Which of these substances undergoes spontaneous combustion in the presence of air?

Options

A. Paper
B. LPG
C. Alcohol
D. Sodium

Answer

To determine which substance undergoes spontaneous combustion in the presence of air, we need to analyze each option provided in the question.

### Step-by-Step Solution:

1. **Identify the substances**: The options given are paper, LPG (Liquefied Petroleum Gas), alcohol, and sodium.

2. **Analyze the first option - Paper**:
- When paper burns, it undergoes combustion, producing carbon monoxide, ash, and smoke.
- This process is characterized as incomplete combustion.
- **Conclusion**: Paper does not undergo spontaneous combustion.

3. **Analyze the second option - LPG**:
- LPG burns with a blue flame, indicating complete combustion.
- The combustion of LPG produces carbon dioxide and water, which means it requires an external ignition source to start burning.
- **Conclusion**: LPG does not undergo spontaneous combustion.

4. **Analyze the third option - Alcohol**:
- Alcohol also burns completely in the presence of air, producing carbon dioxide and water.
- Like LPG, it requires an external ignition source to ignite.
- **Conclusion**: Alcohol does not undergo spontaneous combustion.

5. **Analyze the fourth option - Sodium**:
- Sodium is known to react vigorously with air.
- When sodium is exposed to air, it can ignite spontaneously without the need for an external ignition source.
- This is a characteristic of spontaneous combustion, where the substance ignites on its own due to heat generated from the reaction.
- **Conclusion**: Sodium undergoes spontaneous combustion.

### Final Answer:
The substance that undergoes spontaneous combustion in the presence of air is **sodium**.

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