Melting Point, the temperature at which a substance passes from the solid to the liquid state. Each chemical element has a specific melting point. The melting points of the elements range from —453.5 F. (-269.7 C.) for helium to 6,740.6 F. (3,727 C.) for carbon (graphite). Chemical compounds also have specific melting points, but some compounds decompose before the necessary temperature is reached.
Mixtures of substances, such as butter or paraffin, do not have specific melting points, but melt within a range of temperatures. Each substance in a mixture retains its own melting point. Since the substances with lower melting points melt before those with higher melting points, a mixture tends to become soft before changing into a liquid.
The freezing point, or the temperature at which liquids become solids, occurs at the same temperature as the melting point. The difference between the two is that the temperature is rising when the melting point is reached, and falling when the freezing point is reached.
A substance that is melting stays at the same temperature (its melting point) as long as any of it remains in the solid state. For example, if ice is melted in a glass, both the ice and the water stay at 32 F. (0 C.) until all of the ice is melted. As soon as there is only water in the glass, the temperature can begin to rise.
Pressure affects melting points. Most substances expand as they melt. An increase of pressure retards the melting of these substances. For such substances, increasing the pressure raises the melting point. A few substances, ice for example, contract as they melt. An increase of pressure makes it easier for the substance to melt, and therefore lowers the melting point.
For the melting points of specific chemical elements, see articles on the elements in question.