Density

Definition

We know from everyday life that the same volumes of different substances do not have the same mass. A liter measure full of down is much lighter than a liter measure filled with sugar. Fresh snow is lighter than ice, and ice is slightly lighter than water, although they are all H2O. Density is used as a measure of these different substance properties. Density (ρ, Rho) is defined as the quotient of the mass (m) of a substance and its volume (V):

The SI unit of density is kilograms per cubic meter (kg/m3). Alternatively, it is also often g/cm3 is also frequently used. One cubic centimeter (cm3) corresponds to one milliliter. The higher the density, the heavier the same volume. For example, the heavy precious metal gold has a density of 19.3 g/cm3. Density depends, among other things, on the chemical elements and isotopes in the substance, on compression, on the state of aggregation, and on how closely the molecules or particles in the substance exist together.

Calculation of mass

The mass of a substance can be calculated from its density and volume as follows:

Calculation of the volume

Conversely, the volume is calculated like this:

  • Volume (V) = mass (m) / density (ρ).

The density of water is 1 g/cm3

The density of water at 3.98 °C (i.e., about 4 °C) and a pressure of one atmosphere is 1000 kg/m3 or 1 g/cm3. Thus, because the density of water is 1 g/cm3, the mass and volume are the same. The volume of 1 liter of water corresponds to the mass of 1 kg. Whereby these data are dependent on external influences as already mentioned. The density depends on the temperature. Because substances usually expand with increasing temperature, i.e. the volume becomes larger, the density decreases with increasing temperature. Water is a prominent exception. As a liquid, it is heavier than ice.

Density as a substance property

Density is used as a characteristic property of a substance in analytics, for example. By measuring density, together with other clues, a conclusion can be drawn about the nature of a substance.

Density during filling

Ethanol 70% with camphor has a density of 0.88 g/cm3. It is lower than that of water. Therefore, 100 g has a larger volume of 113.6 ml. If filled as 100 g, a vessel of 100 ml is not enough to contain this mass. This applies to all liquids with a density of less than 1. Density also plays an important role when filling medicinal drugs. For example, roots have a higher density than flowers. The same mass therefore takes up a very different volume.

Examples in pharmacy

  • Water: 1.0 g/cm3
  • Fatty oils: approx. 0.9 g/cm3
  • Ethanol 70%: 0.88 g/cm3
  • Viscous kerosene: 0.86 g/cm3
  • Ethanol 96%: 0.81 g/cm3
  • Vaseline: approx. 0.8 g/cm3
  • Isopropanol: 0.78 g/cm3
  • Rubbing alcohol, spot benzine: 0.68 g/cm3