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Why is water's volume smallest at 4 Celsius?


At temperatures above absolute zero (0 K, -273 C), all molecules are in incessant random motion. Such movements rather interfere with their ability to hold on to each other. For example, at the boiling point of water (100 C) the molecules are able to tear away from each other and move from a liquid phase into the gaseous phase that you recognize as vapor.

As the temperature of liquid water is reduced, random movements of the water molecules are likewise reduced. At 4 C, molecular motion has been diminished to the point where the hydrogen bonds that held them together in the liquid phase are beginning to settle into an optimum (best) arrangement that will ultimately bind them together in the ice latticework at 0 C. From about 4 C to water's freezing point at 0 C, the molecules are no longer able to so easily slip past each other as they did in the liquid phase. They begin to really "feel" the intermolecular attraction of hydrogen bonding which occurs between the hydrogen atoms of one water molecule and the oxygen atoms of nearby molecules. Thus, they begin to take on an orderly crystalline arrangement that we recognize as ice. The ice latticework simply takes up more space than the slightly more compact and disordered liquid state. Water expands when it freezes.

Water is unusual in this regard. Most substances shrink when they pass from the liquid to solid state.

Check introductory chemistry textbooks under the heading of "hydrogen bond" for a more detailed explanation.


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