<P> In addition to the above - mentioned types of phase diagrams, there are thousands of other possible combinations . Some of the major features of phase diagrams include congruent points, where a solid phase transforms directly into a liquid . There is also the peritectoid, a point where two solid phases combine into one solid phase during cooling . The inverse of this, when one solid phase transforms into two solid phases during cooling, is called the eutectoid . </P> <P> A complex phase diagram of great technological importance is that of the iron--carbon system for less than 7% carbon (see steel). </P> <P> The x-axis of such a diagram represents the concentration variable of the mixture . As the mixtures are typically far from dilute and their density as a function of temperature is usually unknown, the preferred concentration measure is mole fraction . A volume - based measure like molarity would be inadvisable . </P> <P> Polymorphic and polyamorphic substances have multiple crystal or amorphous phases, which can be graphed in a similar fashion to solid, liquid, and gas phases . </P>

Temperature and pressure control the phase of a substance