What will happen to water if you change the atmospheric pressure?

This is an interesting one since water is one of the very few substances which actually gains in volume as it freezes (due to the way the water molecules are arranged in ice). Atmospheric pressure has little effect on melting points due to the small volume change as solids melt (it affects boiling points much more - hence pressure cookers). Anyway in the case of water an increase in atmospheric pressure should encourage ice to melt i.e. lower the melting point. And a decrease would cause the melting point to increase.

Hope this helps.