Determining the melting point of a compound is one way to test if the substance is pure. A pure substance generally has a melting range (the difference between the temperature where the sample starts to melt and the temperature where melting is complete) of one or two degrees. Impurities tend to depress and broaden the melting range so the purified sample should have a higher and smaller melting range than the original, impure sample.
Figure 1. A Fisher-Johns melting point apparatus.
Figure 2. Crystals on the lens cover Figure 3. Compound after being melted
A. SublimationB. Melting C. Evaporation
There are a variety of methods by which a sample's melting point can be measured, with the newest being electrical probes (e.g. Vernier MeltStation). Presented in this section are traditional methods that use an electrical melting point apparatus and Thiele tube. Both methods use capillary samples that are prepared in the same manner. Sample Preparation
Melting Point Apparatus
Thiele Tube Method
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