This unit highlights the preparation of sulphur dioxide in the laboratory and on a large industrial scale. Discusses the properties of sulphur dioxide and its uses such as bleaching of straws and sponges, fumigation of our houses to kill insects (i.e. black ants), preservation of some liquids (i.e. orange juice). Sulphur dioxide is also used in the manufacture of sulphuric acid. Sulphuric acid is widely utilized to make paints, plastics, detergents (i.e. Omo, Nomi, Aerial, etc.) and fertilizers.

Sulphur dioxide could be obtained on a large scale from burning sulphur that is itself obtained from natural gas, a component of crude petroleum. In Uganda, the discovery of oil in the western part of the country will in the near future become a source of sulphur.

Sulphur dioxide is a major pollutant of atmospheric air. It is toxic, causes respiratory ailments, damages plant life and corrodes metals. The gas is soluble in water forming what is known as ‘acid rain’. Acid rain erodes marble and limestone. In industries where this gas is evolved it is recommended to bubble the gas through an alkali to form sulphites and thus control its polluting effect on the environment.

Sulphites are useful as mild oxidising agents but more often they act as reducing agents. They are oxidised to sulphates by substances such as potassium manganate (VII), potassium dichromate (VI), chlorine, bromine, hydrogen peroxide, oxygen and iron (III) chloride.

A mixture of sodium sulphite and sulphur when heated react to form sodium thiosulphate. A solution of a ‘fixer’ containing sodium thiosulphate is added to developed films to remove unreduced silver ions to form a soluble complex, which is washed away. This prevents unreduced silver ions from becoming reduced and darkening slowly over time. This is useful in photography, which is a very vital practical application.


This sub-topic deals with

  • Methods of preparing sulphur dioxide in the laboratory and on a large scale.

  • The physical and chemical properties /reactions of sulphur dioxide gas.

  • Uses of sulphur dioxide

  • Preparation and properties of sulphite salts.

Demonstration Experiments:

(a) Laboratory methods of preparing sulphur dioxide

Sulphur dioxide in the laboratory is prepared by:
(i) Reacting dilute hydrochloric acid or dilute sulphuric acid with sodium sulphite crystals
Diagram of the apparatus used

Equation of reaction taking place

(ii) Heating a mixture of copper turnings and concentrated sulphuric acid
Diagram of Apparatus used is the same as the one above except a source of heat must be applied.

Equation of reaction taking place

Uses of sulphur dioxide:

Sulphuric acid used to make soap, paints and pigments

(i) Soap and Detergents

(ii) Fertilizers

(iii) Bleaching agent for wool, straws and sponges.

(iv) Preservative for some liquids e.g. orange juice.

(v) Preparation of calcium hydrogen sulphite used for bleaching wood-pulp in the manufacture of papers.

(vi) Rrefrigerant.

(g) Preparation of sulphite salts for example sodium sulphite and zinc sulphite.

A: Learning Outcome

By the end of this sub-section a student (you) should be able to:

(a) Describe the two methods by which sulphur dioxide is prepared in the laboratory.

(b) State possible methods of preparation of sulphur dioxide gas on a large scale.

(c) State the method of collection of sulphur dioxide gas in the laboratory.

(d) State the physical and chemical properties of sulphur dioxide.

(e) Describe a confirmatory test for sulphur dioxide.

(f) State the observations made in the oxidising and reducing properties of sulphur dioxide.

(g) Write equation(s) for the reaction(s) of sulphur dioxide with various substances.

(h) State the uses of sulphur dioxide.

(i) Describe the preparation of sodium sulphite and zinc sulphite in the laboratory

(j) State chemical properties of sulphite salt.





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