A pure substance is stuff that's made up out of only one kind of
molecule. Adding something else usually gives you a mixture of the two
substances, but other stuff can happen too. Here are some examples.
Pure substance: liquid water. Additional stuff: salt. The salt will
dissolve in the water, and dissociate into its ions. If the water is
frozen, the salt will sit on top, and may lower the melting point below
the outside temperature. (Then it could slowly dissolve into some
liquid- see our other answers on salt and ice.) If the temperature is
really cold, the salt will just sit on top of the ice.
In general, if the pure substance and the extra stuff are nonreacting solids, they will just sit on top of each other.
They could form a suspension -- bits of the stuff floating around
in the pure substance. Example: oil and water. Or dust in the air.
They could undergo a chemical reaction. Example: water and metallic
sodium (please don't try this! Not only does this combination explode
by burning hydrogen which is a product, but it throws off sodium
hydroxide, which is caustic and poisonous.)
A trivial example: if the "something" is just more of the same pure
substance, you just have a bigger collection of the substance.
(republished on 07/13/06)