How Does Salt Affect the Density of Water?

Adding salt to water increases the density of the whole solution. As a result, the solution has a lower freezing point, as the example of seawater shows. On average, seawater has approximately 3.5 percent salinity, and its freezing point is approximately -1.9 degrees Celsius, whereas pure water freezes at 0 degrees Celsius.

The chemical composition of salt features a lattice with chlorine and sodium ions stacked together in a 1:1 ratio. Adding salt to water leads to an attraction between water molecules and either the sodium or chlorine ions, depending on the particular molecule, and this attraction is stronger than the bond between sodium and chlorine. As a result, the water pulls the lattice apart, dissolving the salt.

In the polar regions, the seawater is saltier than it is at other points on the globe. When the ice forms, the salt cannot join the ice crystals, and so the ice is virtually fresh. The salt that does not join the ice mixes in with the rest of the salt water still there, forming a brine that becomes so dense that it sinks beneath less-dense layers of seawater. At more temperate spots in the ocean, none of the water leaves the solution in the form of ice, and so the salinity remains more constant.