Melting point

The melting point of a solid is the temperature at which it changes state from solid to liquid.

When considered as the temperature of the reverse change, it is referred to as the freezing point. For example, the melting point of the element mercury is 234.32 kelvins (−38.83 °C or −37.89 °F). Certain materials, such as glass, may harden without crystalizing; this is called an amorphous solid.

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

The material with the highest known melting point at atmospheric pressure is graphite, with a melting point of 3948 kelvins.

Melting point is often used to ascertain purity of and characterise organic compounds. The melting point of a pure substance is always higher than the melting point of an impure sample of that particular substance. When two chemical substances are mixed, the melting point of the resultant mixture will be lower than the melting point of either constituent. The mixing ratio that results in the lowest possible melting point is known as the eutectic point.

DNA analysis

When referring to oligonucleotides or DNA, the melting temperature T_m refers to the temperature where 50% of the DNA is in a duplex form. In other words, 50% of the DNA has been denatured into single strands.

See also

• phases of matter
• triple point
• Freezing-point depression