A salt solution is not a pure substance because it can be broken down into salt and water by a physical change.
Because it contains more than one distinct chemical species.
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
But common salt solution is a mixture of water molecules and solvated