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Q & A: boiling and pressure

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Most recent answer: 06/24/2011
Q:
Why the boiling point of water is much lower in conditions where atmospheric pressure is very low
- Lucy red (age 17)
victoria
A:
Water (or any liquid) boils when bubbles of vapor inside the liquid grow rather than shrink. That means that more molecules leave the liquid and go into the vapor than the opposite. The rate at which molecules leave the liquid usually depends mostly on the temperature. For a given temperature, the density of molecules in the vapor is approximately proportional to the pressure. At high pressure, more molecules from the vapor strike the liquid and enter it. So to get more molecules to leave the liquid for the vapor, you have to raise the temperature. Likewise, as the pressure is lowered, it takes a lower temperature to get enough molecules to leave the liquid.

There are more elegant ways to understand this in terms of equilibrium statistical mechanics, but perhaps the pictorial description above is best for starters.

mike w.

(published on 10/22/2007)

Follow-Up #1: boiling point

Q:
I know that boiling point is: the fixed temperature at which a (pure) substance changes from liquid to gas. But I have seen in various books another definition which i cant quite understand: The pressure of the gas created above the liquid equals atmospheric pressure.Please explain....Thank you.
- Mohammed Yahia (age 15)
Jeddah, KSA
A:
These are really the same thing.

You can start with one of our old answers:



Say that you just have the one pure substance in a sealed container with fixed volume bigger than the liquid. If there is no gas above, any molecules that escape (via thermal energy) into that vacuum are very unlikely to return. The gas buildup will continue until the rate of molecules returning to the liquid from the gas equals the rate at which they hop out of the liquid. The pressure of the gas is called the equilibrium vapor pressure. It goes up as the substance is heated, primarily because the extra energy helps molecules escape from the liquid.

If instead you put the liquid in an atmosphere of other gases, it will gradually evaporate at any temperature, since the escaping molecules have a huge space in which to wander off.  However, if you look at what happens to a vapor bubble inside the liquid, it's like the gas in the fixed-pressure case.  Bubbles will keep growing if it's above the boiling temperature, and that's just what we call "boiling".

What if we replaced the top of the container with a fixed-pressure sliding piston? If its pressure were greater than the vapor pressure, it would just squash the gas back into the liquid. If its pressure were less than the vapor pressure, the gas would keep pushing it up, expanding until all the liquid is used up. The temperature where that happens is the boiling point for that substance at that pressure.


Mike W.

(published on 06/01/2010)

Follow-Up #2: boiling point and pressure

Q:
why does the boiling point of the liquid increases when pressure is increased?
- gaurab (age 16)
nuwakot, nepal
A:
This is really close to questions that we've answered, so I marked it as a follow-up.

Mike W.

(published on 06/24/2011)

Follow-up on this answer.