Question

What is difference in the meaning of a pure substance to a common man and a chemist?

Answer 1

for a common man it means without any foreign material .like for a common man,milk should not has water.
but in chemistry a pire substance means.
any substance which has single composition of particles is called a pure substance.

Answer 2

A pure substance, for a layman can be anything that is not pure irrespective of how it is formed.

Whereas for a chemist purity of a substance will depend on the composition of a substance.

For example for a layman tea may be sure if it is nicely made but on the other hand, a chemist can never consider it as a pure substance because it was formed by a mixture of water, sugar and milk.