Chemguide: Support for CIE A level Chemistry


Learning outcomes 13.2(a) and 13.2(b)

These statements are about the environmental consequences of the formation of sulphur dioxide.

Before you go on, you should find and read the statements in your copy of the syllabus.


Statement 13.2(a)

This statement is about the formation of atmospheric sulphur dioxide from burning fossil fuels.

Fossil fuels like coal and oil all contain sulphur compounds, and when the coal or the oil product (petrol or diesel or whatever) are burned, sulphur dioxide is produced.

This leads to acid rain unless some method is used to prevent it.

Burning coal in power stations was a major source of sulphur dioxide, but there are ways of removing this from the flue gases, known as "flue gas desulphurisation". The best of these methods allow the sulphur dioxide to be recovered and then converted into sulphuric acid.

Sulphur compounds in petrol (gasoline) or diesel oil were also an important source of sulphur dioxide pollution. Nowadays, these sulphur compounds can be removed at the refinery to produce low-sulphur (or even ultra-low sulphur) fuels.


Statement 13.2(b)

This statement deals with the role of sulphur dioxide in producing acid rain, and some consequences of acid rain.

Apart from the consequences of acid rain, a lot of this has been dealt with either above or in statement 13.1(f).

The role of sulphur dioxide includes:

  • its formation during the combustion of fossil fuels contaminated with sulphur compounds.

  • the oxidation of sulphur dioxide to sulphur trioxide catalysed by nitrogen dioxide (see statement 13.1(f)).

  • the reaction of the sulphur trioxide with water to produce very dilute sulphuric acid (see statement 13.1(f)).

The important equations for this have already been given.

The consequences of acid rain include:

  • the corrosion of limestone buildings as the calcium carbonate reacts with the acid.

  • the corrosion of ironwork as the iron reacts with the acid.

  • the acidification of lakes and rivers leading to the death of aquatic life. This is complicated by the fact that a fall in pH dissolves aluminium ions from the soil. Aluminium ions are toxic to fish.

  • damage to trees. This again is partly the result of aluminium ions being toxic to plants.


Note:  If you are interested in this (as opposed to learning just enough to get a good grade at A level), you might like to explore the US Environmental Protection Agency site. In my experience, by the time they do A level chemistry, most students have been taught about acid rain in so many different subjects (geography, biology, general studies, etc) that they are totally bored by it!



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© Jim Clark 2011 (last modified June 2014)