Phase Diagrams

Phase diagrams give the state of a substance (solid, liquid, gas) given a specific set of conditions such as temperature and pressure. Below is an example of a simple phase diagram for water.¹

It is well known that water exists as a solid at or below its freezing point of 0 °C. However, this is not necessarily true as it depends on the external pressure. The phase diagram (above) indicates that water can be a gas (at 0 °C) at very low pressures.