Saltwater is considered a homogeneous mixture because it has a uniform composition throughout. When salt (sodium chloride) is dissolved in water, it breaks down into individual ions (sodium and chloride ions) that are evenly distributed in the water.
This uniform distribution means that every sample of saltwater will have the same proportions of salt and water, making it indistinguishable in terms of composition. You can’t see the salt crystals in the water, which contributes to its uniform appearance. If you were to take a sample of saltwater from one part of the ocean and another from a different part, they would have the same composition, provided they both had the same concentration of salt.
In contrast, heterogeneous mixtures have visibly different substances or phases. For example, if you mix sand and water, the sand does not dissolve and can be clearly seen, resulting in distinct layers. This is what separates homogeneous mixtures like saltwater from heterogeneous mixtures.