purity checks by melting point

Meaning: A pure crystalline compound usually melts sharply, over a range of about 1–2 °C, close to its literature value. A soluble impurity disrupts the crystal lattice and dissolves in the melt, so melting begins at a lower temperature and finishes over a wider range. This is the same idea as freezing point depression.

Equation: ΔT = Kf × m, where ΔT is the lowering of the melting point (K), Kf is the cryoscopic constant of the solvent (K·kg/mol), and m is the molality of impurity (mol/kg). This applies roughly to a small amount of impurity.

On the MCAT: A passage gives melting ranges for a crude product and for the same product after recrystallization. The purer sample has the higher, narrower range. In a mixed melting point test, the unknown is ground with an authentic sample. If the melting range stays sharp and unchanged, the two are probably the same compound. If it drops and broadens, they are different compounds. Heating too fast can also widen the observed range.

Common confusion: Students think an impurity raises the melting point, as a nonvolatile solute raises the boiling point. Impurities lower the melting point and broaden the range. A sharp melting point shows purity but does not prove identity, since different compounds can melt at similar temperatures. Rare cases such as a eutectic mixture can also melt sharply.