Background Essay: Density and Buoyancy: Testing Liquids

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 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. 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.

If they don't mix, fluids will also either sink in or float on top of other fluids. Just as with solids, the densities of the two fluids determines the buoyancy of one in the other. If a given volume of one fluid is heavier than the same volume of another fluid, the first fluid will sink in the second. If it is lighter, it will float.