The melting point is a temperature at which the solid and liquid phases of a substance are in equilibrium at a specified pressure (normally taken to be atmospheric unless stated otherwise). A pure substance under standard condition of pressure has a single reproducible melting point; the melting point rises slightly with pressure and chemical purity. The terms melting point and freezing point are often used interchangeably, depending on whether the substance is being heated or cooled, although they are not in fact always the same.