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

Question 1. Which of the statements about the reaction below are incorrect?

$2 \mathrm{PbO}(s)+\mathrm{C}(s) \rightarrow 2 \mathrm{~Pb}(\mathrm{~s})+\mathrm{CO}_2(g)$

(a) Lead is getting reduced.

(b) Carbon dioxide is getting oxidized.

(c) Carbon is getting oxidized.

(d) Lead oxide is getting reduced.

(i) (a) and (b)

(ii) (a) and (c)

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

(iv) all

Answer:

$2 \mathrm{PbO}(s)+\mathrm{C}(s) \rightarrow 2 \mathrm{~Pb}(s)+\mathrm{CO}_2(g)$

In the above reaction, PbO reduces to Pb and C(carbon) gets oxidized to carbon dioxide.

Hence, statements (a) and (b) are correct.

Thus, option (i) is correct.

Question 2. $\mathrm{Fe}_2 \mathrm{O}_3+2 \mathrm{Al} \rightarrow \mathrm{Al}_2 \mathrm{O}_3+2 \mathrm{Fe}$

The above reaction is an example of a

(a) combination reaction.

(b) double displacement reaction.

(c) decomposition reaction.

(d) displacement reaction

Answer :

$\mathrm{Fe}_2 \mathrm{O}_3+2 \mathrm{Al} \rightarrow \mathrm{Al}_2 \mathrm{O}_3+2 \mathrm{Fe}$

The above reaction is an example of a displacement reaction.

Hence, the correct answer is option (d).

Question 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.

Answer:

When dilute hydrochloric acid is added to iron fillings, then hydrogen gas and iron chloride are produced.

The reaction is given as :

$\mathrm{Fe}(s)+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{FeCl}_2(\mathrm{aq})+\mathrm{H}_2(g)$

Thus, option (a) is correct.

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

Answer:

The chemical equation which has an equal number of atoms of all elements on both sides of the reaction is known as a balanced chemical equation.

The law of conservation of mass states that mass can neither be created nor be destroyed so chemical equations should be balanced.

Question 5. (a) Translate the following statements into chemical equations and then balance them.

Hydrogen gas combines with nitrogen to form ammonia.

Answer:

Hydrogen gas combines with nitrogen to form ammonia can be written as :

$3 \mathrm{H}_2(g)+\mathrm{N}_2(g) \rightarrow 2 \mathrm{NH}_3$

Question 5. (b) Hydrogen sulphide gas burns in the air to give water and sulfur dioxide.

Answer:

Hydrogen sulphide gas burns in air to give water and sulfur dioxide can be written as :

$2 \mathrm{H}_2 \mathrm{~S}(\mathrm{~g})+3 \mathrm{O}_2(\mathrm{~g}) \rightarrow 2 \mathrm{SO}_2(\mathrm{~g})+2 \mathrm{H}_2 \mathrm{O}(\mathrm{l})$

Question 5. (c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

Answer:

Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate can be written as :

$3 \mathrm{BaCl}_2(\mathrm{aq})+\mathrm{Al}_2\left(\mathrm{SO}_4\right)_3(\mathrm{aq}) \rightarrow 2 \mathrm{AlCl}_3(\mathrm{aq})+3 \mathrm{BaSO}_4(s)$

Question 5. (d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Answer: Potassium metal reacts with water to give potassium hydroxide and hydrogen gas is given by :

$2 \mathrm{~K}(s)+2 \mathrm{H}_2 \mathrm{O}(l) \rightarrow 2 \mathrm{KOH}(\mathrm{aq})+\mathrm{H}_2$

Question 6. (a) Balance the following chemical equations.

(a) Balance the following chemical equations.

$\mathrm{HNO}_3+\mathrm{Ca}(\mathrm{OH})_2 \rightarrow \mathrm{Ca}\left(\mathrm{NO}_3\right)_2+\mathrm{H}_2 \mathrm{O}$

Answer:

The balanced chemical equation is given as :

$2 \mathrm{HNO}_3+\mathrm{Ca}(\mathrm{OH})_2 \rightarrow \mathrm{Ca}\left(\mathrm{NO}_3\right)_2+2 \mathrm{H}_2 \mathrm{O}$

Question 6. (b)Balance the following chemical equations.

$\mathrm{NaOH}+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{Na}_2 \mathrm{SO}_4+\mathrm{H}_2 \mathrm{O}$

Answer:

The balanced chemical equation is given as :

$2 \mathrm{NaOH}+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{Na}_2 \mathrm{SO}_4+2 \mathrm{H}_2 \mathrm{O}$

Question 6. (c) Balance the following chemical equations.

$\mathrm{NaCl}+\mathrm{AgNO}_3 \rightarrow \mathrm{AgCl}+\mathrm{NaNO}_3$

Answer:

The balanced chemical equation is given as :

$\mathrm{NaCl}+\mathrm{AgNO}_3 \rightarrow \mathrm{AgCl}+\mathrm{NaNO}_3$

Question 6. (d) Balance the following chemical equations.

$\mathrm{BaCl}_2+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{BaSO}_4+\mathrm{HCl}$

Answer:

A balanced chemical equation is given as :

$\mathrm{BaCl}_2+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{BaSO}_4+2 \mathrm{HCl}$

Question 7. (a) Write the balanced chemical equations for the following reactions.

(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Answer:

Calcium hydroxide + Carbon dioxide →Calcium carbonate + Water

The balanced chemical equation for the above reaction is given as :

$\mathrm{Ca}(\mathrm{OH})_2+\mathrm{CO}_2 \rightarrow \mathrm{CaCO}_3+\mathrm{H}_2 \mathrm{O}$

Question 7. (b) Zinc + Silver nitrate → Zinc nitrate + Silver

Answer:

Zinc + Silver nitrate → Zinc nitrate + Silver

The balanced chemical equation for the above reaction is given as :

$\mathrm{Zn}+2 \mathrm{AgNO}_3 \rightarrow \mathrm{Zn}\left(\mathrm{NO}_3\right)_2+2 \mathrm{Ag}$

Question 7. (c)Aluminium + Copper chloride → Aluminium chloride + Copper

Answer:

Aluminium + Copper chloride → Aluminium chloride + Copper

The balanced chemical equation for the above reaction is given as :

$2 \mathrm{Al}+3 \mathrm{CuCl}_2 \rightarrow 2 \mathrm{AlCl}_2+3 \mathrm{Cu}$

Question 7. (d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Answer:

Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

The balanced chemical equation for the above reaction is given as :

$\mathrm{BaCl}_2+\mathrm{K}_2 \mathrm{SO}_4 \rightarrow \mathrm{BaSO}_4+2 \mathrm{KCl}$

Question 8. (a) 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)

Answer:

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

The balanced chemical equation for the above reaction is given as :

$2 K B r(a q)+B a I_2(a q) \rightarrow 2 K I(a q)+B a B r_2$

It is a double displacement reaction.

Question 8. (b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)

Answer:

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

The balanced chemical equation for the above reaction is given by :

$\mathrm{ZnCO}_3(s) \rightarrow \mathrm{ZnO}(\mathrm{s})+\mathrm{CO}_2(\mathrm{~g})$

It is a decomposition reaction.

Question 8. (c)Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)

Answer:

Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)

The balanced chemical equation for the above reaction is given by :

$\mathrm{H}_2(\mathrm{~g})+\mathrm{Cl}_2(\mathrm{~g}) \rightarrow 2 \mathrm{HCl}(\mathrm{g})$

It is a combination reaction.

Question 8. (d)Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)

Answer:

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

The balanced chemical equation for the above reaction is given by :

$\mathrm{Mg}(s)+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{MgCl}_2(\mathrm{aq})+\mathrm{H}_2(g)$

It is a displacement reaction.

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

Answer:

Exothermic reactions: Reactions in which heat is given out along with the products are called exothermic reactions.

Example : $\mathrm{CH}_4(\mathrm{~g})+2 \mathrm{O}_2(\mathrm{~g}) \rightarrow \mathrm{CO}_2(\mathrm{~g})+2 \mathrm{H}_2 \mathrm{O}(\mathrm{g})$

Endothermic reactions: Reactions in which energy is absorbed are known as endothermic reactions.

Example: The process of photosynthesis.

$6 \mathrm{CO}_2(\mathrm{~g})+6 \mathrm{H}_2 \mathrm{O}(\mathrm{g}) \rightarrow \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6(\mathrm{aq})+6 \mathrm{O}_2(\mathrm{~g})$

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

Answer:

We know that energy is required to support life. We get energy from the food we eat.

The large molecules of food are broken into simpler substances like glucose during digestion.

Glucose and oxygen react to provide energy to the body. This reaction is a combination reaction named respiration. In this whole process, energy is released, so respiration is considered an exothermic reaction.

$\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6(\mathrm{aq})+6 \mathrm{O}_2 \rightarrow 6 \mathrm{CO}_2(\mathrm{~g})+6 \mathrm{H}_2 \mathrm{O}(\mathrm{l})+$ Energy

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

Answer:

In decomposition reactions, we can observe that a single reactant breaks down to give simpler products. This reaction is a source of energy. Whereas, in a combination reaction, two or more substances combine to give a product and energy is released in this reaction.

Hence, decomposition reactions are called the opposite of combination reactions.

Example : decomposition reaction : $2 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightarrow 2 \mathrm{H}_2(\mathrm{~g})+\mathrm{O}_2(\mathrm{~g})$

And the combination reaction : $2 \mathrm{H}_2(g)+\mathrm{O}_2(g) \rightarrow 2 \mathrm{H}_2 \mathrm{O}(\mathrm{l})+$ Energy

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

Answer:

The decomposition reaction by heat:

$
2 \mathrm{FeSO}_4(s) \rightarrow \mathrm{Fe}_2 \mathrm{O}_3(s)+\mathrm{SO}_2(g)+\mathrm{SO}_3
$

The decomposition reaction by light :

$
2 \mathrm{AgCl}(s) \rightarrow 2 \mathrm{Ag}(s)+\mathrm{Cl}_2(g)
$

The decomposition reaction by electricity :

$
2 \mathrm{Al}_2 \mathrm{O}_3(\mathrm{aq}) \rightarrow 4 \mathrm{Al}(\mathrm{~s})+3 \mathrm{O}_2(\mathrm{~g})
$

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

Answer:

When an element displaces another element from its compound, a displacement reaction occurs.

Example : $\mathrm{Fe}(\mathrm{s})+\mathrm{CuSO}_4(a q) \rightarrow \mathrm{FeSO}_4(a q)+\mathrm{Cu}(s)$
Two different atoms or groups of atoms (ions) are exchanged in a double displacement reaction.

Example : $\mathrm{Na}_2 \mathrm{SO}_4(a q)+\mathrm{BaCl}_2(a q) \rightarrow \mathrm{BaSO}_4(s)+2 \mathrm{NaCl}(\mathrm{aq})$

Question 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:

In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. The reaction involved can be written as :

$2 \mathrm{AgNO}_3(\mathrm{aq})+\mathrm{Cu}(s) \rightarrow \mathrm{Cu}\left(\mathrm{NO}_3\right)_2(\mathrm{aq})+2 \mathrm{Ag}(s)$

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

Answer:

Any reaction that produces a precipitate is called a precipitation reaction.
Example : $\mathrm{Na}_2 \mathrm{CO}_3(\mathrm{aq})+\mathrm{CaCl}_2(\mathrm{aq}) \rightarrow \mathrm{CaCO}_3(s)+2 \mathrm{NaCl}(\mathrm{aq})$
Here, we have $\mathrm{CaCO}_3$ as precipitate so it is a precipitation reaction.

Question 16. (b) Reduction

Answer:

The reaction in which the loss of oxygen or gain of hydrogen can be observed is known as the reduction reaction.

Example : $\mathrm{CuO}+\mathrm{H}_2 \rightarrow \mathrm{Cu}+\mathrm{H}_2 \mathrm{O}$

$
\mathrm{ZnO}+\mathrm{C} \rightarrow \mathrm{Zn}+\mathrm{CO}
$

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

Answer:

A shiny brown-colored element is copper (Cu) and on heating, in the air, it becomes black in colour because of the formation of copper oxide(CuO).

$2 \mathrm{Cu}+\mathrm{O}_2 \rightarrow 2 \mathrm{CuO}$

Question 18. Why do we apply paint on iron articles?

Answer:

To prevent iron from rusting, paint is applied to iron articles. After applying paint iron articles are not in contact with moisture and air and hence rusting is prevented.

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

Answer:

We know that nitrogen is an inert gas and does not react with oil and fat-containing food. Whereas, other gases like oxygen react with the oil and fat-containing food and make them rancid. Hence, to remove oxygen and prevent food from acidic food items is flushed with nitrogen.

Question 20. Explain the following terms with one example each.

(a) Corrosion

Answer:

Corrosion is a process in which metals deteriorate due to chemical reactions with moisture, air, and chemicals. The rusting of iron is a major example of corrosion. Iron corrodes in the presence of moisture and air.

$4 \mathrm{Fe}+3 \mathrm{O}_2+\mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{Fe}_2 \mathrm{O}_3 \cdot \mathrm{H}_2 \mathrm{O}$

Question 20. (b) Rancidity

Answer:

The process of oxidation of fats and oils, which can be noticed by a change in color, smell, and taste, is known as rancidity.

Example: When butter is kept in an open atmosphere,e its smell and taste change which results in rancidity.