + Br_2(g) \\ \underset{\text{Water}}{2H_2O(l)} \xrightarrow{Electricity} \space 2H_2(g) + O_2(g)$$

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

Ans. In displacement reactions, a more reactive metal displaces a less reactive metal from its solution.
For example, Fe (s) + CuSO4 (aq) → Cu (s) + FeSO4 (aq)
This is a displacement reaction where iron displaces copper from its solution. In double displacement reactions, two reactants in solution exchange their ions to form new compounds. For example,
AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq)
This is a double displacement reaction where silver nitrate and sodium chloride exchange Cl and NO3 ions between them to form AgCl and NaNO3.

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

Ans. The reaction involve is displacement reaction. In this reaction Cu replaces Ag from its solution.

$$\underset{\text{Silver nitrate}}{2AgNO_3(aq)}+ \underset{\text{Copper}}{Cu(s)}\rightarrow \underset{\text{Copper nitrate}}{Cu(NO_3)_2} + \underset{{Silver}}{2Ag(s)}$$

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

Ans. A chemical reaction, in which an insoluble solid is formed from the solution. The solid formed is called precipitate and reaction is known as precipitation reaction.

$$Example: Na_2CO_3 (aq) + CaCl_{2} (aq) → \underset{{precipitate}}{CaCO_3 (s)} + 2NaCl (aq)$$

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

Ans. (a) Oxidation: The addition of oxygen to a substance is called oxidation.
Example:
(i) S(s) + O2 (g) → SO2 (g)
(ii) 2Mg(s) + O2 (g) → 2MgO (s)
(b) Reduction: The removal of oxygen from a substance is called reduction.
Example:
(i) CuO + H2 Heat → Cu + H2O
Here, copper oxide is being reduced to copper because oxygen gets removed from copper oxide.
(ii) ZnO + C → Zn + CO
Here, zinc oxide is being reduced to zinc because oxygen gets removed from zinc oxide.

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.

Ans. The shiny elements are metal. The metal with brown colour is Cu so the element X is copper (Cu).
When copper reacts with oxygen it forms a black coloured compound, copper oxide.

$$\underset{\text{Copper (Brown)}}{2Cu} + \underset{\text{Oxygen}}{O_2} \rightarrow \underset{\text{Copper Oxide (Black)}}{2CuO}$$

18. Why do we apply paint on iron articles ?

Ans. Paint is applied on iron to prevent rusting which is caused by the oxygen and moisture present in air. Paint acts as a barrier and does not allow oxygen and moisture present in air to react with iron, hence saves iron from rusting.

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

Ans. To keep food items fresh and save from getting oxidised, food items are flushed with nitrogen because nitrogen is a non-reactive gas that does not combine with oil and fats.

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

Ans. (a) Corrosion : It is the process in which metals are eaten up gradually by the action of air, moisture or a chemical (such as an acid) on their surface.
Example: When iron is exposed to moist air for a long period of time, its surface acquires a coating of a brown, flaky substance called rust. Rust is mainly hydrated iron (III) oxide [Fe2O3.xH2O].

$$4Fe + 3O_2 + 2x H_2O → \underset{\text{Hydrated iron (III) oxide (Rust)}}{2Fe_2O_3.xH_2O}$$

(b) Rancidity: The process of oxidation of fats and oils which can be noticed by change in colour, smell and taste is known as rancidity.
Example: When butter is kept in open atmosphere then its smell and taste changes which results in rancidity.