# Question #e788e

Jul 6, 2017

A salt solution is not a pure substance because it can be broken down into salt and water by a physical change.

#### Explanation:

Pure substances can only be separated by a chemical change, and elements not even by a chemical change.

Mixtures and solutions are not pure substances. Mixtures and solutions can be separated by physical changes.

Jul 6, 2017

Because it contains more than one distinct chemical species.

#### Explanation:

A pure substance contains only one type of atom, or one type of molecule (or in the case of ionic compounds, one specific ion pair).

So common salt itself is a pure substance (it contains only sodium chloride, NaCl) and pure water (as opposed to sea water, or water with additives in it) is a pure substance, it contains only ${H}_{2} O$ molecules.

But common salt solution is a mixture of water molecules and solvated $N {a}^{+}$ and $C {l}^{-}$ ions, more than one distinct chemical species, and therefore not a pure substance.