Definition and properties
Melting point is a characteristic temperature at which a substance changes from solid to liquid state. At this temperature, the solid and liquid occur in equilibrium. A typical example is ice, which melts at 0 °C and becomes liquid water. The melting point is slightly dependent on atmospheric pressure, which is why a pressure specification is required. It can be influenced by the addition of other substances and by impurities. For example, the melting point of ice is lowered by sodium chloride or ethanol. This is referred to as a lowering of the melting point. Many substances decompose or burn when heated, for example organic molecules such as sugar (sucrose). Some also pass directly into the gas phase (sublimation). And mixtures of substances such as natural substances (e.g. waxes, fats) melt in a certain temperature range and not at a defined temperature. The melting point often, but not always, coincides with the freezing point (solidification point). Some liquids can be supercooled and, for example, solidify in seconds on contact with a crystallization nucleus (“supercooling”). The melting point is higher the stronger the bonding forces between the particles or molecules in the solid. For example, sodium chloride (table salt) has a high melting point of 801 °C due to the strong ionic bonds in the crystal. Water has a lower melting point because the strength of the intermolecular interactions (hydrogen bonds) is lower.
Applications
Melting point is used in analytical applications, identification, characterization, and quality assurance, among others.
Examples
Melting point of selected substances at normal pressure:
- Tungsten (W): 3422 °C
- Iron (Fe): 1538 °C
- Copper (Cu): 1084 °C
- Gold (Au): 1064 °C
- Sodium chloride (NaCl): 801 °C
- Aluminum (Al): 660 °C
- Tin (Sn): 231 °C
- Sodium (Na): 98 °C
- Beeswax: approx. 61 °C
- Phenol: 40.8 °C
- Cocoa butter: approx. 35 °C
- Coconut fat: approx. 27 °C
- Dimethyl sulfoxide (DMSO): 18 °C
- Water (H2O): 0 °C
- Mercury (Hg): -38 °C
- Ethanol (EtOH): -114 °C
- Hydrogen (H): -259 °C
