Chemical Reactions and Equations

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Assertion and Reason for Ch 1 Chemical Reactions and Equations Class 10 Science

Important Questions

A

Directions: In the following questions, a statement of assertion (A) is followed by a statement of reason (R). Mark the correct choice as: 

Answer

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion  (A). 

(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A). 

(c) Assertion (A) is true but reason (R) is false.

(d) Assertion (A) is false but reason (R) is true.

1

Assertion (A): Carbon dioxide turns lime water milky.

Reason (R): Carbon dioxide sullies the water. 

Answer

(c) Assertion (A) is true but reason (R) is false.

Carbon dioxide reacts with lime water (calcium hydroxide) to form milky precipitate of calcium carbonate. 

2

Assertion (A): A chemical reaction becomes faster at higher temperatures.

Reason (R): At higher temperatures, molecular motion becomes more rapid. 

Answer

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

A chemical reaction becomes faster at higher temperatures because, at high temperature, the movement of particles is greater.

3

Assertion (A): The balancing of chemical equations is based on the law of conservation of mass.

Reason (R): Total mass of reactants is equal to the total mass of products.

Answer

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A). 

4

Assertion (A): Fe2O3 + 2Al → Al2O3 + 2Fe

The above chemical equation is an example of displacement reaction.

Reason (R): Aluminium being more reactive than iron, displaces Fe from its oxide.

Answer

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

Fe2O3 + 2Al → Al2O3 + 2Fe
It is а displacement reaction. Here, a highly reactive element (Al) displaces Fe from Fe2O3.

5

Assertion (A): In the following chemical equation,

CuO (s) + Zn(s) → ZnO (s) + Cu (s)

Zinc is getting oxidised and copper oxide is getting reduced.

Reason (R): The process in which oxygen is added to a substance is called oxidation whereas the process in which oxygen is removed from a substance is called reduction.

Answer

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

The reaction involves both oxidation and reduction in which, CuO is reduced to Cu and Zn is oxidised to ZnO.

6

Assertion (A): Sodium metal is stored under Kerosene.

Reason (R): Metallic sodium melts when exposed to air. 

Answer

(c) Assertion (A) is true but reason (R) is false.

Sodium is a very reactive metal.  It is kept in kerosene to prevent it from coming in contact with oxygen and moisture. If this happens, it will react with the moisture present in the air and form sodium hydroxide. This is a strongly exothermic reaction, and a lot of heat is generated.

7

Assertion (A): Calcium corbonate when heated gives calcium oxide and water.

Reason (R): On heating calcium carbonate, decomposition reaction takes place.

Answer

(d) Assertion (A) is false but reason (R) is true.

8

Assertion (A): A reducing agent is a substance which can either accept electrons.

Reason (R): A substance which helps in oxidation is known as a reducing agent.

Answer

(d) Assertion (A) is false but reason (R) is true.

A reducing agent is a substance which oxidizes itself but reduces others i.e., looses electrons.

9

Assertion (A): To dilute sulphuric acid, acid is added to water and not water to acid.

Reason (R): The specific heat of water is quite large. 

Answer

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

Mixing water to an acid is highly exothermic in nature. If water is added to an acid, it produces very large amount of heat which can break the container and some times even cause burnings. So, it is advised to add concentrated acid to water very slow manner.

10

Assertion (A): Pungent-smelling gas is produced when sulphur burns in the air.

Reason (R): Sulphur trioxide is formed on the reaction of sulphur with oxygen.

Answer

(c) Assertion (A) is true but reason (R) is false.

Sulphur reacts with oxygen to form sulphur dioxide.

11

Assertion (A): Corrosion of iron is commonly known as rusting.

Reason (R): Corrosion of iron occurs in presence of water and air.

Answer

(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).

Corrosion occurs due to oxidation of iron.

12

Assertion (A): Quicklime reacts vigorously with water releasing a large amount of heat.

Reason (R): The above chemical reaction is an exothermic reaction.

Answer

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

In exothermic reactions, heat is released along with the formation of products.

13

Assertion (A): Brown fumes are produced when lead nitrate is heated.

Reason (R): Nitrogen dioxide gas is produced as a by-product due to the decomposition of lead nitrate.

Answer

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

14

Assertion (A): Photosynthesis is considered as an endothermic reaction.

Reason (R): Energy gets released in the process of photosynthesis.

Answer

(c) Assertion (A) is true but reason (R) is false.

Photosynthesis is considered as an endothermic reaction because energy in the form of sunlight is absorbed by the green plants.

15

Assertion (A): When HCl is added to zinc granules, a chemical reaction occurs.

Reason (R): Evolution of a gas and change in colour indicate that the chemical reaction is taking place.

Answer

(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).

The reaction is Zn + HCl → ZnCl2 + H2. Only the evolution of gas takes place. There is no change in colour.

16

Assertion (A): Calcium carbonate when heated gives calcium oxide and water.

Reason (R): On heating calcium carbonate, decomposition reaction takes place.

Answer

(d) Assertion (A) is false but reason (R) is true.

Calcium carbonate on heating gives calcium oxide and carbon dioxide gas.

17

Assertion (A): After white washing the walls, a shiny white finish on the walls is obtained after two to three days.

Reason (R): Calcium oxide reacts with carbon dioxide to form calcium hydrogen carbonate which gives shiny white finish.

Answer

(c) Assertion (A) is true but reason (R) is false.

Calcium oxide reacts with carbon dioxide to form calcium carbonate which gives white shiny finish to walls.

18

Assertion (A): The reaction

MnO2 + 4HCl → MnCl2 + 2H2O + Cl2

is an example of a redox reaction.

Reason (R): In this reaction, HCl is reduced to Cl2 whereas MnO2 is oxidised to MnCl2.

Answer

(c) Assertion (A) is true but reason (R) is false.

MnO2 is reduced to MnCl2 and HCl is oxidised to Cl2.

19

Assertion(A): Following is a balanced chemical equation for the action of steam on iron:

3Fe + 4H2O → Fe3O4 + 4H2

Reason (R): The law of conservation of mass holds good for a chemical equation.

Answer

(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).

It is a balanced chemical equation because the number of atoms of Fe, H and O are equal on both the sides.

20

Assertion (A): White silver chloride turns grey in sunlight.

Reason (R): Decomposition of silver chloride in presence of sunlight takes place to form silver metal and chlorine gas.

Answer

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A). 

21

Assertion (A): In the reaction,

Zn (s) + 2H+ (aq) → Zn2+ (aq) + H2(g),

Zinc acts as an oxidising agent and H+ acts as a reducing agent.

Reason (R): An oxidising agent accepts electrons while a reducing agent loses electrons.

Answer

(d) Assertion (A) is false but reason (R) is true.

Zn (s) + 2H+ (aq) → Zn2+ (aq) + H2(g)

In this reaction, zinc loses electrons so it is a reducing agent, while H+ gains electrons, so it is an oxidising agent.

22

Assertion (A): The reaction during which hydrogen is lost is called oxidation reaction.

Reason (R): Reducing agent removes hydrogen from another substance.

Answer

(c) Assertion (A) is true but reason (R) is false.

Reducing agent gives hydrogen to another substance.

23

Assertion (A): In a balanced chemical equation, the total mass of the reactants is equal to the total mass of the products.

Reason (R): Mass can neither be created nor destroyed during a chemical change.

Answer

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

This is according to the law of conservation of mass.

24

Assertion (A): When calcium carbonate is heated, it decomposes to give calcium oxide and carbon dioxide.

Reason (R): The decomposition reaction takes place on the application of heat, therefore, it is an endothermic reaction.

Answer

(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).

CaCO3 on heating gives CO2 and CaO.

25

Assertion (A): AgBr is used on photographic and X-ray film.

Reason (R): AgBr is photosensitive and changes to Ag and bromine in presence of sunlight and undergoes a decomposition reaction.

Answer

(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

AgBr is a chemical compound. It is widely used in photography as photographic emulsions.

2AgBr → 2Ag + Br2

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