How does boiling point evolve with decreasing pressure?

1 Answer
Sep 8, 2016


The boiling points of ALL volatile compounds decrease with decreasing pressure...........


And first off, I should define #"boiling point"#. The boiling point is the temperature at which the vapour pressure of the liquid is equal to that of the ambient pressure, and bubbles of vapour form directly in the liquid. The #"normal boiling point"# is defined when the vapour pressure of the liquid (and necessarily the ambient pressure) is at #"1 atmosphere"#.

So if we reduce the ambient pressure, by means of a vacuum pump or water aspirator, we should substantially reduce the boiling point of the liquid, as less heat has to be pumped into the system in order that the vapour pressure of the liquid becomes equal to the (reduced!) ambient pressure. This underlies the principle of #"vacuum distillation"#.

I take it you are at A-level or 1st year university. If you are, you should definitely take this definition of boiling point on board. It is a fact that when we boil water in a kettle, the vapour pressure of the liquid is at #"1 atmosphere"# by definition (or at least close enough to it, as barometric pressure varies slightly day by day).

If you lived in Denver, Colorado (elevation 1610 m above sea level), what would you expect the temperature of boiling water to be? Why?