1. The problem statement, all variables and given/known data What is the effect of an insoluble impurity such as sodium sulfate, on the observed melting point of a impure compound? 2. Relevant equations 3. The attempt at a solution My answer is, there is no effect on the melting point of this particular compound. Because it has to be soluble in order to depress the melting point. If it is insoluble it is just like having 100 % of compound X and 0 % of compound Y, assuming there is only two compounds in this impure compound. Thus the vapor pressure is not effected and melting point will not change. Is this a correct answer?