Question

what is the difference in meaning of a pure substance to a common man than to a chemist

Answer 1

in a common man view pure substance is substance without adulteration

but in a chemist view pure substance is a substance in which all molecules are identical

Answer 2

For a common man pure substance is anything which is unadulterated.For a chemist or scientist pure substance should be a pure single form of matter (anything that has homogenous composition at particle level)