Have you ever noticed that it's easier to float in the ocean than in fresh water? In this video segment adapted from ZOOM, two cast members add salt to tap water to illustrate the effect of salt water on the buoyancy of an egg.
There's nothing magical about density. Simply put, it is the mass of a substance or object relative to the amount of space it takes up.
If two substances differ enough, their relative densities are easy to determine. For example, a piece of lead, which is a dense metal, feels much heavier than a piece of cork of about the same size. An analysis of the densities of the two objects would show that one cubic centimeter of lead, about the size of a small marble, has a mass of more than 11 grams, while one cubic centimeter of cork weighs about one-quarter of a gram.
When materials have such different densities, it is easy to predict which will sink and which will float. But what about other substances? Is there a way to predict whether or not a substance will sink or float in water or any other fluid?
Indeed there is. An object or substance will sink in a fluid if it weighs more than the fluid it displaces when fully submerged. In other words, if a one-cubic-centimeter object weighs more than one cubic centimeter of a fluid -- the amount it would push out of the way if submerged -- it will sink in that fluid. If it weighs less than the same amount of fluid, it will float.
Unlike the densities of solids, which remain relatively constant, the densities of many fluids, in particular gases, can be changed. Heating a gas and allowing it to expand lowers its density. Cooling it or compressing it causes its density to increase.
Dissolving a substance in a fluid also alters the fluid's density. When salt is dissolved in fresh water, for example, the density of the resulting saltwater solution is greater than that of the fresh water. An object that has a density only slightly higher than that of fresh water, such as an egg, will float in salt water if there is enough salt in the solution to make it denser than the egg.