Electrolysis of aqueous solutions of which of the following substances results in only the decomposition of water ?
1) Potassium chloride
2) Zinc sulphate
3) Potassium hydroxide
4) Sodium phosphate

Options

A. Potassium chloride
B. Zinc sulphate
C. Potassium hydroxide
D. Sodium phosphate

Explanation

To determine which of the following substances results in only the decomposition of water during electrolysis, we need to analyze each option based on the behavior of their ions during the electrolysis process.

### Step-by-Step Solution:

1. **Understanding Electrolysis**: Electrolysis involves the decomposition of a substance into its components by passing an electric current through it. In aqueous solutions, both cations and anions can participate in the reactions.

2. **Analyzing the Options**:
- **Potassium Chloride (KCl)**:
- Cation: K⁺ (potassium)
- Anion: Cl⁻ (chloride)
- During electrolysis, K⁺ does not get reduced (it has a higher standard reduction potential than H⁺), while Cl⁻ can be oxidized to Cl₂ gas. Thus, KCl does not lead to only the decomposition of water.

- **Zinc Sulfate (ZnSO₄)**:
- Cation: Zn²⁺ (zinc)
- Anion: SO₄²⁻ (sulfate)
- Zn²⁺ can be reduced to Zn, and SO₄²⁻ cannot be oxidized further (as it is at its highest oxidation state). Hence, water is not solely decomposed in this case.

- **Potassium Hydroxide (KOH)**:
- Cation: K⁺ (potassium)
- Anion: OH⁻ (hydroxide)
- K⁺ does not get reduced, but OH⁻ can be oxidized to O₂ gas. Therefore, KOH does not lead to only the decomposition of water.

- **Sodium Phosphate (Na₃PO₄)**:
- Cation: Na⁺ (sodium)
- Anion: PO₄³⁻ (phosphate)
- Na⁺ does not get reduced, and PO₄³⁻ is at a high oxidation state and cannot be oxidized further. Thus, the electrolysis of sodium phosphate leads to the decomposition of water only.

3. **Conclusion**: The only substance among the options that results in only the decomposition of water during electrolysis is **Sodium Phosphate (Na₃PO₄)**.

### Final Answer:
**4) Sodium phosphate**