Pure substance (one component) phase diagram in equlibrium

The properties of a given substance depend on the state functions: pressure and temperature. For example water can be ice, liquid water and water vapour, so there is 3 matter of state: solid, liquid and gas.

We can easily plot these different states on a temperature-pressure diagram, where the borders can be seen between the different matter of states. There are two interesting points on this diagram, one where the border lines cross each other, called triple point and the critical point where the phase boundary disappears between liquid and gas phase. Furthermore there are some exciting video links on the topic.