Which of the following acids is formed when sulphur dioxide is dissolved in water?

Sulphurous acid

The correct answer is option 2. Sulphurous acid.

When sulfur dioxide (\( \text{SO}_2 \)) dissolves in water (\( \text{H}_2\text{O} \)), it undergoes a chemical reaction to form sulfurous acid (\( \text{H}_2\text{SO}_3 \)). This reaction occurs because sulfur dioxide is acidic in nature, capable of reacting with water to produce an acidic solution.

The chemical equation for the reaction between sulfur dioxide and water can be represented as follows:

\[ \text{SO}_2(g) + \text{H}_2\text{O}(l) \rightarrow \text{H}_2\text{SO}_3(aq) \]

In this reaction, sulfur dioxide reacts with water to produce sulfurous acid, which is a weak acid. Sulfurous acid is a diprotic acid, meaning it can donate two protons (\( \text{H}^+ \)) per molecule in solution.

Sulfurous acid is commonly used as a reducing agent and preservative in various industrial processes. It is also formed in the atmosphere through the oxidation of sulfur dioxide emitted from volcanic eruptions, combustion of fossil fuels, and other sources. Additionally, sulfurous acid plays a role in atmospheric chemistry and the formation of acid rain.

Therefore, when sulfur dioxide is dissolved in water, it forms sulfurous acid, making it the correct answer among the options provided.