What is the difference between a pure substance and an impure substance, on a molecular scale?

pure substance:
elements:
they are made by one kind of atoms,, have their own properties cannot be broken down into simpler substances,,
compounds:
made by chemical reactions of elements joined together,,they have different properties has compared to the elements which make them in compounds elements may be separated by chemical means..
impure substances :
they are made by physical combinations,,and they may be separate by physical means ,,mixture is the example ,, also their properties do not change after becoming a mixture,,

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Well technically there are two correct answers b and d. but for you its probably b in general.good day sir/ma'am : ) (i know not gender)