Melting point is the temperature at which a substance changes from a solid to a liquid. The melting points of different substances vary considerably. For example, tungsten has an extremely high melting point, 3422 °C, but solid hydrogen melts at the low temperature of -259 °C. The melting point of a material depends partly on whether the material is a pure substance or a mixture. A pure substance is either a pure element, such as iron, or a simple compound, such as water. A mixture consists of two or more substances that are not chemically combined.
A pure substance melts at a definite temperature or within a narrow temperature range. For example, when iron is heated, its temperature rises until the metal reaches its melting point of 1535 °C. The iron remains at that temperature until all the metal has melted.
Mixtures do not melt at a specific temperature. Simple mixtures, such as brass and steel, melt over a range of temperatures. For example, steel, which is a mixture of iron and other elements, has a melting point of 1400 °C to 1500 °C. Thus, the temperature of steel rises 100 °C during the melting process instead of remaining constant. Such complex mixtures as glass and tar do not melt over a specific temperature range. Instead, these substances gradually become softer and more fluid as their temperature increases.
The melting point of most simple mixtures differs from that of any of the pure substances in the mixtures. Brass, an alloy of copper and zinc, melts over a range of 900 °C to 1000 °C. However, the melting point of copper is 1083.4 °C, and that of zinc is 419.58 °C.
The melting point of a substance is affected by atmospheric pressure. An increase in this pressure raises the melting point of most substances. But an increase in atmospheric pressure lowers the melting point of water and of the few other substances that expand when they freeze. All freezing points specified in this article are based on a pressure of 1 atmosphere (14.696 pounds per square inch [101.325 kilopascals]), the pressure of the atmosphere at average sea level.
The liquid form of a pure substance freezes at the same temperature at which its solid form melts. Thus, the solid and liquid forms can exist together at the melting point without any temperature change in either. For example, if any amount of ice whose temperature is 0 °C, the melting point of ice, is added to any amount of water whose temperature is also 0 °C, the temperature of the resulting ice water will remain 0 °C. Furthermore, the relative proportions of ice and water will remain the same unless heat is added to the ice water or removed from it. However, if heat is added, the ice will begin to melt. If heat is removed, the water will begin to freeze.
The solid and liquid forms of a mixture can exist together over a range of temperatures. This range is determined by the type and amount of each of the pure substances in the mixture.
Some pure solids do not melt when heated. Instead, they change from a solid to a gas. Such substances as arsenic, dry ice, and iodine go through this process, which is called sublimation.
