2.16 recall the gases present in air and their approximate percentage by volume
|This is for unpolluted, dry air. Normally air contains a little water vapour too!|
Sulphur dioxide dissolves in water in the atmosphere to form sulphurous acid (H2SO3). In the presence of oxygen in the air, this acid is slowly oxidised to sulphuric acid (H2SO4).
Oxides of nitrogen also contribute to acid rain. In the presence of oxygen and water, nitrogen dioxide is converted to nitric acid.
The pH value of unpolluted rainwater is usually slightly below 7. This is because carbon dioxide in the air dissolves in rainwater to form carbonic acid, which is a weak acid.
However, acid rain is much more acidic than rain that only contains carbonic acid. Acid rain has a pH value of 4 or less. Which is slightly less acidic than vinegar, which is at around pH 3.
So, what are the effects of acid rain?
- Acid rain reacts with metals and with carbonates in marble and limestone (calcium carbonate). When this happens, metal bridges and stone buildings are damaged, even statues if they're made of limestone.
- Acid rain can reduce the pH of natural water bodies from between 6.5 and 8.5 to below 4, which will kill fish and other aquatic life. The water is then too acidic to support life.
- Acid rain also leaches important nutrients from the soil and destroys plants. Without these nutrients, plant growth is stunted. In some cases, acid rain dissolves aluminium hydroxide (Al(OH)3) in the soil to produce Al3+ ions, which are toxic to plants. Forests throughout most of Central and Eastern Europe have been destroyed in this manner by acid rain. The plants are literally 'sick' and dying.
|acid rain eroding a statue|