Question

The pH of rainwater, is approximately?
A. 6.5
B. 7.5
C. 5.6
D. 7.0

Answer 1

Hint: The pH scale measures how acidic or basic a solution is. The scale has values from 0 to 14. Less than 7 pH is acidic and more than 7 pH are basic. Pure water has pH of 7. And this value is considered as neutral, neither acidic nor basic.

Complete answer:
Air is the mixture of gases that form the atmosphere of the earth. Dry air is composed of nitrogen, oxygen, argon, carbon dioxide and some amount of other gases.
Earth’s atmosphere contains many layers. Troposphere, stratosphere, mesosphere, thermosphere, and ionosphere. The troposphere starts at the earth’s surface and extends to 8 to 14 kilometers high. Almost all weather is in this region. That means clouds are present at the troposphere.
The rain water coming from the clouds is acidic in nature. The acidity of rain water comes from the natural presence of three main gases like carbon dioxide, nitric oxide, and sulphur dioxide found in the troposphere. As out of these three gases the concentration of carbon dioxide is the greatest and therefore carbon dioxide contributes the most in the natural acidity of rainwater.
Carbon dioxide reacts with water and form carbonic acid as
 $C{O_2} + {H_2}O \to {H_2}C{O_3}$
When the carbonic acid dissociates, it produces hydrogen ion and hydrogen carbonate ion as
${H_2}C{O_3} \to {H^ + } + HC{O_3}^ - $
Since carbonic acid is producing hydrogen ions on dissociation, therefore it is classified as an acid. And it will lower the pH of the rain water.
Nitric oxide also contributes to the acidity of rain water. Nitric oxide forms during lightning, by the reaction between oxygen and nitrogen (both are abundant in the air).
${N_2} + {O_2}\xrightarrow{{lightening}}2NO$
In air the nitric oxide then oxidizes to nitrogen dioxide.
$2NO + {O_2} \to 2N{O_2}$
This nitrogen dioxide when it reacts with water droplets forms nitric acid. $3N{O_2} + {H_2}O \to 2HN{O_3} + NO$
This acid when dissociates will produce hydrogen ion and nitrate ion and hence contributes to acidity of rain water. Similarly, when sulphur dioxide reacts with water it produces sulphuric acid. This all happens in clouds. When the water falls to the ground in the form of rain it is acidic in nature. The pH of this natural rain is approximately 5.6.

Hence the correct option is C. 5.6

Note:
Due to increased human industrial activity, acid forming compound’s concentration increased in the atmosphere than the natural source. And therefore the pH of rain decreased further. When the rain is too acidic, it can cause problems ranging from killing freshwater fishes, damaging crops, and eroding buildings and monuments.