phase diagram, graph that shows the relation between the solid, liquid, and gaseous states of a substance (see states of matter) as a function of the temperature and pressure. The graph is divided into three regions, one for each of the physical states, and it specifies the range of temperatures at which the substance exists in each state for any value of the pressure. For example, a phase diagram for water shows that at a pressure of 1 atmosphere water is a solid up to a temperature of 0°C, a liquid from 0°C to 100°C, and a gas above 100°C. At a pressure of 0.5 atmospheres, the graph shows that although the melting point of ice remains 0°C, the boiling point of water is lowered to 82°C. Each substance has its own phase diagram, which must be determined experimentally. The border between two regions on the graph represents an equilibrium state, such as a melting point or boiling point, at which two states can coexist (see chemical equilibrium). The point at which all three regions meet is called the triple point; at these conditions of temperature and pressure, the solid, liquid, and gaseous states can coexist in equilibrium. The triple-point conditions for water are a temperature of .01°C and a pressure of .006 atmospheres.
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