Fortuitous Properties of Cold Water

As cold water starts to get close to the freezing point, it behaves differently than most other substances in that it stops getting denser and actually gets less dense as it gets colder.  In this post, we’ll look at the variation of density with temperature, and talk about why it is lucky for all life on earth that it behaves this way.

Most liquids get denser as they get colder, and are more dense in the solid state than in the liquid state.  Pure water does get denser with decreasing temperature down to a point. 

However, at about 4 deg C, the trend reverses, and the density actually decreases with decreasing temperature.  Very pure water, if kept very still, can actually be cooled well below the usual freezing point without solidifying.  The decreasing density with decreasing temperature continues well into the supercooled region.  

Notice that at 0 deg C, the typical freezing point, the density of water has decreased by about 0.013% compared to 4 deg C.  On freezing into a solid, however, the density decreases by about 9%--an enormous additional decrease.

With the lower density of water near the freezing point, and the much lower density of ice, natural bodies of water tend to freeze from the surface downwards in cold weather.  Since both ice and liquid water are relatively good thermal insulators, large bodies of water rarely freeze clear to the bottom in even the coldest weather allowing many forms of aquatic life to survive the winter. Notice that if it weren’t for this unusual density behavior, ice would fill in from the bottom of large bodies of water, and except for a relatively thin layer of water that would melt every summer (and then insulate the ice below from further melting) most of the world’s water would be permanently locked up as ice.

more about water density:

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