In order for a phase change to occur, a certain temperature must be reached (at a certain pressure), and a certain amount of heat energy must be added of removed from the material. This is called the latent heat of a material. For instance, the latent heat of fusion (melting) of water is about 80 cal/g, and its fusion (melting) point is 0 deg. C. This means that water at 0 deg. C must have 80 cal per gram of substance taken away from it in order to freeze, or ice at 0 deg. C must have 80 cal per gram of substance added to it in order to melt. If the latent heat of the substance is not met, the substance will remain at its equilibrium temperature with proportional amounts of the two phases of matter the substance is changing to and from, depending on the heat energy added.