2012: Advent Chemistry: Water

by on December 9, 2012

A drawing of a water molecule, with the length of the bonds and the angle between them shown

That up there is a water molecule.

Now, I know you think water isn’t very interesting, but bear with me. There’s a reason I like water, and it’s not just because it makes me live. It’s a really interesting and unusual chemical.

Water is extremely polar – extremely magnet-like. The oxygen molecule is very negative and the hydrogens are very positive. That’s quite normal. But water is also very small, and the polarity of the oxygen-hydrogen bond isn’t countered by any other atoms. What that adds up to mean is that water doesn’t behave like other liquids.

Water has a very high melting and boiling point, much higher than it should. Ethanol, which we’ve looked at already, is basically water with an ethyl (CH2CH3) group instead of one of the hydrogens, and is therefore much heavier. Being heavier, it ought to have higher melting and boiling points – but ethanol, as you probably know, boils before water does, and freezes later, which is why freeze distillation works.

So why does water boil at such a high temperature? Boiling and melting points are controlled by two things: the weight of the molecules, and how strongly the molecules stick together. Water’s very light, but it clings to itself with extraordinary strength. That clinging is down to the polarity again, but in particular, to the fact that the polar atoms involved are hydrogen and oxygen.

Oxygen has something called “lone pairs”, pairs of electrons that aren’t involved in any bonding. Hydrogen, when it’s bonded to something as electronegative as oxygen, has almost no electrons. So the spare electrons on oxygen and the very positive hydrogen form a bond. One molecule of water is bound to all those around it by these ‘hydrogen bonds’. They aren’t as strong as a covalent bond, but they’re much stronger than ordinary intermolecular interactions. So water is very strongly bound together, and doesn’t boil very easily.

When water freezes, something even more unusual happens. Everything else, both elements and compounds, gets denser when it freezes. Not so water. Water ice floats, and that’s very weird. The solid form is less dense – weighs less for the same volume – than the liquid form. Downright bizarre.

It’s down to those hydrogen bonds again. They change the shape of frozen water to a more structured form, almost like an ionic lattice, and that structured form takes up more space.

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