Ice, water,steam

[pompei595]

Water freezes at 0 degrees, and becomes a gas at 100 degrees. How does water evaporate even when the surrounding air temperature may only be a few degrees above zero?

[Whickerman]

Water neither freezes at 0�C nor becomes a gas at 100�C.

It begins to melt, becomes a gas and can stay frozen at 0.01�C

This is known as the 'triple point' of water.


http://en.wikipedia.org/wiki/Triple_point

[heathfield]

All those little molecules are dashing about in liquid water. Some move fast enough to break free of the surface of the liquid, even at low temperatures.

[Teddio]

Water boils when its vapour pressure is equal to the ambient air pressure. At sea level the air pressure is 101kPa and this corresponds to a vapour pressure of water of 101kPa at a temperature of 100C.
Cold water will evaporate because a small proportion of the molecules have sufficient kinetic energy to break free from the liquid surface and enter the gas phase.

[Space]

this has nothing to do with the triple point of water
because it is at a pressure of 0.006 atmosphere,
so it clearly doesn't explain usual evaporation.

[Space]

mmmh... maybe I'm wrong and it does have something to do with the triple point...
If the proportion or water in air is 0.6%, then the partial pressure of steam is 0.006 atmosphere...
so it will behave as if it was at the triple point, and there is an equilibrium...
sorry, I have vague memories of this theory....

[jake-the-peg]

The words "boiling point" do not well describe what happens

Think of it like this

The boiling point of something is actually the maximum temperature that you can heat something to before it becomes a gas.

At lower temperatures some of the molecules will break free and become a gas - because these are the "hotter" ones what's left is colder and that's why evaporating water (or sweat cools you down)

When you heat something to it's boiling point this happens more and more until the temperature stays the same and all the energy goes into the liquid becoming a gas - this is it's boiling point.

Hope that helps

[pompei595]

Thanks everyone - we had a heated (although not steaming) discussion around the dinner table this evening about all this - your contributions were well received

[newtron]

Evaporation also depends on relative humidity, which is a measure of how much water vapor is present in air compared the maximum amount of vapor that the air can hold at a specific temperature. If the relative humidity is 100% (air is saturated with water vapor), then liquid water and water vapor are in equilibrium, meaning that the rate at which liquid water is changing to vapor equals the rate at which water vapor is changing to liquid water, resulting in no net evaporation.