[adultstudent]

One of the experiments I had to do in the Lab. Techniques subject was that of determining the melting point of different substances using a Thiele. Theoretically, if a substance has impurities its melting point will go down, since such impurities will weaken intermolecular forces. Thus, one of the exercises involved mixing a substance whose melting point had been determined with a little of another that would play the role of impurity. To my surprise, not only was not the melting point depressed but it was actually elevated. Why does this happen? Could it be that impurities only lower the melting point when they have a melting point lower than the substance you have to assess? Then why do all text books say that impurities lower the melting point instead of saying that they can either lower or raise it?