Write two reactions to explain amphoteric nature of water .

Explanation

To explain the amphoteric nature of water, we can write two reactions that demonstrate how water can act both as an acid and as a base.

### Step-by-Step Solution:

1. **Understanding Amphoteric Nature**:
- Amphoteric substances can act as both acids and bases. Water (H₂O) is a classic example of an amphoteric substance.

2. **Reaction of Water as an Acid**:
- When water reacts with ammonia (NH₃), it donates a proton (H⁺) to ammonia.
- **Reaction**:
\[ \text{H}_2\text{O} + \text{NH}_3 \rightarrow \text{NH}_4^+ + \text{OH}^- \]
- In this reaction, water acts as an acid because it donates a proton, forming the ammonium ion (NH₄⁺) and hydroxide ion (OH⁻).

3. **Reaction of Water as a Base**:
- When water reacts with hydrogen sulfide (H₂S), it accepts a proton (H⁺) from hydrogen sulfide.
- **Reaction**:
\[ \text{H}_2\text{O} + \text{H}_2\text{S} \rightarrow \text{H}_3\text{O}^+ + \text{HS}^- \]
- In this reaction, water acts as a base because it accepts a proton, forming the hydronium ion (H₃O⁺) and hydrosulfide ion (HS⁻).

4. **Autoprotolysis of Water**:
- Water can also undergo autoprotolysis, where two water molecules react with each other.
- **Reaction**:
\[ \text{H}_2\text{O} + \text{H}_2\text{O} \rightleftharpoons \text{H}_3\text{O}^+ + \text{OH}^- \]
- In this reaction, one water molecule acts as an acid (donating a proton) and the other acts as a base (accepting a proton).

### Summary of Reactions:
1. **Water as an Acid**:
\[ \text{H}_2\text{O} + \text{NH}_3 \rightarrow \text{NH}_4^+ + \text{OH}^- \]
2. **Water as a Base**:
\[ \text{H}_2\text{O} + \text{H}_2\text{S} \rightarrow \text{H}_3\text{O}^+ + \text{HS}^- \]
3. **Autoprotolysis of Water**:
\[ \text{H}_2\text{O} + \text{H}_2\text{O} \rightleftharpoons \text{H}_3\text{O}^+ + \text{OH}^- \]