Melting point

Here is a look at the factors that affect the melting point, how the melting point differs from the freezing point, and tables of melting point values for elements and other substances.

Factors affecting the melting point

Pressure is the main factor affecting the melting point. For this reason, melting points often include pressure values. Substances with high melting points have strong intermolecular forces that bind atoms or molecules together and consequently low vapour pressures. For example, water has a higher melting point than comparable compounds because the intermolecular bonds between atoms or molecules are stronger than those between atoms or molecules. hydrogen bonds help ice to 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 liquid to solid. You might think that the melting point and the 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 due to supercooling. An overcooled liquid does not solidify because it lacks nucleation sites that allow crystals to form. Essentially, its liquid state is more stable than its solid state, even below its melting point.

Supercooling occurs with water. The melting point of ice is 0 °C (32 °F or 273.15 K), but the freezing point of water can go as low as -40 °C or -40 °F!

The freezing point also depends on purity. Impure substances experience a depression of the freezing point. Here again, the freezing point can be lower than the melting point.

Melting points of the elements

The element with the highest melting point is the tungstenwith a melting point of 3,414°C (6,177°F; 3,687K). 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 in liquid at high pressures (10 MPa or 99 atm). At these extreme conditions, carbon is estimated to have a melting point of 43030-430°C (7290-8010°F; 4300-4700K).

The element with the lowest melting point is heliumwith a melting point of 0.95 K (-272.20 ° C, -457.96 ° F) at a pressure of 2.5 MPa. This is very close to absolute zero. The metal with the lowest melting point is mercurywith 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. Non-metals tend to have relatively low melting and boiling points.

Table of melting point values for example substances.

The substance with the highest known melting point is hafnium carbide tantalum (Ta ₄ HfC ₅ ). Tantalum hafnium carbide is a refractory metal with a melting point of 4,215 K (3,942 °C; 7,127 °F). Computer models predict that the alloy HfN _0.38 C _0.51 has an even high melting point of around 4400 K.

Chemical	Melting point (K)
Helium	Does not melt at normal pressure.
Coal	Does not melt at normal pressure.
Hydrogen	14.01
Oxygen	54,36
Chlorine	171,6
Mercury	234,4
Water	273
Gallium	302,9
Cocoa butter	307.2
Paraffin wax	310
Potassium	336,5
Iodine	386,9
Welding of lead	456
Address	600,6
Silver	1234,9
Gold	1337.3
Copper	1357,8
Iron	1811
Tungsten	3695

Melting points at ordinary pressures

How the melting point is measured

When a substance melts, its solid transforms into a liquid. The phase change is endothermic because the chemical bonds absorb energy to break their rigid structure and change from solid to liquid. So, measuring the melting point works in two ways:

• Slowly increase the temperature of a solid and observe the formation of liquid.
• Heat a material and monitor the temperature of its black body with a pyrometer.