Almost all elements as well as compounds (read: virtually all materials) contract when they freeze or turn from a liquid to a solid. Another behavior exibited by <almost> everything is that when materials do solidify from the liquid, they do so from the outside- in. A good example of this is welding; welding can only work because the weld puddle solidifies or 'freezes' from the outside- in, if it did not, the last part of a weld to solidify would be where the weld joins the parent metal and due to size change, it would effectively crack and fall out. Put simply, welding would not work. Luck for us that it does freeze from the outside in, and shrinks as it solidifies so that the weld bead causes all tensile (pulling) stresses in the weld, rather than expanding outwards, which would again cause a weld to fail the moment it solidified.
In the same vein, most materials expand as they are heated.
But there are exceptions, exactly two that I know of: Plutonium and Water. We need not concern ourselves with plutonium too much, mostly 'cause we are not likely to run into any of the stuff in a normal lifetime and even if we did, it is improbably that almost anyone would want to melt the stuff anyway. Water is another matter through and again, lucky for 'us' (read; everything that lives on Earth), it does not follow the rules. Oh sure, water contracts as it gets cold but then when it starts to freeze, it goes through a sudden and significant expansion..... this is why water based liquids in glass containers will break the container if allowed to freeze. So water shrinks as it cools but then suddenly reverses this process and significantly expands as it solidifies. This has a number of odd results, such as water has two points in temperature where it will be the same density, one close to boiling and one at freezing which basically makes no sense at all. Well, it does with a little more information: water forms a crystalline structure when it is a solid (ice), and to 'form up' in that way, the molecules of water actually have to move apart a bit to align with each other and form that crystalline structure. So once slightly below boiling temp. water continues to contract until reaching about 4 degrees C, at which point it begins to expand again as the temperature drops. This is because by 4C, there are enough ice crystals in water to overcome the waters' natural contraction and start to cause the entire quantity to expand, on average, as more and more ice crystals appear IN the water but before it is all frozen.
This has one other really great benefit, at least one that I really like: the reduced density of solid water vs. liquid water causes it to solidify (freeze) from the top down, rather than the bottom up like virtually all, or all other materials. This is why a pond freezes on top and yet leaves the great majority of the pond in the form of water, remember, virtually all other materials would freeze from the bottom up. The big benefit of this is that it allows life to exist wherever there is water. See, you like that too, right?
All food cycles start deep in the water on this planet, with plankton and microscopic organisms living on the bottoms of the world's bodies of water, whether sweet water or salt water. Now, if water froze from the bottom up as it <should>, the first mild freeze of any body of water would kill off these small, simple forms of life, and they are the ones that start the food chain that feeds everything else on the planet. So if water was not such an odd- ball and behaved all wrong, life may have started on Earth but a few freezing cycles would have wiped it out long before those simple forms of life even had a chance to grow in size and complexity. Nothing that moves of its own force anywhere on Earth would have evolved.
Or so science would have us believe.
Yep, Dihydrogen Monoxide: good stuff, no matter what the nay- sayers would have you believe!
http://www.dhmo.org/truth/Dihydrogen-Monoxide.htmlBrian