Why don't saltwater and freshwater mix, even though salt can dissolve in freshwater to create saltwater?

Saltwater and freshwater don't mix easily because of the differences in their density caused by their differing amounts of dissolved salts. This creates a layer of water with different properties and composition, called a halocline, which acts as a barrier to the mixing of the two types of water.

When salt dissolves in freshwater, it raises the salinity and density of the water. However, this does not cause it to sink because the temperature and pressure are constant. In contrast, saltwater is denser than freshwater, so it sinks and forms a layer at the bottom.

Due to the differences in density, the two types of water will form distinct layers, with the saltwater on the bottom and freshwater on top. The boundary between them is known as the interface, and it is where the mixing of the two types of water occurs. However, this mixing is slow and limited due to the halocline and the differences in density, which results in the maintenance of two separate layers of water.