substance’s melting and boiling points. Reduced pressure makes it easier for
solids to melt and liquids to vaporize.
An Exception to the Rule
The exception to this rule is water. Increasing pressure on ice or water
lowers the freezing or melting point. One commonly cited example
illustrating the relationship between pressure and the freezing point of water
is ice-skating. The pressure of the blade of the skate pushing down on the
ice causes the ice to melt. The layer of water that results under the blade
allows the blade to slide along with little friction. Most substances tend to
want to freeze under greater pressure. Skating is really possible then only
on the few substances which share water’s property of tending to melt
under greater pressure.How come? Just imagine pressure as something that’s pushing down on the solid
or liquid, tending to prevent its molecules from moving around. If we increase
that downward push, melting and boiling are harder to achieve; melting and
boiling points, therefore, increase. If we reduce that downward push, melting
and boiling are easier to achieve; melting and boiling points, therefore, decrease.
Another Type of Phase Diagram
We mentioned that whether a particular substance is a solid, liquid, or gas
depends on both its pressure and temperature. The relationship among pressure,
temperature, and phase can be neatly shown in the following type of phase
diagram, which is a graph of pressure versus temperature:
Phase Diagram for Substance X