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 the options given: (i) Ionisation, (ii) Neutralisation, (iii) Dilution, (iv) Formation.

### Step-by-Step Solution:

1. **Understanding Ionisation**:
- When an acid, such as hydrochloric acid (HCl), is added to water, it dissociates into its ions. HCl breaks down into H⁺ (hydrogen ions) and Cl⁻ (chloride ions). This process of breaking down into ions is known as ionisation.
- **Conclusion**: Ionisation occurs when acid is added to water.

2. **Understanding Neutralisation**:
- Neutralisation is a reaction between an acid and a base to form salt and water. In this case, since we are only adding acid to water and not mixing it with a base, neutralisation does not occur.
- **Conclusion**: Neutralisation does not occur when acid is added to water.

3. **Understanding Dilution**:
- When a small amount of acid is added to water, the concentration of the acid decreases as it is mixed with water. This process is called dilution, where the solute (acid) is mixed with a solvent (water) to reduce its concentration.
- **Conclusion**: Dilution occurs when acid is added to water.

4. **Understanding Formation**:
- Formation typically refers to the creation of new substances. In this context, adding acid to water does not lead to the formation of new substances but rather the dissociation of the acid into ions.
- **Conclusion**: Formation does not occur in this scenario.

### Final Answer:
The phenomena that occur when a small amount of acid is added to water are:
- (i) Ionisation
- (iii) Dilution