Concentrated sulphuric acid is not used to dry hydrogen.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Role of Drying Agents:**
- A drying agent is a substance that removes moisture from gases or liquids. In the case of drying hydrogen gas, the drying agent should absorb water without reacting with hydrogen.

2. **Properties of Concentrated Sulphuric Acid:**
- Concentrated sulphuric acid (H₂SO₄) is known to be a very strong dehydrating agent and a powerful oxidizing agent. This means it has a strong tendency to remove water from substances and can also cause oxidation reactions.

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