Late night talk show host David Letterman has a long running segment entitled “Will it Float?” where an object presented and Letterman and his on-air staff debate and then guess whether it will float in a tank of water. If the tank happened to be filled with salt water, more of the objects Letterman used would have, in fact, floated. Adding salt to water changes the physical forces that water exerts on objects, making them float, a concept that you can demonstrate in your own home.
The Principle of Buoyancy
Buoyancy is the upward force that a fluid exerts on an object. When an object is dropped into a fluid, the force of gravity pulls the object down towards the Earth. The magnitude of that force depends on the mass of the object. The fluid pushes back up on the object and the magnitude of the force is dependent on the mass of the displaced fluid. If the mass of the object is less than the mass of the displaced fluid, the object will float. Buoyancy is influenced by the density of the object and the density of the fluid, that is the mass and volume of the object and the fluid it displaces.
How Salt Water Influences Buoyancy
Adding salt increases the density of water. As the salt dissolves in the water, the ions fit into the spaces between the water molecules, similar to marbles filling the spaces if you poured them into a bucket full of tennis balls. The mass of the saltwater is much higher and the volume only slightly greater, so the saltwater is more dense than freshwater. If the same volume of water is displaced by an object, the weight of saltwater displaced is greater and thus the force of buoyancy is proportionally greater.
Swimming in the Dead Sea
The Dead Sea, in Israel, is a fun place to experience the buoyancy effects of saltwater first hand. The Dead Sea is a dead end; it is the lowest point on Earth and the end of the Jordan River. The river carries salt into the sea and the evaporating water concentrates it. The salt concentration in the Dead Sea is 300 parts per thousand, by contrast, ocean water is 35 parts per thousand. The high salinity means that swimmers float easily and a popular tourist activity is to recline effortlessly on top of the water while reading a newspaper or magazine.
Science Project: Floating an Egg
You don’t need to travel to Israel to explore why salt makes things float, you can do a science project. All you need is a peeled hard-boiled egg, a jar of warm water and salt. Place the egg in the jar of water. It will sink to the bottom. Take the egg back out and stir in as much salt as will dissolve in the water. Try to place the egg in the jar again and this time, the salt has increased the density of the water enough to float the egg.