Question

why is mixture called impure substance

Answer 1

A mixture is an impure substance. It consists of different elements and substances combined together physically in an unknown ratio. Mostly mixtures are heterogenous. Their constituents are not combined chemically but only physically. The mixture still shows all the properties of its constituents. A mixture doesn't have a definite boiling or freezing point.
All these properties of a mixture make it an "Impure substance".

Answer 2

Mixtures are formed by mixing 2 pure substances in any proportion such that they do not undergo any chemical reaction and retain their individual properties .They can be homogeneous or heterogeneous ; with heterogeneous being more in number.Since they are made up of  2 pure substances mixed together in any proportion and can be separated by simple physical method, they are treated as impure substances and their melting and boiling points vary.
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