The temperature at which a solid substance undergoes fusion, i.e. melts, changes from solid to liquid form. All substances have their characteristic melting points. For very pure substances the temperature range over which the process of fusion occurs is very small. The melting point of a pure crystalline solid is a process of pressure. It increases with increasing pressure for most substances. However in the case of ice (and a few other substances) the melting point decreases with increasing pressure. Under a pressure of one standard atmosphere, the melting point of pure ice is the same as the ice point, that is 0°C or 32°F.