Freezing and Boiling water at the same time

[AbsoluteZer0]

Hi,

As I understand, by placing water in a vacuum and decreasing pressure, vapor pressure will subsequently be increased causing the water to boil. The water will then evaporate, which will cool the water that is left behind. Further evaporation over a period of time will drop the water temperature to a level suitable for freezing, and the water will then freeze at the top.

It seems pretty clear-cut, but I am struggling to understand how decreasing atmospheric pressure increases vapor pressure. What is the cause of this?

Thanks,

 

[Borek]

I would not say "decreasing pressure increases pressure". If anything, low pressure means there is a "place" for water vapor, so water will happily boil. This boiling will increase the pressure at the expense of the water internal energy - that's why the water temperature drops down. But: this increased pressure is lower than the initial pressure.

 

[tiny-tim]

Hi AbsoluteZer0!

… I am struggling to understand how decreasing atmospheric pressure increases vapor pressure.

No it doesn't, the vapour pressure depends only on the temperature of the water …

at a particular temperature, the vapour pressure is fixed.

Lowering the atmospheric pressure simply enables the vapour pressure to "win", see http://en.wikipedia.org/wiki/Boiling …

While below the boiling point a liquid evaporates from its surface, at the boiling point vapor bubbles come from the bulk of the liquid. For this to be possible, the vapor pressure must be sufficiently high to win the atmospheric pressure, so that the bubbles can be "inflated".​

 

[AbsoluteZer0]

So essentially, when their is less atmospheric pressure it lowers the boiling point of water, so vapor pressure is able to cause boiling faster?

 

[tiny-tim]

So essentially, when their is less atmospheric pressure it lowers the boiling point of water, so vapor pressure is able to cause boiling faster?

Yes.