boiling point of water

[drowsyneil]

why does the boiling point of water decrease with a drop in pressure

[Gef]

When water boils (or evaporates) water molecules are escaping from the surface of the liquid. To do this they must have enough energy to "push aside" the molecules in air.

At lower pressures there are fewer air molecules to push aside.

This is a fairly simplistic answer but I hope it helps.

[drowsyneil]

thanks

[benaddam]

You've got it wrong. The higher the pressure the lower the point of boiling. that's the secret of all pressure cookers.It's been decades since I took Physics but I think it's called Boyle's (nothing to do with boil) Law.

[kwicky]

As the answer above, Boyle's Law I believe

P1V1/T1 = P2V2/T2

or something similar

[HWKE]

I agree with Gef.The reason for pressure cookers is to allow elevated temperatures through increasing the pressure, allowing the food to cook more quickly.
The link below gives a chart relating pressure and boiling point.
http://www.engineeringtoolbox.com/boiling-poin t-water-d_926.html

[nightmare]

benaddam
No, you've got it totally wrong.
Boyle's law is pretty irrelevant here

[gix2fir8as]

The lower the pressure the lower the boiling point. the boiling point of water drops 1oC for every 1000ft you go up. so the boiling point of water on top of Everest would be approx. 70 oC. Less pressure means less air molecules preventing the water molecules from escaping. so it boils at a lower temperature

[gix2fir8as]

benadam...... A pressure cooker increases the pressure, less molecules can escape from the water. More energy is therefore needed for the water molecules to escape. The temperature of the water goes up and the food cooks quicker!

[Muppit]

PV=NRT

where p=pressure, v=volume, n=number of molecules, r=gas constant and t=temperature.

Boiling water is changing it from a liquid state to a gas state.

A gas has a greater volume than a liquid.

Therefore, by rearranging the formula above, you can see that in order to obtain a greater volume, with a lower pressure, you will need a lesser temperature increase.

Get it?