Asked by: Anonymous
Water has the highest density at 4°C. As you cool it below this temperature, the molecules begin to assemble into an ordered crystalline lattice that takes up more space and so the density drops.
If you put water in a very strong, rigid container and continue to cool it, the pressure will begin to rise as more and more molecules adopt the lattice formation and press against the remaining molecules still in the free liquid state. If the container doesn’t break, the pressure will rise very fast until eventually at around 200 megapascals (roughly 2000 Atmospheres), the atoms begin to rearrange again into a new, more compact configuration.
There are 13 known forms of ice that are stable at different temperatures and pressure. Ordinary ice is called ice Ih, whereas the most dense of the high pressure varieties is ice III. In an enclosed container, the expansion pressure will reach an equilibrium point and the water will freeze as a mixture of ice Ih and ice III.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.