Density, the weight of an object divided by its volume, is a property of all matter, including solids, liquids and gases. The value of an object’s density depends on what it’s made of as well as its temperature; for example, lead weights are denser than feathers, and cold air is denser than hot air. Because scientists use it so frequently, density has its own mathematical symbol, the Greek letter rho, which resembles a lower case p.
Density is an intrinsic property of every substance, meaning the density of all iron objects is the same regardless of how big they are or what shapes they take. This makes it possible to identify an unknown material by determining its density, then comparing it to a list of known substances and their densities.
The Greek philosopher Archimedes was given the difficult task of finding out if King Hiero’s goldsmith was stealing gold and replacing it with a cheaper metal in a valuable object. Archimedes realized, while taking a bath, that he could determine the volume of the suspected object by the amount of water it displaced. Then, by dividing the weight by the volume, then comparing the resulting density with that of gold, he could determine if the object was gold or a cheaper substitute. According to legend, the thought so thrilled Archimedes that he ran through town shouting “Eureka!,” a Greek word meaning “I have found it.”
Changes in Density
Changing the pressure or temperature of an object will generally change its density. As temperature decreases, the motion of the molecules in a substance slows down; as they slow, they require less space, causing density to increase. Conversely, an increase in temperature usually results in a decrease in density. There are exceptions to the temperature rule: Water, for example expands slightly when it freezes, so ice is less dense than liquid water. Ice floats on water because the density of ice is lower.
Floating and Sinking
Relative density determines whether an object will float in a liquid; for example, a tree branch floats on a river if the wood is less dense than water. On the other hand, an iron cannonball sinks into water because its density is greater than water. Keep in mind that an object’s whole density plays an important role in floating and sinking. An iron ship, for example, floats in an ocean because, although iron is denser than water, most of the ship’s interior is filled with air, reducing the vessel’s density overall. If the ship were a solid block of iron, it would sink like a stone.
Density measurements are used when weight and weight distribution are important. This may include the construction of ships, buildings, airplanes and other modes of transportation. Density measurements are also useful when determining how much force is required to move a liquid through piping or tubing.