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The experiment aims to clarify the meaning of purity in the chemical sense ; apply the method of recrystallization in the purification of a solid sample; recognize melting point as a physical property that can serve as an index of purity; determine the melting point of a substance using a simple apparatus; and compare the melting point of a pure versus impure sample of a substance.
Pure, crystalline solids have a characteristic melting point, the temperature at which the solid melts to become a liquid. The transition between the solid and the liquid is so sharp for small samples of a pure substance that melting points can be measured to $\pu{0.1^\circ C}$. The melting point of solid oxygen, for example, is $\pu{-218.4^\circ C}$. Liquids have a characteristic temperature at which they turn into solids, known as their freezing point. In theory, the melting point of a solid should be the same as the freezing point of the liquid. In practice, small differences between these quantities can be observed. It is difficult, if not impossible, to heat a solid above its melting point because the heat that enters the solid at its melting point is used to convert the solid into a liquid.