What Is Density?

Density is an important concept in science, because in a universe where gravity is not uniform, weight is actually the least useful way to measure the physical property of matter. The classic definition of "density" is that it is the mass present in matter per unit of volume. However, density is a bit more complicated than that.

Identification

As previously noted, density is how much mass is in a given volume. Expressed as an equation, it is density = mass/volume. Basically, it is how tightly packed matter is into a given space.

History

The Greek philosopher, inventor, and mechanic Archimedes is commonly regarded as having discovered the concepts of both density and displacement at the same time. As the story goes, Archimedes of Syracuse was assigned by his ruler, the king Hiero II, to figure out if the king's goldsmith was stealing gold but diluting it with silver. Archimedes was given a golden crown, but was forbidden to crush it into something that could be easily measured. One night while puzzling over the problem, Archimedes got into the bath and observed how the water level rose when his body entered and left it. From that he deduced that denser objects displace more water. For example, despite being a solid, wood floats because it contains less overall matter per unit of volume than water. Pure gold is very dense, but it becomes less dense when it is diluted in an alloy with other metals. Thus, Archimedes determined he could use a comparable, pure gold object, he could determine the density of the original by observing how much water the two objects displaced.

Liquids, Solids, and Gases

The density of matter is not a constant, which can be easily observed with water. As water changes from vapor (steam) to liquid to solid (ice), its density changes, too. This is best observed when liquid water expands as it is frozen into a solid, become less dense as the same amount of mass occupies more volume. An elementary mistake is to believe that solids are always the densest form of matter, but water is actually at its highest density in a very cold, but unfrozen, liquid form.

Importance

Understanding density is important because helps us understand why small, heavy objects weigh more than big, light ones. Density is also very important for measuring matter. Because gravity is not the same throughout the universe, weight is a poor means of measuring an object. A 120 pound woman only weighs 20 pounds on the Moon, where the gravity is only one-sixth that of Earth. However, her mass, density, and volume will all remain the same. That makes density one of three very reliable statistics for the measurement of matter.

Other Misconceptions

An important thing to avoid misunderstanding about density is that it does not alter the effects of gravity. Gravity pulls on mass, not density. Galileo proved that with his experiments, depicted in legend as him dropping a bag of feathers and a cannonball off the Leaning Tower of Pisa. Two objects of equal mass accelerate at the same rate while falling, regardless of density. Another common misconception is that density automatically confers certain physical properties, like toughness. This is just as wrong as the idea of solids automatically being denser than liquids, and can be observed in how soft lead and gold (two extremely dense metals) are relative to iron.