Saltwater is considered a mixture because it consists of two or more substances that are physically combined, but not chemically bonded. In the case of saltwater, the main components are water (H2O) and salt (sodium chloride, NaCl). When salt is added to water, it dissolves, and the individual salt ions disperse throughout the water. However, this does not change the chemical structure of the water or salt itself; they retain their individual properties.
In a mixture, the components can be separated by physical means. For instance, if you evaporate the water from saltwater, the salt can be recovered. This is a key distinction from a substance, which has a uniform composition and specific properties throughout. Pure substances have fixed ratios of their components and cannot be separated into other substances through physical methods.
In summary, saltwater is a mixture because it combines water and salt without altering their individual identities, allowing for separation by physical means.