Sulphur dioxide

Sulphur dioxide is a colourless gas which is present in the exhaust gases coming out of automobiles and factories. It is a poisonous gas having molecular formula SO₂. It can cause inflammation of the lungs when inhaled in the body. It is the main source of air pollution.

Sulphur dioxide is highly soluble in water. It dissolves in rainwater to form sulphuric acid and hence causes acid rain which is responsible for damaging historical buildings, bridges, metal works and plants.

Preparation of Sulphur Dioxide

In the laboratory, sulphur dioxide is prepared by heating copper turnings with concentrated sulphuric acid.

Cu    +    2H₂SO₄     →    CuSO₄     +     SO₂     +     2H₂O

(Copper + Sulphuric acid → Copper sulphate + Sulphur dioxide + Water)

Physical Properties of Sulphur Dioxide

1. It is a colourless and poisonous gas having a pungent smell.
2. It is highly soluble in water.
3. It is heavier than air.

Chemical Properties of Sulphur Dioxide

1. Sulphur Dioxide is Acidic in Nature

Sulphur dioxide is acidic in nature because solution of sulphur dioxide in water turns blue litmus red.

2. Reaction with Lime Water

Sulphur dioxide turns lime water (calcium hydroxide) cloudy due to formation of calcium sulphite.

SO₂     +     Ca(OH)₂     →     CaSO₃     +     H₂O

(Sulphur dioxide + Calcium hydroxide  → Calcium sulphite + Water)

3. Reaction with Sodium Hydroxide

Sulphur dioxide produce sodium sulphite when passed through sodium hydroxide solution.

2NaOH     +     SO₂     →     Na₂SO₃     +     H₂O

(Sodium hydroxide + Sulphur dioxide → Sodium sulphite + Water)

4. Reaction with Oxygen

When sulphur dioxide is heated to 450°C with oxygen in the presence of vanadium oxide, sulphur trioxide is formed.

2SO₂     +     O₂     →     2SO₃
(Sulphur dioxide + Oxygen → Sulphur trioxide)

5. Reaction with Water

Sulphur dioxide dissolves in water to form sulphuric acid.

2SO₂     +     2H₂ O     →     H₂SO₄     +     2[H]
(Sulphur dioxide + Water → Sulphuric acid + Hydrogen)

Uses of Sulphur Dioxide

1. It is used in the manufacturing of sulphuric acid.
2. It prevents the growth of bacteria. So, it is used as a preservative for the preservation of dry fruits, jams and lemon and orange squashes.
3. It is used for bleaching wood pulp, straw, wool and silk.
4. It is also used as a disinfectant.

Test your understanding and answer these questions:

1. What are chemical properties of sulphur dioxide?
2. How sulphur dioxide is prepared?