The melting point is the temperature at which a substance changes from a solid to a liquid. At the melting point, the solid and liquid states both exist and are at equilibrium. Melting point is a physical property of matter.
Here is a look at the factors that affect melting point, how melting point differs from freezing point, and tables of melting point values of elements and other substances.
Factors That Affect Melting Point
Pressure is the primary factor affecting melting point. For this reason, melting points typically include pressure values. Substances with high melting points have strong intermolecular forces binding atoms or molecules together and consequently low vapor pressures. For example, water has a higher melting point than comparable compounds because hydrogen bonding helps ice maintain its structure. Ionic compounds generally have higher melting points than covalent compounds because ionic bonds are stronger than covalent bonds.
Difference Between Melting Point and Freezing Point
Freezing is the reverse process of melting where a substance changes state from a liquid to a solid. You might think melting point and freezing point are the same temperature. Usually, the two values are close enough that they are essentially the same. But, sometimes the freezing point is lower than the melting point because of supercooling. A supercooled liquid doesn’t solidify because it lacks nucleation sites that allow for crystal formation. Essentially, its liquid state is more stable than its solid state, even below its melting point.
Freezing point also depends on purity. Impure substances experience freezing point depression. Here again, freezing point can be lower than melting point.
Melting Points of the Elements
The element with the highest melting point is tungsten, with a melting point of 3,414 °C (6,177 °F; 3,687 K). Tungsten is a transition metal. Many references cite carbon as the element with the highest melting point (3642 °C, 6588 °F, 3915 K), but carbon actually sublimes from a solid directly to a gas at ordinary pressures. It is only at a liquid at high pressures (10 MPa or 99 atm). Under these extreme conditions, it’s estimated carbon has a melting point of 4,030–4,430 °C (7,290–8,010 °F; 4,300–4,700 K).
The element with the lowest melting point is helium, with a melting point of 0.95 K (−272.20 °C, −457.96 °F) at 2.5 MPa pressure. This is very near absolute zero. The metal with the lowest melting point is mercury, with a melting point of 234.3210 K (−38.8290 °C, −37.8922 °F). Mercury is a liquid at room temperature.
In general, metals tend to have high melting and boiling points. Nonmetals usually have relatively low melting and boiling points.
Table of Melting Point Values for Example Substances
The substance with the highest known melting point is tantalum hafnium carbide (Ta4HfC5) . Tantalum hafnium carbide is a refractory metal with a melting point of 4,215 K (3,942 °C; 7,127 °F). Computer models predict the alloy HfN0.38C0.51 has an even high melting point of around 4400 K.
|Chemical||Melting Point (K)|
|Helium||Will not melt at ordinary pressure|
|Carbon||Will not melting at ordinary pressure|
How Melting Point Is Measured
When a substance melts its solid changes into a liquid. The phase change is endothermic because chemical bonds absorb energy to break their rigid structure and change from solid to liquid. So, measuring melting point works one of two ways:
- Slowly raise the temperature of a solid and watch for liquid formation.
- Heat a material and monitor its black-body temperature with a pyrometer.
- Agte, C.; Alterthum, H. (1930). “Researches on Systems with Carbides at High Melting Point and Contributions to the Problem of Carbon Fusion”. Z. Tech. Phys. 11: 182–191.
- Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). CRC Press. ISBN 1439855110.
- Hong, Q.-J.; van de Walle, A. (2015). “Prediction of the material with highest known melting point from ab initio molecular dynamics calculations”. Phys. Rev. B. 92 (2): 020104(R). doi:10.1103/PhysRevB.92.020104
- Ramsay, J. A. (1949). “A new method of freezing-point determination for small quantities.” J. Exp. Biol. 26 (1): 57–64.