Salt water is a mixture and not a pure substance. A pure substance has a uniform composition. In composition, a mixture does not have a uniform composition of its constituents and can be divided into them by simple physical means. Salt water does not have a uniform composition and we can divide salt and water by simple physical process of evaporation. Thus, salt water is a mixture, with salt as the solute and water as the solvent. In fact, salt water is a homogeneous mixture and can be termed as a solution.