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

Options

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

Answer

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

### Step-by-Step Solution:

1. **Understanding Spontaneous Combustion**:
- Spontaneous combustion refers to a process where a substance ignites and burns without an external ignition source. This occurs due to the heat generated from the substance itself when it reacts with air.

2. **Analyzing the First Option: LPG (Liquefied Petroleum Gas)**:
- LPG burns in the presence of oxygen to produce a blue flame, indicating complete combustion. This means it requires an external ignition source and does not undergo spontaneous combustion.
- **Conclusion**: LPG does not undergo spontaneous combustion.

3. **Analyzing the Second Option: Paper**:
- When paper burns in the presence of oxygen, it typically produces smoke, ash, and carbon monoxide, indicating incomplete combustion. It also requires an external ignition source.
- **Conclusion**: Paper does not undergo spontaneous combustion.

4. **Analyzing the Third Option: Alcohol (Ethanol)**:
- Ethanol, when burned in the presence of oxygen, produces carbon dioxide and water, indicating complete combustion. Like the previous substances, it also requires an external ignition source.
- **Conclusion**: Alcohol does not undergo spontaneous combustion.

5. **Analyzing the Fourth Option: Sodium**:
- Sodium is known to undergo spontaneous combustion when it comes into contact with air (oxygen). It reacts vigorously and can ignite spontaneously, producing sodium oxide (Na2O).
- **Conclusion**: Sodium undergoes spontaneous combustion.

### Final Answer:
The substance that undergoes spontaneous combustion in the presence of air is **Sodium** (Option 4).

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