Question

Rain water has a pH of $5.6$due to the presence of:

Answer 1

Rainwater is acidic in nature:

• The pH of any solution determines the relative strength of the acidic or basic nature of any solution.
• If a solution has a higher concentration of ${\mathrm{H}}^{+}$this indicates that the solution has less pH and it is acidic in nature.
• A pH of $5.6$ is not considered strongly acidic rather it is more towards the neutral side and it is because of the presence of ${\mathrm{H}}^{+}$ions.
• Rain water has a pH of $5.6$ due to the presence of ${\mathrm{H}}^{+}$ions formed by the reaction of rainwater with carbon dioxide.
• The acidity of unpolluted rainwater is due to its presence of $C{O}_2,NO\text{and S}{\text{O}}_2$ in the troposphere.
• As carbon dioxide is present in the 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 ions thus making it acidic.
  $$\begin{array}{rl}& C{O}_2+H_{2}O\to H_{2}C{O}_3\\ & H_{2}C{O}_3\to H^{+}+HC{O_3}^{-}\end{array}$$
• Also, Nitric oxide is oxidized to nitrogen dioxide in the air which reacts with water and gives nitric acid, thus making rainwater acidic.
  $$\begin{array}{rl}& N_2(g)+O_2(g)\stackrel{lightning}{\to }2NO(g)\\ & 2NO(g)+{\displaystyle \frac{1}{2}}{O}_2(g)\to N{O}_2(g)\\ & 3N{O}_2(g)+H_{2}O(aq.)\to 2HN{O}_3(aq.)+NO(g)\end{array}$$
• However, when pH of the rainwater drops below $5-6,$ then it is called acid rain.

Thus, rainwater has a pH of $5.6$due to the presence of:$C{O}_2,NO\text{and S}{\text{O}}_2$ in the troposphere.