Q & A: Water at 4 deg C

What is so special about water at 4 degrees celsius?
- Qihan and Wufan

4 degrees C turns out to be the temperature at which liquid water has the highest density. If you heat it or cool it, it will expand. The expansion of water when you cool it to lower temperatures is unusual, since most liquids contract when they're cooled.

An interesting consequence of this peculiar feature of water is that the temperature of water at the bottom of a lake in the winter is almost always 4 degrees C, since the densest water will settle to the bottom -- if it gets any colder or warmer, it will rise. Ice floats on top of lakes, preventing evaporation (and convection in the frozen layer), and lakes stay liquid underneath, allowing fish and other life to survive.

Mike W. and Tom J.

(published on 10/22/2007)

WHY DOES ICE EXPAND BELOW AND ABOVE 4 DEGREES CELSIUS? WHY AT THIS PARTICULAR TEMPERATURE? WHAT IS SO SPECIAL ABOUT 4 DEGREES?
- ROHAN
INDIA

Rohan- That’s a very nice question. I assume you are referring to liquid water, not ice, since 4°C is about the temperature (T) at which liquid water has a minimum volume, at atmospheric pressure.
The expansion of water at lower T results from the water molecules arranging themselves to minimize the energy of their interactions. For most molecules, that typically involves squeezing together a bit, but water happens to have a low-energy arrangement that’s rather open. When things cool, they settle into lower energy arrangements. At higher T, the molecules sample even higher energy states, and a majority of them are loosely packed, so the liquid expands- the more typical behavior.

I haven’t said why 4°C is special. Nothing really special happens there in the structure of the water. 4°C just happens to be the T at which the expansion, due to settling into the special low-energy states, and the contraction, due to the general trend to spend more time in contact at low T, cancel.

0°C is another matter. At that T, the tendency toward settling into a special low-energy structure runs away in a cooperative manner, and ice crystals form. Water samples just above and below 0°C are very different. Just above and below 4°C they are almost the same.

Mike W.

(published on 10/22/2007)