We explain what density is and some characteristics of this property. In addition, other types of density exist.
What is density?
The term “density” comes from the field of physics and chemistry and refers to the relationship between the mass of a substance (or a body) and its volume. It is an intrinsic property of matter since it does not depend on the amount of substance considered.
Density, a property usually expressed in kilograms per cubic meter (kg/m3) or grams per cubic centimeter (g/cm3), varies to a greater or lesser extent depending on pressure and temperature and with changes in condition.
Due to the low cohesion between their particles, gases generally have a lower density than liquids, and liquids have a lower density than solids.
The density of matter is often associated with the story of the Greek philosopher Archimedes, who was tasked with determining whether his king’s crown had been forged using pure gold or made from an alloy of other metals.
During an immersion bath, Archimedes realized that he could calculate the volume of the crown by immersing it in water and measuring the displacement of the liquid without having to melt or break it and that knowing the density of gold (which is a constant), he could then weigh the crown and determine (using the formula) whether it was pure gold or an alloy (the density of gold would have changed by mixing it with other metals).
Although there are exceptions, in general, as the temperature increases, the density decreases.
Density can be defined in several ways:
Density or absolute density. It is the mass’s ratio to a substance’s volume, be it solid, liquid, or gas. It is represented by the Greek letter rho ():
DensityWhere m is the mass of a substance and V is its volume.
Relative density. It is the ratio of one substance’s thickness to another substance’s density.
DensityWhere (substance X) is the density of substance X, and (substance Y) is the density of substance Y, relative to which the relative density of X is calculated.
Apparent density. It applies to porous materials that may have air or other substances between their pores. It is calculated similarly to density, but you have to add the air mass of the importance that occupies the pores. You also have to increase the volume of the essence, incorporating the volume occupied by the sense that occupies the pores.
The density of water is 1 g/cm3, and that of lead is 11.35 g/cm3. These two examples show how the density can take very different values in different materials.
Other types of density
Population density. It is a demographic concept that refers to the number of inhabitants per square kilometer. China and India are countries with very high population densities, while the Nordic countries and Oceania have low population densities. Highly populated areas are often associated with housing problems, air pollution, and insufficient public service infrastructure.
Optical density. It is a physical parameter that constitutes the absorption of an optical element at a given wavelength per unit distance. This data is used to assess the content of cells, the quality of smoke generated by different substances, laser power, filters, etc.
Electric current density. It is the relationship between the intensity of the electric current flowing through a conductor per unit of time and per unit of the cross-section.
Magnetic flux density. Also called “magnetic induction,” it is the magnetic flux that causes an electric charge to move for each unit area normal to the direction of change.
Bone mineral density. In medicine, this measure refers to the number of minerals per unit area. It is usually expressed in g/cm2 and is explicitly established for certain bones, such as the femur or the lumbar spine. Low bone mineral density can trigger osteoporosis, a disease in which the bones have a deficient proportion of minerals (mainly calcium), so they become too porous and, therefore, fragile and brittle, thus increasing the risk of people suffering fractures.
Examples of the density of some compounds and elements at 20°C
- Magnesium (Mg). 1.738g/cm3
- Calcium (Ca). 1.54g/cm3
- Iron (Fe). 7.874g/cm3
- Molybdenum (Mo). 10.22g/cm3
- Silver (Ag). 10.5g/cm3
- Gold (Au). 19.3g/cm3
- Water (H2O). 1g/cm3
- Oil 0.92g/cm3
- Air. 1.225 x 10-3g/cm3