Water As A Solid

As water loses its heat and the molecules move more slowly another form of water begins to appear and the water freezes into a solid. The individual molecules lock up into an interlocking crystalline grid of six-sided rings. These ring structures are a bit different from the ones found in cool water. They are more stable - instead of continually forming and reforming as they do in the liquid, the hydrogen bonds of the crystal are more abundant and stable and the water molecules are more locked in place.

Pressure exerted by hydrogen bonding with adjacent molecules actually forces the angle between the hydrogen atoms of the water molecule apart slightly - it is 109^o 28' instead of 104^o 31'. In this crystalline configuration virtually all of the molecules are hydrogen bonded to adjacent molecules.

The diagram below tries to illustrate the molecular structure of the ice crystal: the oxygen atoms are blue-grey, the hydrogen atoms and their bonds to the oxygen are red, the hydrogen bonds are grey. The size of the electron clouds around each atom has been reduced to allow you to see through the crystal matrix.

As you go around a ring of molecules in the crystal you'll see that each water molecule alternates with the adjacent ones in sticking up slightly higher or slightly down from the plane of the hexagon. Also notice that each water molecule is also a part of an adjacent hexagon. The whole linkage of hexagons spreads out in a bumpy layer, sort of like a sheet of bubble-wrap.

Each of the sheets of hexagons is also hydrogen bonded to parallel sheets of hexagons that lie above and below it. Note that the sheets do not nestle together like spoons - instead a "down" molecule from one sheet is opposite (and hydrogen bonded to) an "up" molecule in the sheet below it. These hydrogen bonds attach the adjacent layers to each other.

In this crystalline structure there is a good bit of molecular space between the molecules when compared to liquid water. Therefore ice is less dense than water and floats. To be precise, at 0^o C the density of ice is 0.9170 grams/cubic centimeter and that of water is 0.9999 grams/cubic centimeter. The ice weighs only about 92% as much as the water, so about 8% of the ice floats above the water surface.

When water changes from a liquid to a solid it loses heat; conversely when it changes back to a liquid it needs to regain heat. This heat is called the latent heat of fusion (or melting). In water it equals 80 calories per gram. Compared to other chemical compounds this is very high (exceeded only by ammonium). And this high latent heat of fusion has some very important consequences.

Because of it the mixture of water and ice on the earth's surface acts as a temperature buffer. During the fall and winter the northern hemisphere loses heat - and the temerature goes down. But when ice starts to form part of the loss from the earth is replaced by the heat released from water as it turns to snow and ice. If this were not so the temperature would go down much further and faster than it does. The reverse is true in the spring and summer. The northern hemisphere gains heat, but the temperature doesn't go up as much as it would if part of the heat were not used in melting the ice. In this way temperature changes are buffered.

The same ice-water temperature buffering system applies to the global warming problem. Because of the increased levels of carbon dioxide in the atmosphere the earth is retaining more heat energy than before. If it were not for part of the energy being sucked up to melt ice on the polar caps the temperature would be rising more rapidly than it is.