Physical properties

Sulphur dioxide is:

(i) a colourless gas with an irritating smell.

(ii) Two and half times denser than air.

(iii) soluble in water forming an acidic solution.

Confirmatory test for sulphur dioxide

To confirm the presence sulphur dioxide, we use:

(i) Acidified orange potassium dichromate (VII) solution that changes from orange to green.

(ii) A purple solution of potassium manganate (VII) that turns to colourless.

Chemical properties (Reactions)

As a reducing agent:

Sulphur dioxide acts as a reducing agent by supplying electrons or gaining oxygen from other substances. Examples include:

The bleaching action: This is due to sulphurous acid which removes oxygen from the dye.
Equations of reaction

With acidified dichromate(VI) solution

Equation of reaction:


Solution turns from orange to green.

With potassium manganate(VII) solution

Equation of reaction

Solution turns from purple/pink to colourless

With iron (III) sulphate solution

Iron (III) ions in solution are reduced by sulphur dioxide to iron (II) ions.

Equation of reaction


Solution turns from brown to green

With bromine solution

Bromine in reduced to bromide ions.

Equation of reaction


Solution turns from brown to colourless

With iodine solution

Iodine is reduced to iodide ions.

Equation of reaction


Solution turns from brown to colourless.

With sodium hydroxide solution

With a little sulphur dioxide bubbled through an excess of the alkali, a normal salt is formed but equimular quantities react to form an acid salt.

Equation of reaction

(ii) Examples in which sulphur dioxide acts as an oxidising agent include:

With burning magnesium ribbon

Equation of reaction


Mixture of white and yellow solids deposited at the bottom and sides of the jar.


The burning metal gives out a lot of heat that decomposes the gas into its elements sulphur and oxygen. The metal then continues to burn in the oxygen formed.

With hydrogen sulphide gas

Equation of reaction


Yellow solid of sulphur formed

Large scale production of sulphur dioxide gas

By burning sulphur in air

By burning iron (II) sulphide or zinc sulphide in air

Teachers' Notes
Scheme of work
Lesson Plan
Learners' activities
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Sulphur dioxide