"Mixed melting point" is a technique used to identify chemical compounds. It is particularly used for organic compounds, where a sample with a known identity and melting point is mixed with an unknown purified sample to determine the melting point.
Glass capillaries are used to determine the mixed melting point. An equal amount of the unknown and known substance are mixed and placed in one capillary. The two substances are also placed in separate capillaries. All three capillaries are heated simultaneously in the melting point apparatus. The melting point of the capillary with the mixed substances should not differ by more than 4 degrees Celsius from the melting point of the substances placed in separate capillaries.