We’ve all done the experiment. Take an aluminium tray divided into
sections, and fill with water. Place in freezer and wait a few hours. Remove
and tap firmly on a hard surface. Out fall perfectly formed ice cubes.
As you put your feet up and sip your Scotch on the rocks, do you ever
wonder how the tray of water becomes transformed into a solid block? Clearly,
it only takes place at subzero temperatures. And if you remove the tray
from the freezer too soon you’ll find, not a growing cube, but a skin of
ice with liquid water below it. During freezing, this boundary moves through
the liquid – it is at this boundary that the ice grows.
But any closer investigation of ice and its remarkable properties has
to start with water molecules – each with one oxygen atom joined to two
hydrogen atoms. Of the two types of atom, oxygen has more pull on the electrons
that they share to form the bonds between them, so the oxygen becomes slightly
negatively charged and the hydrogens slightly positive. Opposite charges
attract, so water molecules tend to stick to each other as a positive hydrogen
atom of one molecule attaches itself to the oxygen atom of its neighbour.
These bonds between molecules, which are called hydrogen bonds, dominate
the properties of water and ice. In water, the molecules jostle around,
continually breaking and making hydrogen bonds – although according to
a theoretical model developed last year by Sydney Benson and Eleanor Siebert
of the University of Southern California, Los Angeles, they like to form
short-lived networks of cubes and rings along the …
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