Freezing water is an example of a phase transition -- a change in the physical properties of a substance when the temperature or pressure are changed. Phase transitions are often accompanied by either the absorption or release of thermal energy.
Water molecules have electric dipole moments -- the oxygen atoms are more negatively charged than the hydrogen atoms, and the molecule is in a bent shape, with hydrogen atoms not quite on opposite sides of the oxygen. This means that water molecules strongly attract each other electrostatically (opposite charges attract each other). If there isn't too much random motion of the molecules (that is, the water isn't too hot), then the molecules prefer to line up in an orderly fashion, with the positively-charged part of one molecule next to the negatively-charged part of another molecule and so on, held together in a rigid crystal. If the molecules have more thermal energy, they shake around and break free of their neighbors. They still like to stick to one another, but because they are moving so much, they constantly change their neighbors and bounce off of each other. This is the liquid phase.
Ice is actually less symmetrical than liquid water. In the liquid state, all directions appear the same, and all places in the liquid have the same properties as all others. Not so with ice -- the crystals point in definite directions over long ranges (you may still need a microscope to see them well for a snowflake, for example, but the crystals still are very large compared to the molecular-scale randomness in liquid water). Freezing transitions of substances involve a reduction in their spatial symmetry, or otherwise said, an increase in how orderly the molecules are arranged.
When water freezes, the molecules give up some of their energy to their environment (by conduction or radiation, helped on macroscopic scales by convection, of course) and slow down. They begin to stick to each other, and as permanent bonds form, additional energy is released (it takes energy to pull the molecules apart, and you get the energy back when you let them stick to each other). The amount of energy released is 80 calories per gram of water when it freezes.
A curious thing about water: liquid water with a temperature close to the freezing point is actually more dense than ice, due to the fact that the crystalline arrangement of water molecules in ice is not the closest packing possible because of the shape of the molecules. You can even melt ice under some circumstances by exerting pressure on it
. I suppose it also goes in the opposite direction -- water may re-freeze once the pressure is gone. But of course, as always, the 80 calories per gram must be added when the ice melts and removed when it re-freezes.
If you dissolve something in the water then the freezing point of the solution will be lower than for the water alone. Salt can be used to lower the melting point in many practical situations
. The reason for this is that the salt, actually, sodium and chloride ions in solution, "gets in the way" of the water's ability to make a nice, orderly crystal, so when the ice crystal forms, almost all the salt gets left behind in the liquid. That means that freezing the water not only lines up the water molecules but also limits the room the salt ions have for moving around. That makes it harder to freeze. Other additions to the water will have a similar effect. You can try experimenting with sugar, for example, to see how the concentration affects the freezing point of water. Some substances won't dissolve in water (like oil) and won't have an effect on the freezing point, although a coating of oil on top of the water may have an effect on the rate at which heat flows into and out of a container of water.
(published on 10/22/2007)