[maccha]

My textbook gives a chart of vapor pressures for various temperatures of water.. I don't understand how there is vapor pressure at temperatures less than 100? Isn't that the only temp at which vapor is created?

[Borek]

No, there is always vapor over every liquid. If you leave water in an open vessel it dries out - that's because it was vaporizing all the time, even if the temperature was well below 100 deg C. If you take cold bottle from the freezer it becomes covered with mist - that's because there was water vapor present in the air, even if the temperature was well below 100 deg C.

At 100 deg C (and assuming 1 atm) pressure of vapor of water becomes identical with ambient pressure and water starts to boil. It doesn't mean there was no vapor present earlier.

[maccha]

Okay, thanks very much that really helps. I'm just a little confused still by why phase diagrams show that water will be completely liquid until reaching 100 deg C?

[Borek]

No, phase diagram doesn't show there is only liquid below 100 deg C, it shows that water can exist as a liquid below 100 deg C and can not exist as a liquid above (that is, for 1 atm). This is not the same as being "completely liquid".

[maccha]

Oh okay- so, let's say, if a sample of pure water was confined to a cylinder with a freely moving piston with weights, maintaining it at a pressure of 1 atm.. would any vapor exist until it reached 100 deg C if the cylinder is filled completely with water?

[Borek]

No, if you keep it pressurised at 1 atm there will be no gas phase until vapor pressure of the liquid becomes 1 atm.

[mbeychok]

Vapor pressures of water below 100 deg C simply means that the pressure of the surroundings is less than atmospheric pressure. Water will boil at an infinite number ot temperatures because there are an infinite number of environmental (surrounding) pressures ... some less than atmospheric pressure and some more than atmospheric pressure. This applies to all liquids.

Read these two articles in the Citizendium online encyclopedia:

   Click here ==> Vapor pressure
   Click here ==> Boiling point