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Why boiling points and melting points can be used to identify substances?



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Because they are unique to each pure substance.

Different substances usually have different melting points, so determining the melting point of a substance can narrow down the possibilities for the identity of the substance. Likewise different substances usually differ in their boiling points, so boiling point can narrow down the possible identities of a substance. Although two substances can have nearly identical melting points or have nearly identical boiling points, it is extremely rare for two different pure substances to have BOTH the same melting point AND the same boiling point, so taking the two together is usually enough to uniquely identify a substance, or at the very least narrow it down to a very small list.