Think of chromatography as a race and you'll find it's much simpler than it sounds. Waiting on the
starting line, you've got a mixture of chemicals in some unidentified
liquid or gas, just like a load of runners all mixed up and bunched together. When
a race starts, runners soon spread out because they have different
abilities. In exactly the same way, chemicals in something like a
moving liquid mixture spread out because they travel at different
speeds over a stationary solid. The key thing to remember is that chromatography is a surface effect.
As the liquid starts to move past the solid, some of its
molecules (energetic things that are constantly moving about) are
sucked toward the surface of the solid and stick there temporarily before being
pulled back again into the liquid they came from. This exchange of
molecules between the surface of the solid and the liquid is a kind of adhesive or
gluing effect called adsorption (with a d—don't confuse it with
absorption, with a b, where molecules of one substance are permanently
trapped inside the body of another). Now remember that our liquid is actually a
mixture of quite a few different liquids. Each one undergoes
adsorption in a slightly different way and spends more or less time
in either the solid or the liquid phase. One of the liquids might
spend much longer in the solid phase than in the liquid, so it would
travel more slowly over the solid; another one might spend less time
in the solid and more in the liquid, so it would go a bit faster.
Another way of looking at it is to think of the liquid as a mixture
of glue-like liquids, some of which stick more to the solid (and
travel more slowly) than others. This is what causes the different
liquids within our original liquid mixture to spread out on the
solid.
