When sulphur dioxide (SO2) is dissolved in water, it undergoes a chemical reaction to form sulphurous acid (H2SO3). This reaction can be represented by the following equation:

SO2 + H2O → H2SO3

Explanation:

1. Dissolution of Sulphur Dioxide:
When sulphur dioxide gas (SO2) is dissolved in water (H2O), the following reaction takes place:

SO2 + H2O → H2SO3

2. Formation of Sulphurous Acid:
The reaction between sulphur dioxide and water results in the formation of sulphurous acid (H2SO3). Sulphurous acid is a weak acid and is characterized by the presence of the sulfite ion (SO32-). The chemical formula of sulphurous acid can be represented as H2SO3.

3. Reaction Mechanism:
The dissolution of sulphur dioxide in water involves the transfer of a proton (H+) from water to the sulphur dioxide molecule. This proton transfer leads to the formation of the sulphurous acid molecule. The detailed reaction mechanism is as follows:

SO2 + H2O → H+ + HSO3- → H2SO3

4. Properties of Sulphurous Acid:
Sulphurous acid is a colorless liquid with a characteristic odor. It is a weak acid and can act as a reducing agent. It is commonly used in various industrial processes, including the production of paper, textiles, and food preservation.

5. Uses of Sulphurous Acid:
Sulphurous acid has several applications, including:

- As a food preservative: Sulphurous acid is used to preserve foods such as dried fruits, wine, and canned goods. It acts as an antioxidant and prevents the growth of microorganisms.
- In water treatment: Sulphurous acid is used to remove excess chlorine from water, which helps in maintaining the water quality.
- In the production of chemicals: Sulphurous acid is used as a precursor in the production of various chemicals, including sulphuric acid and sulphur dioxide.

In conclusion, when sulphur dioxide is dissolved in water, it reacts to form sulphurous acid. This reaction is important in various industrial processes and has several applications in food preservation and water treatment.