Salt water is a mixture and not a pure substance. We define pure substances as those that contain atoms or molecules of the same type. Examples of pure substances are elements (such as iron, silver, gold, etc.), compounds (such as water, sodium chloride, etc.), etc. A pure substance has a uniform composition. In comparison, 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 (by boiling the salt water). 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.
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