How does the boiling point of water evolve with ambient pressure?

1 Answer
Sep 15, 2017


Well the water should boil at a lower temperature.......


But first let's define #"boiling point"#. And it's a good definition to learn off by heart. The #"boiling point"# is defined as 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"# occurs when the ambient pressure (and thus the vapour pressure of the boiling liquid) is ONE ATMOSPHERE. I would get this straight in your head first, because there is a lot going on here.

So we put the kettle on, and boil for a cup of tea. The water boils at #100# #""^@C#, and the vapour pressure of the steam is 1 atmosphere, and it pushes the atmosphere back..... If we have an involatile liquid, we can perform a vacuum distillation, and reduce the pressure to #10^-3-10^-4*atm#, we can substantially reduce the boiling point of the liquid.

There are several so-called temperature/pressure nomographs that predict the boiling point of a liquid under reduced pressure (or under elevated pressure), that can give you an idea. Have a play with this site and report how you get on.