Melting point

The melting point of a crystalline solid is the temperature at which it changes state from solid to liquid. When considered as the temperature of the reverse change from liquid to solid, it is referred to as the freezing point.

For most substances, melting and freezing points are equal. For example, the melting point and freezing point of the element mercury is 234.32 kelvins (−38.83 °C or −37.89 °F). However, certain substances possess differing solid-liquid transition temperatures. For example, agar melts at 85°C (185°F) and solidifies from 32 to 40°C (89.6 to 104°F); this process is known as hysteresis.

Unlike the boiling point, the melting point is relatively insensitive to pressure.

The melting point of water is 0°C (32°F, 273 K). In the presence of nucleating substances the freezing point of water is the same as the melting point. But in the absence of nucleators water will supercool to −42°C (−43.6°F, 231 K) before freezing.

Melting points are often used to ascertain the purity of and to characterise organic compounds. The melting point of a pure substance is always higher than the melting point of that substance when a small amount of an impurity is present. The more impurity is present, the lower the melting point. Eventually, a minimum melting point will be reached. The mixing ratio that results in the lowest possible melting point is known as the eutectic point.

The chemical element with the highest melting point is carbon, (4300–4700 K) (4027–4427 °C,7280–8000 °F).

Note: Certain materials, such as glass, may harden without crystallizing; this is called an amorphous solid.

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