Re: When a beaker of water is put in a Bell Jar... is it really boiling?

Bill, sorry I’ve taken a while to get back to you.

Boiling happens when the vapour pressure of the liquid is the same as the surrounding air pressure, so molecules can escape from the liquid freely. They will also form bubbles of vapour in the liquid.

At normal atmospheric pressure, the water molecules only reach the energy to have a vapour pressure equal to atmospheric at 100 deg Celsius. In the bell jar the pressure is very low, so the water molecules have sufficient energy (and therefore pressure) to evaporate into the surroundings at a much lower temperature and spontaneously form vapour within the body of the liquid too. The liquid is indeed boiling. Of course, all tap water does contain some dissolved air (only a small amount) and this will make the first few bubbles only.

A little subscript from an Englishman and avid tea drinker - I believe it is hard to make a good cup of tea on Mt Everest. The atmospheric pressure is so low there that water boils at a lower temperature and does not brew the tea sufficiently.

Hope that helps,

Kevin