<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> <P> Some organic materials pass through intermediate states between solid and liquid; these states are called mesophases . Attention has been directed to mesophases because they enable display devices and have become commercially important through the so - called liquid - crystal technology . Phase diagrams are used to describe the occurrence of mesophases . </P>

When solid and liquid phases are in equilibrium