`H_(2)O` can act as acid as well as base.

Answer

**Step-by-Step Solution:**

1. **Understanding Amphoteric Substances:**
- Water (H₂O) is classified as an amphoteric substance, meaning it can act both as an acid and as a base.

2. **Dissociation of Water:**
- Water can dissociate into hydronium ions (H₃O⁺) and hydroxide ions (OH⁻):
\[ H_2O \rightleftharpoons H_3O^+ + OH^- \]

3. **Acid-Base Behavior According to Bronsted Theory:**
- According to Bronsted-Lowry theory:
- An acid is defined as a substance that donates a proton (H⁺).
- A base is defined as a substance that accepts a proton (H⁺).

4. **Water Acting as an Acid:**
- When water donates a proton (H⁺), it forms hydroxide ions (OH⁻):
\[ H_2O \rightarrow H^+ + OH^- \]
- In this reaction, water is acting as an acid because it donates a proton.

5. **Water Acting as a Base:**
- Conversely, when water accepts a proton (H⁺), it forms hydronium ions (H₃O⁺):
\[ H_2O + H^+ \rightarrow H_3O^+ \]
- Here, water is acting as a base because it accepts a proton.

6. **Conclusion:**
- Therefore, water can act as both an acid and a base depending on the reaction it is involved in.

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