Salinity, the concentration of dissolved salts in water, is a major factor determining the density of water. The fundamental relationship is straightforward: as the amount of salt dissolved in water increases, the density of that water also increases. For instance, freshwater has a density of about 1 gram per cubic centimeter (g/cc). Average seawater, with a salinity of around 35 parts per thousand, is slightly denser, measuring between 1.02 and 1.03 g/cc. This difference drives significant movements in the world’s oceans.
The Mechanism of Increased Density
The increase in water density due to salt is a direct consequence of adding mass to the water volume. When sodium chloride—the most common salt—dissolves, it breaks apart into individual sodium and chloride ions. These ions add their mass to the total without causing a proportional increase in the water’s overall volume.
This occurs because the dissolved ions are relatively small and fit neatly into the existing spaces between the water molecules. The ions occupy voids in the liquid structure, unlike adding a solid material like sand, which would significantly increase the total volume. The result is a greater total mass packed into nearly the same volume, which translates to a higher density.
Seawater with a salinity of 35 parts per thousand contains approximately 35 grams of dissolved salts for every 1,000 grams of water. Because this additional mass is integrated without much volumetric change, the salt water is heavier than the same volume of pure water. This measurable increase in density means that salty water will sink when placed next to less salty, or fresher, water.
How Temperature Modifies Salinity’s Effect
While salinity adds mass to a fixed volume, temperature changes the volume of a fixed mass of water, creating a complex interplay between the two factors. Generally, as water cools, it contracts and becomes denser, and as it warms, it expands and becomes less dense. Ocean density is thus controlled by both temperature (“thermo”) and salinity (“haline”).
These two properties can either reinforce or counteract each other. The densest water in the ocean is typically cold and salty, as both factors increase the mass-to-volume ratio. Conversely, warm and fresh water is the least dense and tends to remain on the surface.
This combined influence creates water stratification, where the ocean layers itself based on density, with the densest water at the bottom. Layers where density changes rapidly with depth are called pycnoclines, often caused by quick changes in temperature (thermoclines) or salinity (haloclines). Highly saline water can sometimes be less dense than extremely cold, less salty water, illustrating the balance between these variables.
Salinity and Density as Drivers of Ocean Currents
Density differences created by variations in temperature and salinity are the main driver for deep-ocean circulation, known as thermohaline circulation. This vast current system is often called the “global conveyor belt” because it moves water, heat, and nutrients around the planet. While wind drives surface currents, density drives the flow in the deep ocean, affecting about 90% of the ocean’s volume.
The circulation begins in the Earth’s polar regions where surface water becomes intensely cold. As sea ice forms, it pushes salt out into the surrounding water, making the water both cold and highly saline. This cold, salty water becomes exceptionally dense, causing it to sink to the deep ocean floor.
Once this dense water sinks, it initiates a global flow, pulling surface water in to replace the sunken mass. This process creates deep currents that move very slowly, perhaps 10 to 20 kilometers per year. The movement of this dense water from the poles drives the mixing of the world’s oceans over centuries.