Saturday, September 1, 2012

Why does liquid water have a relatively high boiling point?

The graph below shows the boiling points of different molecular liquids of the chemical composition HnX. Note that at room temperature all of them, except H2O would be vapour.
What is so different about water?
It can form much stronger hydrogen bonds than the heavier ones, because X=O is the most electronegative.
H2O has two proton donors and two lone pair orbitals which act as acceptors for hydrogen bonds. Hence, each molecule can form four hydrogen bonds in a tetrahedrally co-ordinated liquid.
In contrast NH3 has three donors and one acceptor. Hence, it can only form two hydrogen bonds with neighbours.

This can all be illustrated with some very cute pictures of little men grabbing hands and legs in Philip Ball's book H2O: a biography of water.
I first saw all this nicely explained by Jim Skinner a few months ago in a Town Talk at the Telluride Science Research Center in Colorado.

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