The Difference in Buoyancy of Fresh & Salt Water

If you're trying to figure out why it's easier to float in the water at the ocean than it is in your backyard pool, the problem has a lot to do with the type of water you're floating in. The molecular makeup of saltwater is somewhat different than fresh water, resulting in a buoyancy difference that you can notice when trying to float.

  1. Buoyancy Defined

    • The physics of buoyancy have much to do with something called displacement. When it comes to water, buoyancy or the ability to float on water occurs when the weight of the object on the water is less than the upward pressure of that same liquid. A cubic foot of water equals a certain amount in pounds. As a result, it can repel the same amount of weight or less. If an object weighs more than that figure per cubic foot, then it will sink in that water.

    Fresh Water Buoyancy

    • A cubic foot of fresh water weighs 62.4 pounds or 28.3 kilograms. This is 1.6 pounds or 0.7 kilograms less than saltwater. Fresh water is found in rivers, creeks, pools and hot tubs. It's the same type of water one drinks after it has been treated through municipal facilities. Most people when young will float easily in their local pool. This is because they weigh less than the space they take up in water. However, as adults, most of us sink in freshwater pools fairly quickly. Again, our weight exceeds the water space we take up, thus eliminating any buoyancy possible.

    Salt Water Buoyancy

    • The most common saltwater available can be found in oceans and large bodies of natural saltwater such as the Dead Sea in Israel. It is exactly the salty components in the water that gives it greater density: 64 pounds per cubic foot. Because of this difference, it is somewhat easier to float in saltwater than in freshwater.

    Diving in Different Waters

    • Due to the difference in buoyancy per water type, divers have to prepare differently with their equipment. Since saltwater has effectively more resistance, heavier weights are needed for scuba divers to travel below water to deeper levels. In fresh water, the diving is far easier because weight penetration works better. This issue and how much weight to use varies from person to person due to weight and diving skill, and the positioning of weights also dictate how the buoyancy will affect a person's dive.