Which of the following phenomena occur, when a small amount of acid is added to water?
(i) Ionisation, (ii) Neutralisation, (iii) Dilution, (iv) Formation

Options

A. (i) and (ii)
B. (i) and (iii)
C. (ii) and (iii)
D. (ii) and (iv)

Explanation

To determine which phenomena occur when a small amount of acid is added to water, we can analyze each option provided:

1. **Ionisation**: When an acid is added to water, it dissociates into its ions. For example, hydrochloric acid (HCl) dissociates into H⁺ and Cl⁻ ions in water. This process is known as ionization. Therefore, ionization occurs when acid is added to water.

2. **Neutralisation**: Neutralization is a reaction between an acid and a base to form water and a salt. Since we are only adding acid to water (and not a base), neutralization does not occur in this scenario.

3. **Dilution**: When acid is added to water, the concentration of the acid decreases because it is being mixed with a larger volume of water. This process is known as dilution. Therefore, dilution occurs when acid is added to water.

4. **Formation**: The term "formation" is vague in this context. It does not specifically refer to a chemical process that occurs when acid is added to water. Therefore, we cannot conclude that formation occurs.

Based on the analysis above, the phenomena that occur when a small amount of acid is added to water are:

- **Ionisation**
- **Dilution**

**Final Answer**: (i) Ionisation and (iii) Dilution.