Page 18 - Curriculum Visions Dynamic Book

Sulphur dioxide
Sulphur dioxide is one of the most important sulphur gases. It is a dense, colourless gas with a choking (pungent) smell.
In large concentrations, sulphur dioxide can cause suffocation, while even small amounts can combine with water droplets to make acid rain. For these reasons, sulphur dioxide
is regarded as a health hazard. In addition it is partly responsible for harming vegetation and corroding limestone buildings.
Sulphur dioxide is a pollutant, but also has many valuable uses. For example, it is a very good bleaching agent and many factories producing paper and textiles rely on it. Sulphur dioxide is also used in food warehouses because it is helpful in preserving both fruit
and vegetables.
Preparing sulphur dioxide
in the laboratory
Sulphur dioxide can be prepared by burning sulphur in oxygen as shown on pages 14 and 15, but more usually it is made by adding dilute sulphuric acid to a salt of sulphurous acid called a sulphite. Sulphites are generally used as bleaching agents.
EQUATION: Preparation of sulphur dioxide gas from a sulphur salt
Sulphuric acid + sodium sulphite ➪ sulphur dioxide + sodium sulphate + water
H2SO4(aq) + Na2SO3(s)
Properties of sulphur dioxide gas
Sulphur dioxide is an acidic gas, that is, when dissolved in water a weak acid
is produced, called sulphurous acid.
Sulphur dioxide is a reducing agent, and will take oxygen from other substances. As a result, it can act as
a bleach (see page 20).
➪ SO2(g) + Na2SO4(aq) + H2O(l)
Making use of sulphur dioxide
Sulphur dioxide can be used to produce sulphuric acid, a chemical used in fertilizer production. Some of the sulphur dioxide created by power stations is now recovered from the smokestacks rather than released into the environment (see page 22).
Sulphur dioxide can also be used as a bleaching agent because of its reducing properties. It is used to bleach wood pulp in making paper.
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