How do mixtures differ from pure substances?

consist of two or more substances.

By definition, a substance is pure.

A substance is matter that has a specific composition and the same properties throughout.

Substances consist of either (where there's one type of atom) or (where there's one type of molecule).

For example, gold and silver are elements and hence are also substances.

Pure water is a compound, so it is also a substance.

A mixture contains more than one substance, and it can have variable composition.

As a result, you can't write a nice formula for a mixture, because the chemical makeup can vary based on what's put into it.

For example, salt water contains both salt and water, and it can have different concentrations of salt.

And the substances can be separated pretty easily by physical methods such as evaporation.

Below, you see an example of the large scale separation of salt from water by evaporation.

The pure salt is a compound and is therefore a substance.