Melting Point

Solids can mainly be found in an ordered structure (crystalline solid) or an unordered structure (amorphous solid). Crystalline solids will organize themselves in a crystal lattice. Removing atoms or molecules from this lattice requires energy. Therefore, transferring an atom or molecule from the crystalline solid phase to the liquid phase takes energy in the form of heat. The melting point is the amount of energy in the form of heat that is necessary to remove atoms or molecules from the crystal lattice.

Not all molecules will crystallize in a single crystal lattice. Some organic molecules are known to crystallize in different lattices that are similar in energy. Theses solids are called polymorph because they do not have a defined melting point but rather a range of temperature of where the solids melts.

Another factor that can influence the melting point are impurities. In most cases the melting point of a sample will decrease when containing an impurity. Measuring the melting point can therefore give a qualitative measure of the purity of a known sample.

Measuring the melting point requires a special apparatus, mostly a copper block that can be heated electronically. The temperature can be taken from an electronic sensor or a thermometer.

A sample of pure benzoic acid will melt at 122 °C.

The melting point is an important property of a crystalline solid. It can be measured easily and gives information about the identity and purity, as well as the bonding strength within the crystal lattice.

Due to its simple determination, the melting point is still an important property that is commonly employed to identify compounds and qualify their purity. The melting point can not only be used to identify solids, but also liquids like isopropanol. Through derivatization of isopropanol with 4-nitrobenzoic acid to its corresponding ester, the derivative becomes a defined solid with a melting point, which now enables the identification.