Chemguide: Core Chemistry 14 - 16

Environmental problems involving the air - acid rain

This page takes a quick look at the causes and effects of acid rain.

The pH scale

The pH scale is used to measure the acidity of a solution. The key points are:

• A pH of 7 shows the solution to be neutral.

• A pH below 7 shows the solution to be acidic. The lower the number, the more acidic a solution is.

• A pH above 7 shows the solution to be alkaline. The higher the number, the more alkaline a solution is.

• Because of the way the scale is defined, each shift of 1 pH unit means a ten times shift in the acidity or alkalinity of a solution. So a solution with a pH of 5 is ten times more acidic than one with a pH of 6.

An introductory video

This 5 minute video is a useful summary of acid rain, although it isn't quite complete enough as it stands. I will give you a summary of the main points and some extra comments.

The causes of acid rain

Formation of sulfuric acid

Fossil fuels such as coal and oil contain sulfur compounds. When these are burned, the sulfur reacts with oxygen in the air to make sulfur dioxide. Further reactions in the air give a very, very dilute solution of sulfuric acid - acid rain.

Some fossil fuels such as petrol (gasoline) and natural gas are treated to remove sulfur to prevent this happening.

Formation of nitric acid

Most nitric acid originates from burning fossil fuels such as coal, petrol (gasoline) and diesel oil - either in vehicles or in power stations.

When fuels burn, at the high temperatures produced, some of the nitrogen and oxygen in the air combine to make nitrogen oxides, collectively know as NO_x - mainly NO and NO₂.

Any NO formed reacts with oxygen in the air to form NO₂.

The NO₂ then undergoes further reactions to give a very, very dilute solution of nitric acid - acid rain.

The effects of acid rain

The effect on vegetation

Acid rain can change soil chemistry. It can free-up aluminium ions previously bound into the soil so that they get taken up by plants and trees. Aluminium ions are poisonous and can kill the plant or tree.

The hydrogen ions in the acid can also free-up metal ions necessary for plant growth and which are also normally bound into the soil - for example, calcium, magnesium and potassium. The rain then washes them away.

The acidification of lakes

Run-off of acid rain into lakes can acidify the lakes to the extent that fish and other aquatic animals and plants are killed.

Weathering of limestone

Limestone, calcium carbonate, is a common building material, but carbonates react with acids.

You can find details on old limestone buildings or statues where the sharp outline is blurred because the surface has weathered due to acid rain.

The calcium carbonate will react with the nitric and sulfuric acids in the rain to give calcium sulfate and calcium nitrate. You will meet these reactions later in the course.

CaCO₃ + H₂SO₄     CaSO₄ + CO₂ + H₂O

CaCO₃ + 2HNO₃     Ca(NO₃)₂ + CO₂ + H₂O

Calcium sulfate is slightly soluble in water; calcium nitrate is soluble in water.