Which of the following non-metals reacts and catches fire on exposure to air?

Options

A. phosphorus
B. sulphur
C. nitrogen
D. hydrogen

Answer

To determine which non-metal reacts and catches fire on exposure to air, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Non-Metals**: The question asks about non-metals that can catch fire when exposed to air. Common non-metals include phosphorus, sulfur, nitrogen, and hydrogen.

2. **Research the Properties of Each Non-Metal**:
- **Phosphorus**: Particularly the white form of phosphorus (P4) is known to be highly reactive. It ignites spontaneously in air, forming phosphorus pentoxide (P4O10).
- **Sulfur**: While sulfur can burn in air, it does not catch fire spontaneously and requires a source of ignition.
- **Nitrogen**: Nitrogen is generally inert and does not react with oxygen in the air under normal conditions.
- **Hydrogen**: Hydrogen can burn in air, but it does not catch fire spontaneously without a spark or flame.

3. **Conclusion**: Among the non-metals listed, white phosphorus is the only one that reacts and catches fire spontaneously when exposed to air. Therefore, the answer is phosphorus.

4. **Final Answer**: The non-metal that reacts and catches fire on exposure to air is **phosphorus**.