14.4 Acid Rain and Its Effects

As you have studied, burning of fossil fuels produces oxides of sulphur and nitrogen in air. Rain water converts ${\mathrm{S}\mathrm{O}}_2$$\mathrm{SO}_2$ into ${\mathrm{H}}_2{\mathrm{S}\mathrm{O}}_4$$\mathrm{H}_2\mathrm{SO}_4$ and ${\mathrm{N}\mathrm{O}}_x$$\mathrm{NO}_x$ to ${\mathrm{H}\mathrm{N}\mathrm{O}}_2$$\mathrm{HNO}_2$ and ${\mathrm{H}\mathrm{N}\mathrm{O}}_3$$\mathrm{HNO}_3$ . Normal rain water is weakly acidic because it consists of dissolved ${\mathrm{C}\mathrm{O}}_2$$\mathrm{CO}_2$ of the air. Its pH is about 5.6 to 6. But rain water on dissolving air pollutants (acids) becomes more acidic and its pH reduces to 4. Thus, acid rain is formed on dissolving acidic air pollutants such as sulphur dioxide and nitrogen dioxide by rain water.

Figure 14.8 shows how oxides of sulphur and nitrogen are converted into acids. These acids dissolve in rain water and damage soil, animals, plants and aquatic life.

Effects of acid rain