The melting point of a in impure compound is normally lower than the pure compound. Also a pure compound usually melts at a sharply defined temperature but an impure compound usually melts over a few degrees range of temperatures. This is a useful way of testing how pure your compound is when you've made it. Melting point depression is due to formation of "eutectics" which are low-melting mixtures of compounds.
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its nt much complicated...simply the melting point of pure substance is greater the impure substance as pure substance have strong inter molecular forces..than impure substance which is nt much tightly packed.