When a liquid and its vapour are at equilibrium and the pressure is suddenly decreased, cooling occurs.

Explanation

**Solution:**

1. **Understanding the System**: We start with a system where a liquid and its vapor are in equilibrium. This means that the rate of evaporation of the liquid equals the rate of condensation of the vapor.

2. **Sudden Decrease in Pressure**: When the pressure in the system is suddenly decreased, the equilibrium is disturbed. According to Le Chatelier's Principle, the system will respond to this change by shifting the equilibrium position to counteract the effect of the pressure decrease.

3. **Shift in Equilibrium**: In this case, the equilibrium will shift to the right, favoring the formation of more vapor from the liquid. This means that more liquid will evaporate to increase the vapor pressure back towards the original equilibrium state.

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