Ncert Exercises & Solutions

Given below are two statements:

Assertion (A):	The pH of acid rain is less than 5.6.
Reason (R):	Carbon dioxide present in the atmosphere dissolves in rainwater and forms carbonic acid.

1.	Both (A) and (R) are true and (R) is the correct explanation of (A).
2.	Both (A) and (R) are true but (R) is not the correct explanation of (A).
3.	(A) is true but (R) is false.
4.	(A) is false but (R) is true.

Hint: Acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air.

Normally rainwater has a pH of 5.6 due to the presence of H⁺ ions formed by the reaction of rainwater with carbon dioxide

present in the atmosphere.

H_{2} O (l) + {CO}_{2} (g) \to H_{2} {CO}_{3} (aq)

H_{2} {CO}_{3} (aq) ⇌ H^{+} (aq) + {HCO}_{3}^{-} (aq)

When the pH of rainwater drops below 5.6 it is called acid rain.

When rain combines with sulfur dioxide or nitrogen oxide produced from power plants and automobiles, the rain becomes much more acidic. Typical acid rain has a pH value of 4.0.

Hence, Both assertion and reason are true and the reason is not the correct explanation of assertion.

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