Question

Assertion (A): Rainwater usually has a pH of 5.6.
Reason (R): This is due to the presence of ${{H}_{2}}S{{O}_{4}}\text{ and HN}{{\text{O}}_{3}}$ produced from oxides of sulphur and nitrogen.
[A] Both A and R is true and R is the correct explanation of A.
[B] Both A and R are true but R is the not correct explanation of A.
[C] A is true R is false.
[D] A is false and R is true.

Answer 1

Hint: Rainwater is naturally acidified due to presence of atmospheric gases in the troposphere. These gases cause natural acidity of rainfall but humans produce these gases in higher concentration by burning fossil fuels and cause un-natural acidity of rainwater.

Complete step by step answer:
We know pH is a scale for the measurement of acidity or basicity. A pH of 7 means it is neutral and lower than 7 proves that the liquid is acidic and a pH above 7 means the liquid is alkaline.
The pH of fresh, pure water is 7 but the pH of unpolluted, natural rainwater is 5.5 - 5.6. This means that natural and unpolluted rainwater is acidic.

Let us discuss the reason for the acidity of unpolluted rainwater.
We know that our atmosphere is surrounded by several gases. The main component of this mixture of gases is carbon dioxide, nitrogen etc. The acidity of unpolluted rainwater is due to presence of $C{{O}_{2}},NO\text{ and S}{{\text{O}}_{2}}$ in the troposphere.
As carbon dioxide is present in highest concentration therefore it contributes the most to the natural acidity. It is the primary source of acidity of pure rainwater.
Carbon dioxide reacts with water to form carbonic acid which dissociates to give hydrogen ion thus making it acidic.
     \[\begin{align}
 & C{{O}_{2}}+{{H}_{2}}O\to {{H}_{2}}C{{O}_{3}} \\
 & {{H}_{2}}C{{O}_{3}}\to {{H}^{+}}+HC{{O}_{3}}^{-} \\
\end{align}\]

Also, nitric oxide which is formed during lightning by reaction of atmospheric nitrogen and oxygen contributes to acidity of rainwater. NO is oxidised to nitrogen dioxide in air which reacts with water and gives nitric acid, thus making rain water acidic.
     \[\begin{align}
 & {{N}_{2}}(g)+{{O}_{2}}(g)\xrightarrow{lightning}2NO(g) \\
 & 2NO(g)+\dfrac{1}{2}{{O}_{2}}(g)\to N{{O}_{2}}(g) \\
 & 3N{{O}_{2}}(g)+{{H}_{2}}O(aq.)\to 2HN{{O}_{3}}(aq.)+NO(g) \\
\end{align}\]
Therefore, the assertion that rainwater usually has a pH of 5.6 is correct.

Now let us see the reason - this is due to the presence of ${{H}_{2}}S{{O}_{4}}\text{ and HN}{{\text{O}}_{3}}$ produced from oxides of sulphur and nitrogen.
As we can see from the above discussion that the majority of acidity of natural rainfall is due to carbon dioxide therefore the reason for the assertion is incorrect.
So, the correct answer is “Option C”.

Note: We cause many combustion processes that increase the acidity of rain water. Internal combustion of automobiles increases the concentration of nitric oxide in the atmosphere un-naturally and thus contributes to man-made acidity of rainfall.
Also, burning of sulphur containing fossil-fuel cause’s reaction of sulphur with atmospheric oxygen and forms sulphur dioxide which forms sulphuric acid with water.
Formation of nitric acid and sulphuric acid decreases pH of rainwater to 4 thus making it highly acidic.