Density

Density, amount of a substance contained within a specific area. In physics, density is the ratio of the mass of a substance to its volume, and it can be calculated by dividing the mass by the volume. Density is often expressed in units such as grams per cubic centimeter (g/cm³) or pounds per cubic foot (lb/ft³).

The density of a substance can vary under different conditions. Substances expand and contract as their temperature changes, and as a result their density also changes. Precise measurements of density therefore include the temperature at which they were taken. For example, 1 cu cm of water at 4°C (39°F) weighs 1 g, so water’s density at that temperature is 1 g/cm³. Hot air balloons are able to fly because the density of air changes as its temperature changes. The balloon rises because the heated air inside its bag is less dense than the cooler air outside. Pressure also affects the density of gases. If a given amount of gas is contained within a smaller volume, its density increases. Air at sea level, for example, is denser than air at the top of Mount Everest because the air pressure at sea level is higher and squeezes the air into a smaller volume.

Scientists often indicate the density of a substance by using another measurement called specific gravity. Specific gravity is the density of a substance divided by the density of another substance that is used as a standard. For solids and liquids, water at 4°C (39°F) is usually the standard. Gold has a density of 19.3 g/cm³, so its specific gravity is 19.3 g/cm³ divided by 1 g/cm³ (the density of water at 4°C), or 19.3.

Density can be measured in a number of ways. Solid objects can be weighed to determine their mass and then immersed in a liquid to determine their volume. The volume of liquid displaced by the object is equal to the object’s volume, and the mass divided by the volume is its density. The density of a liquid may be determined similarly. The liquid’s mass can be found by first weighing an empty container, then weighing the container with the liquid in it, and then subtracting the empty weight from the full weight. The liquid’s volume may be determined by instruments similar to the transparent measuring cups used in cooking. Gases may be weighed in airtight containers of known volume and weight. Since gases are more sensitive to changes in temperature and pressure than are liquids or solids, the temperature and pressure must be included in any measurement of the density of a gas.

The term density is also used in many other ways. Population density, for example, is the number of people living within a certain area. Photographic density refers to the blackness of an image on film or on a photographic plate. Particle density is the number of particles in a given volume divided by that volume. Charge density is the total electric charge contained in a volume divided by that volume.