# Why does the boiling point of a liquid REDUCE, as the pressure of the gas above it is reduced?

Nov 16, 2016

Are you are asking why this is so?

#### Explanation:

The boiling point of a liquid, any liquid, is the temperature at which the vapour pressure of the liquid is equal to the ambient pressure, and bubbles of vapour form directly in the liquid. The $\text{normal boiling point}$ is specified when the ambient pressure (and of course the vapour pressure of the liquid at the boiling point) is $\text{1 atmosphere}$

Clearly, as we reduce the ambient pressure, the boiling point of the liquid will reduce, and this underlies the practice of vacuum distillation, when an otherwise involatile liquid is heated and distilled at high vacuum.

On the other hand, when the ambient pressure is increased, for instance in a pressure cooker, the boiling point of the liquid is RAISED; typically above $100$ ""^@C, and thus pressure cookers are capable of cooking things rapidly with reduced cooking time.

On the other, other hand, when the ambient pressure is reduced, say halfway, or all the way up a high mountain, the reduced pressure means that boiling will occur at A REDUCED temperature; one that may not cook your food completely. And a pressure cooker is used here to increase the boiling point.

I once had a colleague who used to cook curries in his pressure cooker; he usually got beautiful, tender meat from his pressure cooker in 1-2 hours, rather than the 4-5 hours it would take in a normal saucepan.