* Pure substances have a fixed composition and uniform properties throughout. They are made up of only one type of molecule or atom. Examples include pure water (H₂O), pure gold (Au), and pure sugar (C₁₂H₂₂O₁₁).

* Salt solution is a mixture, specifically a homogeneous mixture. It's made up of two or more substances (salt and water) that are evenly distributed throughout. The salt dissolves in the water, forming a solution where the salt molecules are dispersed among the water molecules.

Key takeaway: Because a salt solution contains multiple substances, it doesn't meet the definition of a pure substance.