When you step into the ocean, you likely notice that floating feels much easier compared to swimming in a pool. Your body rises higher in the water with less effort, allowing you to relax on the surface. This effect is a direct result of the dissolved salts changing the physical properties of the water. Understanding this requires looking at the fundamental physics of how objects interact with fluids.
The Concept of Density
The tendency of an object to float or sink is governed by density, which is a measure of how much mass is packed into a specific volume. An object will float in a fluid if its average density is less than the fluid’s density, and sink if it is denser than the fluid surrounding it. The human body is generally composed of tissues slightly denser than freshwater, which is why a person must often tread water to stay afloat in a pool.
Freshwater has a density of approximately \(1.0\text{ g/cm}^3\). Seawater, however, has a higher density, typically ranging from \(1.02\) to \(1.03\text{ g/cm}^3\). This small difference shifts the balance, causing the human body to be less dense than the fluid it is immersed in when in the ocean.
How Salt Increases Water Density
The increase in density is a consequence of how salt interacts with water at a molecular level. Salt, primarily sodium chloride, dissolves in water by breaking apart into constituent ions. These ions are significantly heavier than the water molecules, which are made up of the relatively light atoms of hydrogen and oxygen.
When salt dissolves, the ions settle into the tiny, naturally occurring spaces between the water molecules. This process adds a large amount of mass to the solution without causing the total volume to increase proportionally. Since density is calculated as mass divided by volume, adding mass while keeping the volume relatively constant results in a higher density for the saltwater solution. The dissolved ions essentially fill the gaps and make the water “heavier” per unit of space compared to pure freshwater.
The Buoyant Force Explained
The scientific explanation for why denser water supports an object better is described by Archimedes’ Principle. This principle states that the upward force exerted on an object immersed in a fluid—the buoyant force—is equal to the weight of the fluid that the object displaces. The fluid’s weight is directly tied to its density.
When you enter the water, your body displaces a certain volume of that fluid. In denser saltwater, the volume of water your body pushes aside weighs more than the equivalent volume of less-dense freshwater. Because the buoyant force is equal to the weight of the displaced fluid, the upward push is greater in salty water.
This enhanced buoyant force makes floating easier. In freshwater, the upward force may not be enough to counteract your body’s downward weight, causing you to sink slightly. In saltwater, the stronger upward force more easily overcomes your weight and keeps you near the surface.
Hypersaline bodies of water, such as the Dead Sea, demonstrate this effect. The Dead Sea has a salt concentration nearly ten times that of the average ocean, giving it a density of up to \(1.24\text{ g/cm}^3\). At this density, the buoyant force is so substantial that a person can recline on the surface with almost no effort.