What is the effect of impurity on the melting point of a substance?

Question

What is the effect of impurity on the melting point of a substance?

Answer 1

It tends to lower the melting point of the mixture. Consider the effect of small amounts of urea and hydrocinnamic acid on the melting points of either one.  

Answer 2

If the impurity is less dense than the substance the melting point would be cooler than normal. But if the impurity is more dense than the substance it would make the melting point hotter. And if it the same density it wouldnt change at all.

Answer 3

Solutions containing impurities compared to pure solutions will have higher boiling points, lower freezing and melting points, and lower vapor pressure.