Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: avoiding evaporation

Learn more physics!

Most recent answer: 10/22/2007
Q:
I have a two-part question I would really appreciate some help with.

Background: my goal is to preserve a liquid solution as close to freezing as possible without actually converting it into its solid state. My thought is that keeping the liquid as cold as possible, and thus reducing its molecular movement as much as possible, without actually freezing it solid, will help to preserve its volatile components.

My first question is: What would better allow the liquid to be kept at a temperature at or below its normal freezing point without freezing it, a partial vacuum or added pressure?

And, if you successfully kept a liquid at a temperature below its normal freezing point, would the molecular movement be less than or equal to that same liquid under normal pressure? For example, water freezes at 0C under normal pressure, and water freezes at 3C (for the sake of argument) with increased pressure of XX. What would have the greater molecular activity: water under normal pressure at 1C, or water under an increased pressure of XX at 2C, both samples being 1C away from freezing point?

Thanks in advance.
- steve (age 23)
san francisco
A:
Most liquids contract when they freeze, so squeezing them (applying pressure) tends to make them freeze. Water is unusual because it expands a little when it freezes. Applying pressure to ice tends slightly to melt it.

Generally, the moleular motions will be reduced by cooling. Because the formation of ice is what's called a 'first-order phase transition' the properties of the liquid water are not very sensitive to how close it is to the freezing point.

But your ultimate goal seems to be to avoid losing volatile solutes without freezing the solution. For that it seems most imporatnt simply to keep the solution sealed up as much as possible. Reducing the pressure, say with a vacuum pump, sounds like it would be very counterproductive.

Mike W.

(published on 10/22/2007)

Follow-up on this answer.