Question

Define the term melting point range

Answer 1

The melting point of solid is defined as the temperature at which the solid exists in equilibrium with its liquid under an external pressure of one atmosphere.

The melting point is an important physical property of a compound. The melting point can be used to identify a substance and as an indication of its purity.

A pure crystalline compound usually possesses a sharp melting point and it melts completely over a narrow range of temperature of not more that 0.5-1^oC. The presence of even small amount of impurities usually produces a depression in the freezing points and shows a marked increase in the width of the melting point range. The melting point range of > 5^oC indicates that the substance is impure. For a material whose identity is known, an estimate of degree of purity can be made by comparing its melting point with that of a pure sample.

Melting point is also used for the identification and characterisation of a compound.

The force of attraction between the molecules affects the melting point of a compound. Stronger intermolecular interactions result in higher melting points. Ionic compounds usually have high melting points because the electrostatic forces holding the ions (ion-ion interaction) are much stronger. In organic compounds the presence of polarity, or especially hydrogen bonding, generally leads to higher melting point.