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

Question

Wyatt Adkins · Accepted Answer

Are you are asking why this is so?

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 #"normal boiling point"# is specified when the ambient pressure (and of course the vapour pressure of the liquid at the boiling point) is #"1 atmosphere"# It is obvious that lowering the ambient pressure will lower the liquid's boiling point, which is the basis for the process of vacuum distillation, which involves heating and distilling an otherwise involatile liquid under 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. However, if ambient pressure is dropped, as it might be halfway up a high mountain or more, boiling will occur at a REDUCED temperature, which might not fully cook your food. In these situations, a pressure cooker is used to raise the boiling point. My former coworker used to make curries in his pressure cooker; instead of the 4-5 hours it would take in a regular saucepan, he would typically get gorgeous, tender meat in 1-2 hours.

Cooper Adams · Answer

undefined The boiling point of a liquid reduces as the pressure of the gas above it is reduced because boiling occurs when the vapor pressure of the liquid equals the external pressure. When the external pressure decreases, the vapor pressure required for boiling is reached at a lower temperature, causing the boiling point to decrease.