Let鈥檚 start at the very beginning. As pure water is cooled to 0oC, it begins to form ice. This simply means that the H2O molecules which were randomly scurrying about in the liquid state now assume an orderly pattern and begin to form crystals. Any substance that is dissolved in the water will interfere with this crystallization by getting in-between the water molecules and disrupting the ordered arrangement that characterizes crystal structure. Interestingly enough, the nature of the dissolved substance is not important, the extent of freezing point lowering is determined only by the number of dissolved molecules or ions. Salt, sugar and alcohol are all very effective at lowering the freezing point. Commercial antifreezes which are mixtures of water and methanol or water and ethylene glycol will not freeze until about -40o.
But let's get one thing straight. While salt certainly lowers the freezing point of water, it does not melt ice! Only heat can do that. So why do we throw salt on our driveway? Because salt is hygroscopic. In other words, it has a very strong attraction for water. We've all seen what humidity can do to the salt in a salt shaker. When we apply salt to ice, it begins to tug at the water molecules which make up the ice crystal. As these molecules are liberated, they are absorbed by the salt, which being soluble in water, slowly dissolves and forms a salt solution. But a salt solution has a lower freezing point than water, so unless it is really cold, it will not refreeze. Even if it does, the result is a non-slippery slush. The salt did not melt the ice, it dissolved it! Calcium chloride is even more hygroscopic than rock salt, therefore it will dissolve ice more quickly. A real ice buster!
One of the most interesting properties of ice is that it is less dense than water and therefore floats. This is very important, otherwise lakes and rivers would freeze solid, destroying all aquatic life. But the fact that water occupies more space when it is in the form of a solid than in the form of a liquid can have catastrophic consequences. The sinking of the Titanic is one example. The bursting of our water pipes is another.
The force exerted by the expansion of water as it forms ice is tremendous. I remember seeing an educational film when I was a student which featured a classic demonstration of this phenomenon. A steel bomb made of roughly half an inch thick metal was filled with water and was then closed with a tightly fitting screw cap. The contraption was immersed in liquid nitrogen to freeze the water inside. Before long there was a tremendous bang as the steel container cracked open!
That brings up the question of frozen water pipes, every house owner鈥檚 nightmare. Surprisingly, water is more likely to freeze in hot water pipes than I cold water ones. Why would the hot water pipes freeze first? There are a couple of reasons. Gases, such as oxygen, nitrogen and carbon dioxide are more soluble in cold water than in hot water. Since dissolved gases can depress the freezing point, cold water has a lower freezing point than hot water. Furthermore, as cold water freezes, it releases its dissolved gases which form an insulating layer in the pipe. Hot water has previously had all the dissolved gases driven out by heat and therefore cannot benefit from this type of insulation. I should point out that there are numerous other theories that purport to account for this effect and that there is even some controversy about whether hot water pipes really do freeze first. I hope I never have to make any observations along these lines.
There is certainly plenty of academic interest when it comes to water freezing. The formation of ice crystals and the resulting tissue damage they can cause due to expansion presently precludes the freezing of human organs. If this problem could be overcome, large scale organ banks may be possible. The American Cryonics Society seems to be unaware of this particular problem as they promote the freezing of dead bodies with the hope that at some distant time in the future they can be thawed out and revived. In fact, they maintain that it isn't even necessary to freeze the whole body, the head alone will do. By the time of "reanimation," they hypothesize, science will have progressed so far that not only can all diseases be cured, but the body can be cloned from the preserved cells. Anyone with a good head on their shoulders should be extremely skeptical of this possibility.
If an answer to the problem of tissue damage due to ice is to be found, it may come from a study of fish. Fish are cold-blooded animals, meaning that they have the same internal temperature as their surroundings. Yet many species happily live in icy waters. Why don't they freeze? Research has revealed that these fish produce and store certain proteins in their blood which have been termed "antifreeze" proteins. These not only lower the blood's freezing point to a temperature below that of sea water, but they also inhibit the formation of large ice crystals.
As soon as tiny ice crystals start to form, the antifreeze proteins coat their surface and prevent other water molecules from attaching. No large crystals of ice grow. Some preliminary experiments with rat livers indicate that the addition of such fish proteins may dramatically reduce tissue damage associated with freezing. A more imminent possibility is the use of antifreeze proteins in ice cream manufacture to prevent a phenomenon known as recrystallization. This happens when large ice crystals grow upon storage and produce the disturbing crunchiness of aging ice cream. An even more exciting possibility is the insertion of a gene which codes for the antifreeze protein into the genome of a plant to protect it from freezing.
After all this talk about freezing, would you believe that the formation of ice actually releases heat? Since it takes heat to melt ice, it stands to reason heat must be liberated when water freezes. And indeed it is. This principle is used by farmers who try to protect their crops from freezing by spraying them with water. The common misconception is that the layer of ice that forms acts as an insulating barrier against the cold. This is not so; it is the heat released by the water as it freezes that protects the crop.
There is yet another interesting property of ice: it sublimes. In other words, it can pass straight into the vapour form without going through a liquid state. Even without a thaw, a lot of the ice out there will sublime and disappear!
