Re: freezing point of water

Dear Tony and Mary,

I can think of three cases when water could be liquid below 32°F (0°C):

1. If it's not pure. Dissolving things like sugar or salt in water makes it hard for the water molecules to arrange neatly into a lattice. You then have to make the solution colder in order to freeze it. This is why they put salt on the roads to melt the ice in winter. But you probably anticipated that, since you asked about _pure_ water. So...

2. If the pressure is greater than atmospheric pressure. When you squeeze a substance, it will tend to change into a denser phase if it can. For water, the liquid phase is denser than the solid. (Ice cubes float in water.) So squeezing ice will tend to melt it, even if its temperature is several degrees below freezing. This is why you can ice skate--the pressure under the blade melts the ice, allowing you to glide on a film of liquid water. Water is a very weird substance in this regard; most substances freeze, rather than melt, when compressed. A good way of plotting this behavior is a phase diagram. It maps the pressures and temperatures for which water is solid, liquid, or gas. From the boundaries one can read the melting point and boiling point at any pressure. Check out the excellent description and pictures at The University of Florida. (Scroll down to the very bottom of the page.)

3. If it's very pure (temporarily). When you cool water below its freezing point, it wants to form ice. But it needs someplace to start, or nucleate, the growth of ice crystals. Usually there are dust particles in the water and rough spots on the container walls that can serve as a starting point. However, if you filter out all the dust and use a smooth-walled container, there is no place for the ice to start growing. In this case you can keep the water liquid below the freezing point. This state is called a "supercooled liquid". It only lasts for a little while. Eventually, some water molecules bump into each other in just the right way to make ice, and then the whole pot will freeze at once. See also these pages from Earth and Sky and this discussion forum.

Hope this resolves your questions. Thank you for asking!

Deron Walters
Rice University
Houston, Texas