Explain the principal involved in the liquefaction of air.

Explanation

### Step-by-Step Solution:

1. **Understanding Liquefaction of Air**:
- Liquefaction of air refers to the process of converting air (a gas) into a liquid state. This is achieved through specific physical processes.

2. **Cooling Below Critical Temperature**:
- To liquefy a gas, it must be cooled below its critical temperature. The critical temperature is the temperature above which a gas cannot be converted into a liquid, regardless of the pressure applied.

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