Assertion & Reason Type Questions: Chemical Reactions & Equations - 1 - Science Class 10

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

Question 1: 
Assertion (A): Carbon dioxide turns lime water milky. 
Reason (R): Carbon dioxide sullies the water.
(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.

Correct Answer is Option (c)

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

Question 2:

Assertion (A): After white washing the walls, a shiny white finish on walls is obtained after two to three days.
Reason (R): Calcium oxide reacts with carbon dioxide to form calcium hydrogen carbonate which gives a shiny white finish.
(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.

Correct Answer is Option (c)

Calcium hydroxide is present in whitewash. It reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of white washing. Hence the shiny white finish appears after two to three days on the walls.

Question 3:
Assertion (A): A chemical reaction becomes faster at higher temperatures. 
Reason (R): At higher temperatures, molecular motion becomes more rapid.
(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.

Correct Answer is Option (a)

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

Question 4:
Assertion (A): To dilute sulphuric acid, acid is added to water and not water to acid.
Reason (R): Specific heat of water is quite large.
(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.

Correct Answer is Option (a)
The mixing of water to an acid is highly exothermic in nature. If water is added to an acid it produces a very large amount of heat which can break the container and sometimes even causes burning. So it is advised to add concentrated acid to water in a very slow manner.

Question 5:
Assertion (A): Sodium metal is stored under kerosene.
Reason (R): Metallic sodium melts when exposed to air.
(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.

Correct Answer is Option (c)
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 air and form sodium hydroxide. This is a strongly exothermic reaction, and a lot of heat is generated.

Question 6:
Assertion: Corrosion of iron is commonly known as rusting. 
Reason: Corrosion of iron occurs in the presence of water and air.
(a) If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
(c) If Assertion is correct but Reason is incorrect.
(d) If Assertion is incorrect but Reason is correct.

Correct Answer is Option (b)
Iron when exposed to moist air for a long time acquires a coating of a brown flaky substance called rust. Thus, both statements are true but independent of each other.

Question 7:
Assertion: A reducing agent is a substance which can either accept electrons.
Reason: A substance which helps in oxidation is known as a reducing agent.
(a) If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
(c) If Assertion is correct but Reason is incorrect.
(d) If Assertion is incorrect but Reason is correct.

Correct Answer is Option (d)
A reducing agent is a substance which oxidizes itself but reduces others i.e., loses electrons.

Question 8:
Assertion (A): Carbon dioxide turns lime water milky.
Reason (R): Carbon dioxide sullies the water.
(a) If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
(c) If Assertion is correct but Reason is incorrect.
(d) If Assertion is incorrect but Reason is correct.

Correct Answer is Option (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.

Question 9:
Assertion (A): Sodium metal is stored under Kerosene.
Reason (R): Metallic sodium melts when exposed to air.
(a) If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
(c) If Assertion is correct but Reason is incorrect.
(d) If Assertion is incorrect but Reason is correct.

Correct Answer is Option (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 present. If this happens, it will react with the moisture present in air and form sodium hydroxide. This is a strongly exothermic reaction, and a lot of heat is generated.

Question 10:
Assertion(A): Calcium corbonate when heated gives calcium oxide and water.
Reason (R): On heating calcium carbonate, decomposition reaction takes place.
(a) If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
(c) If Assertion is correct but Reason is incorrect.
(d) If Assertion is incorrect but Reason is correct.

Correct Answer is Option (d)
On heating calcium carbonate, calcium oxide and carbon dioxide are formed.

Question 11:
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.
(a) If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
(c) If Assertion is correct but Reason is incorrect.
(d) If Assertion is incorrect but Reason is correct.

Correct Answer is Option (a)
Silver chloride turns grey when kept in sunlight. This is because a photolytic decomposition reaction takes place where the white silver chloride decomposes in the presence of sunlight to form white silver and chlorine gas which escapes.

Decomposition of silver chloride 

Question 12:
Assertion(A): In a reaction of copper with oxygen, copper serves as a reducing agent. 
Reason (R): The substance which gains oxygen in a chemical reaction is a reducing agents. 
(a) If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
(c) If Assertion is correct but Reason is incorrect.
(d) If Assertion is incorrect but Reason is correct.

Correct Answer is Option (a)
Both A and R are true and R is the correct explanation of A.

Question 13: 
Assertion: Fe₂O₃ + 2Al → Al₂O₃ + 2Fe 
The above chemical equation is an example of displacement reaction. 
Reason: Aluminium  being more reactive than iron, displaces Fe from its oxide
(a) If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
(c) If Assertion is correct but Reason is incorrect.
(d) If Assertion is incorrect but Reason is correct.

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

Question 14:
Assertion: Quicklime reacts vigorously with water releasing a large amount of heat. Reason: The above chemical reaction is an exothermic reaction.
(a) If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
(c) If Assertion is correct but Reason is incorrect.
(d) If Assertion is incorrect but Reason is correct.

Correct Answer is Option (a)
If both Assertion and Reason are true and Reason is the correct explanation of Assertion. Because in exothermic reactions, hear is released along with the formation of products.

Question 15: 
Assertion: Photosynthesis is considered as an endothermic reaction.
Reason: Energy gets released in the process of photosynthesis.
(a) If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
(c) If Assertion is correct but Reason is incorrect.
(d) If Assertion is incorrect but Reason is correct.

Correct Answer is Option (c)
If Assertion is true, but Reason is false. Photosynthesis is considered as an endothermic reaction because energy in the form of sunlight is absorbed by the green plants.