The melting point range is the span of temperature from the point when the solid starts melting to the point at which the entire solid is in a liquid state.

This technique can be used to determine the purity of a solid. If the melting point range of a pure solid is known, we can estimate the purity of our impure solid as the difference in the melting point compared to the pure solid. An impure solid melts over a wide range of temperatures and at a temperature lower than that of the pure solid. The closer the melting point range of the impure solid gets to the melting point of the pure solid, the fewer impurities it will contain.

To perform a melting point range in a solid we should follow the following steps:

1. Grind your crystals until you have a fine powder
2. Tap a capillary tube on top of your sample to load it and then tap it against the workbench to make it go to the other end
3. Insert the melting capillary tube in the melting point apparatus and turn it on
4. Determine the temperature at which the solid starts melting
5. Determine the temperature at which the solid is completely melted

If at the end of the protocol, the melting point range of the impure solid is below the melting point of the pure solid, it means that we still have impurities in our solid.

Figure 1: Melting Point Range protocol.