Question

What possible impurities does rainwater contain?

Answer 1

• Rainwater may contain impurities in the form of dust particles and dissolved gases like Oxygen, Nitrogen and Carbon dioxide.
• These gaseous impurities tend to mix with rainwater and may form acid rain while raining on earth initially but washes away after few showers.
• Thus, making rainwater not so poisonous.
• Acid rain occurs when the pH of rain water falls between 2.0 to 5.5. This is due to the presence of Sulphurous acid, Sulphuric acid, Nitric acid and Carbonic acid in rain water.
• Sulphur dioxide accounts for most of the acid rain problems.

(a) When Sulphur dioxide dissolves in water, Sulphurous acid is formed.
$\underset{\mathrm{Sulphur}\mathrm{dioxide}}{{\mathrm{SO}}_2\left(\mathrm{g}\right)}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)}\to \underset{\mathrm{Sulphurous}\mathrm{acid}}{{\mathrm{H}}_2{\mathrm{SO}}_3\left(\mathrm{aq}\right)}$

(b) Sulphur dioxide can react with oxygen and water to form Sulphuric acid.

$\underset{\mathrm{Sulphur}\mathrm{dioxide}}{2{\mathrm{SO}}_2\left(\mathrm{g}\right)}+{\mathrm{O}}_2\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{I}\right)\to \underset{\mathrm{Sulphuric}\mathrm{acid}}{2{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)}$

(c) When Nitrogen dioxide reacts with water, Nitrous acid and Nitric acid are formed.
$2{\mathrm{NO}}_2\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to \underset{\mathrm{Nitrous}\mathrm{acid}}{{\mathrm{HNO}}_2\left(\mathrm{aq}\right)}+\underset{\mathrm{Nitric}\mathrm{acid}}{{\mathrm{HNO}}_3\left(\mathrm{aq}\right)}$

(d) When CO₂ reacts with water, Carbonic acid is formed.

$\underset{\mathrm{Carbon}\mathrm{dioxide}}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to \underset{\mathrm{Carbonic}\mathrm{acid}}{{\mathrm{H}}_2{\mathrm{CO}}_3\left(\mathrm{aq}\right)}$