The process of testing the melting point of a compound is to measure its purity. But if the compound is impure, then the results won't be accurate and reliable. When testing for the melting point of a compound, the impurity has the ability to:
- lowers the general melting point of the compound; when an impure in a compound is soluble, it lowers the melting point of the compound
- increase the range of temperatures at which the compound is completely melted
Hope this helps :)