MadSci Network: Chemistry |
Cold water is denser than hot water for the same reason that cold gases are
denser than hot gases or cold solids are denser than hot solids. In hot
water, the molecules are bouncing off each other more vigorously, and need
more space than in cold water -- the result is that you have fewer
molecules per unit volume in something hot than in something cold.
This effect is much, much bigger for gases than for condensed phases (liquids or solids). A gas at room temperature will expand by about 0.3% for every 1° rise in temperature. A solid or liquid expands much less; mercury by about 0.01% and water by about 0.03%. But the expansion is significant: mechanical thermostats work by exploiting the different thermal expansion rates of two different (solid) metals. Incidentally, water reaches its maximum density at 4° C. This is because solid water (ice) has an open structure. In ice, water molecules are distributed in hexagons around empty regions. When the ice melts, the water molecules are able to approach each other more closely. This overcomes thermal expansion until the temperature rises enough (to, say, 5°). A Quicktime™ movie depicting the phase changes in water, at a molecular level, is available at molecules.com. Thanks to Professor Terry Helser, SUNY Oneonta, who gave the gist of this answer on the CHEMED discussion list.
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