The figure below shows an example of a phase diagram, which summarizes the effect of temperature and pressure on a substance in a closed container.
Every point in this diagram represents a possible combination of temperature and pressure for the system. The diagram is divided into three areas, which represent the solid, liquid, and gaseous states of the substance.
The figure below shows what happens when we draw a horizontal line across a phase diagram at a pressure of exactly 1 atm.
Point B in this phase diagram represents the only combination of temperature and pressure at which a pure substance can exist simultaneously as a solid, a liquid, and a gas. It is therefore called the triple point of the substance, and it represents the only point in the phase diagram in which all three states are in equilibrium.
Point C is the critical point of the substance, which is the highest temperature and pressure at which a gas and a liquid can coexist at equilibrium.
