How Does the Density of Saltwater Compare to Freshwater

Temperature

Temperature is a major determinant of water density. As water cools, it contracts, and thus density increases. At 4 degrees Celsius, water reaches its maximum density, however, and with further cooling density decreases. The solid form of water--ice--is actually less dense than the liquid form, the effects of which are easily observed with ice cubes floating in a glass of water or ice chunks on the surface of a lake.

Salinity

The dissolved salts in ocean water add to its density. Seawater is thus denser than freshwater. Adding a drop of saltwater with food coloring to a jar of freshwater demonstrates this phenomenon: the colored saltwater sinks.

Estuaries

Runoff at major river mouths results in a stratification of fresh and saltwater, often in estuaries. The degree to which the layers intermix depends on a number of factors, including the relative volumes of the river outflow and the ocean tide. Where the former is much greater than the latter, as at the mouth of the Mississippi River, there is often less mixing, and a layer of freshwater will extend over a saltwater "wedge" (these are called salt-wedge estuaries).

Temperature and Salinity

Temperature and salinity are always dual factors on oceans, and their effects on density can be additive or opposite. In the world's subtropical belt, high temperatures would tend to decrease density, while the usual low precipitation (lower influx of freshwater) encourages higher density. In the tropics, high temperatures and high rainfall both work to decrease density.

References

• The Expansion of Water Upon Freezing
• Oceanography; M. Grant Gross; 1985.
• Estuaries

Resources

• Graphic Explaining Salt Wedge Estuaries