Don’t take liquids for granted

2006-01-28 Drop-impact
Water is everywhere – we have been inundated with the stuff over the last few months. But maybe we should not take it for granted. In Britain water is everywhere but in the universe as whole liquids of any sort are extremely rare. And even on Earth, water is pretty much the only liquid around.

So we should not take water for granted. A prominent 20th century physicist called Victor Weisskopf suggested that if we gave the laws of physics to intelligent aliens from another universe, then they would be able to predict both gases and crystals, but would probably miss liquids altogether.

Liquids are a delicate, intermediate state. If molecules or atoms repel each other then at atmospheric pressure they expand into a gas. If atoms or molecules strongly attract each other they pull together and pack efficiently into a crystal. It is only if molecules or atoms attract each quite weakly but not too weakly that liquids form. This attraction energy should be about equal to the typical kinetic energy due to thermal motion, which is about 10-21 J per molecule. If the energy is a tenth of this or less you have a gas, and if it is ten times, or more, larger you have a crystal. Liquids only exist in a narrow window in between.

So apart from our very wet corner, there is little water in the universe. Of course as water is absolutely essential to all life on Earth, if the Earth was not wet we would not be around to complain about the floods.

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