Originally posted by michaelab
A strange fact about water is that it's at its densest at 4 deg C. Don't know why. The fact that it expands when it freezes is also odd.
Water is not as boring as it would seem.
Michael.
Dihydrogen monoxide is actually one of the most bizarre substances in the universe. It simply doesn't behave how it should. Consider its relative in the Periodic Table, hydrogen sulphide, "rotten egg gas" - it's a gas, even though sulphur has a higher atomic weight than oxygen.
Much of the secret lies in the molecule itself. It's extremely polar, which means that it tends to like its own company. It goes in for hydrogen bonding in a big way. Now, while hydrogen bonding is nowhere near as strong as covalent bonding (which holds molecules together), it still exerts a force that is far from negligible. That hydrogen bonding makes water a liquid when all the rules suggest it should be a gas. The hydrogen bonding seems to like groups of six, which reflects (sort of) on the six-fold symmetry frequently noted in the frost patterns on windows.
Water also has the highest latent heat of fusion and latent heat of evaporation of any known liquid, that is, the energy needed to change the state of the stuff, from ice at 0°C to liquid water at 0°C, and from liquid water at 100°C to steam at 100°C is very high. In other words, it's not that easy to turn water into ice at one end or steam at the other, so it remains a liquid over a considerable range of temperatures.
The bizarre behaviour is fundamental to the existence of life as we know it. Because the maximum density is reached at a temperature above freezing point, it means that ponds freeze from the surface downwards, not from the bottom up, making life in water possible all year round, even in the coldest places. The polarity also makes it a very good solvent for all sorts of natural things and the perfect transport medium for materials in cells.
So, next time you quaff a glass of the stuff, think how magnificent it is and say thank you to it, because without it, you wouldn't be here and neither would your hi-fi.
P.S. So firmly stuck in gasbag mode that I forgot to aswer the question, why does water expand on freezing? It's that hydrogen bonding again, plus the fact that the molecule is a particular shape, with its hydrogen-oxygen bonds sticking out behind it, making an angle of 104.5 degrees with each other under normal circumstances. When water reaches 4°C, the molecules have been pushed as close to one another as they will be. Below 4°C, the water molecules begin to align themselves into the crystal structure of ice. This requires the water molecules to widen the angle between the oxygen- hydrogen bonds from the usual 104.5 degrees. When this happens the water molecules take up more space, hence expansion.