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The addition of an impurity to a pure substance will cause its melting point to decrease...

The addition of an impurity to a pure substance will cause its melting point to decrease in proportion to the amount of impurity. Explain this phenomenon.

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Answer #1

A pure crystalline compound usually possesses a sharp melting point and it melts completely over a narrow range of temperature of not more that 0.5-1oC.

The presence of even small amount of impurities usually produces a depression in the freezing points and shows a marked increase in the width of the melting point range. The melting point range of > 5oC indicates that the substance is impure. For a material whose identity is known, an estimate of degree of purity can be made by comparing its melting point with that of a pure sample.

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