Assertion. Dihydrogen oxidises sodium to sodium hydride.
Reason. Hydrogen can act only as a reducing agent.

Explanation

To solve the assertion and reason question, we will analyze both the assertion and the reason step by step.

### Step 1: Understand the Assertion
The assertion states that "Dihydrogen oxidizes sodium to sodium hydride."

- **Explanation**: When sodium (Na) reacts with dihydrogen (H₂), it forms sodium hydride (NaH). In this reaction, sodium goes from an oxidation state of 0 (elemental sodium) to +1 (in sodium hydride), indicating that sodium is oxidized.

### Step 2: Write the Reaction
The reaction can be represented as:
\[ 2 \text{Na} + \text{H}_2 \rightarrow 2 \text{NaH} \]

### Step 3: Analyze Oxidation States
- Sodium (Na) starts at an oxidation state of 0 and ends at +1 in sodium hydride (NaH).
- Hydrogen (H) starts at an oxidation state of 0 in dihydrogen (H₂) and ends at -1 in sodium hydride (NaH).

### Step 4: Determine Oxidation and Reduction
- **Oxidation**: Sodium is oxidized from 0 to +1.
- **Reduction**: Hydrogen is reduced from 0 to -1.

### Step 5: Conclusion for Assertion
Since dihydrogen causes sodium to be oxidized, the assertion is **correct**.

### Step 6: Understand the Reason
The reason states that "Hydrogen can act only as a reducing agent."

- **Explanation**: This statement is incorrect because hydrogen can act as both a reducing agent and an oxidizing agent depending on the reaction conditions. In this case, hydrogen acts as an oxidizing agent because it oxidizes sodium.

### Step 7: Conclusion for Reason
Since hydrogen can act as both a reducing agent and an oxidizing agent, the reason is **incorrect**.

### Final Conclusion
- Assertion: Correct
- Reason: Incorrect

Thus, the final answer is that the assertion is correct, but the reason is incorrect.

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