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Lesson: Chapter 15 — The biosphere

15.3. Pollutants and their sources 6 of 6

Acid rain

Because carbon dioxide in the air dissolves in it, rainwater is naturally slightly acidic, with a pH of about 5.6. But at times the pH of rainwater has been found to fall below this value — that is, to become more acidic.

The main cause has been found to be a rise in the concentration of sulphur dioxide, sulphur trioxide and nitrogen dioxide in the atmosphere.

  • Sulphur dioxide dissolving in water forms sulphurous acid (H2SO3), which is further oxidised to sulphuric acid (H2SO4).
  • Sulphur trioxide dissolving in water also forms sulphuric acid (H2SO4).
  • Nitrogen dioxide forms nitric acid (HNO3) with rainwater.

Water mixed with these acids falling as rain is called acid rain. Some of its harmful effects:

  • Forests and crops are destroyed.
  • Aquatic life dies as the acidity of lakes and reservoirs rises.
  • Rising acidity affects the uptake of minerals by plants.
  • Rocks such as limestone dissolve.
  • Metal structures, buildings, statues and ruins are damaged.
  • Poisonous heavy metals dissolve, raising their ion concentrations in reservoirs to harmful levels.

Figure to be added

Figure 15.23 — damage caused by acid rain: a forest of dead, bare trees.

Activity 15.2

Test the acidity of everyday rain, and of the rain that falls after a drought, with an indicator.

What to expect: the first rain after a long dry spell is usually the more acidic, because the gases and dust that built up in the air are washed out at once.

All rain is a little acidic

Rain at pH 5.6 is normal, because of dissolved CO2. It is called acid rain only when SO2 and NO2 push the pH lower. That is also why damaged iron and marble corrode faster, as in chapter 12.

Sulphur and nitrogen oxides make acid rain

SO2, SO3 and NO2 dissolve in rain to form sulphuric and nitric acids, harming forests, lakes, soil and buildings.