|MadSci Network: Chemistry|
Before discussing water in particular, let's briefly go over density in
general terms. Density is the mass per unit volume, which means how many
molecules of a substance can be found in a specific space. Something is
described as "less dense" if fewer molecules are present in a given area.
One point to keep in mind is that when heat is added to a material, its
molecule move faster and faster until eventually, as in the case of liquid
water, a phase change occurs forming steam. Because this steam has
significantly more heat than water, its molecules move faster and are
located further apart. As a result less water molecules can be found in a
specific space and the steam is LESS dense than water.
Now, when water is cooled, its molecules slow down until solid ice is formed. Logically, it would seem that this solid ice (with slower moving molecules) should be more dense. However, surprisingly, ice is LESS dense than water. This effect is caused by the charge polarity of water molecules, which, upon being slowed down orient themselves is a particular pattern. This pattern causes each molecule to be slightly further apart from its neighboring molecule than in the case of water. As a result, in ice, fewer water molecules are found in a given area and it is LESS dense.
This unusual phenomena allows life to exist on Earth. In the winter, when water freezes, the less dense ice floats to the top, insulating the deeper waters. Were this not the case, the water would freeze, sink to the bottom, and the new top layer would freeze and also sink. Eventually, the entire lake would be completely frozen.
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