With a homogeneous mixture, if you get down far enough and look at
small enough samples, you could reach the point where you could find
tiny samples of different substances. The first example I can think of is
mayonnaise. If you look at amounts of it that are small enough, you'll
find tiny samples of egg and oil.
With a pure substance, no matter how tiny a sample you look at, it's
always the same substance, all the way down until you're looking at
single molecules of . . . the same substance.
Salt is a simple example. No matter how far you break it down, to tinier
and tinier samples, each and every one of them is still salt, until you have
a single molecule of salt. If you cut that molecule up somehow, then you
have one atom of sodium and one atom of chlorine. But until you do that,
you always have salt, because salt is a pure substance.
