We explain what density is and what types of density exist. Examples of the absolute density of different substances.
What is the density of matter?
Density is a scalar quantity, frequently used in the physical and the chemistry, which refers to the amount of mass present in a body or a substance determined by unit volume. It is usually represented by the symbol ρ.
Two bodies or substances of the exact same size and proportions can present densities different, and this is measured through the average density, which is the ratio between the mass of a body and the volume that it occupies in space, according to the following formula:
where m is the mass and V the volume, so the unit of measurement of the density in the International system It will be the kilogram per cubic meter (kg / m3) or equivalent units of measurement.
Variations of temperature Y Pressure affect the measurement of the density of a substance.
The density of matter is often associated with the story of the Greek philosopher Archimedes, who was supposedly tasked with determining whether the king's crown had been forged using pure gold or whether it had been made from a alloy with others metals.
During an immersion bath, Archimedes realized that he could calculate the volume of the corona by immersing it in Water and measuring the displacement of liquid, without having to melt or break it. Then he could weigh the crown (to have its mass) and determine, using the formula detailed above, the density of the crown. He compared the values of the calculated density of the crown with the density of pure gold (which is a constant), and this way he knew if it was pure gold or an alloy, since the density of gold would have varied when mixed with other metals.
Density types
There are several types of density of matter:
- Absolute density. We generally speak of absolute density when we use the term density, and it is an intensive magnitude calculated, as we said above, from the volume and the dough. Its units of measurement in the SI are (kg / m3).
- Relative density. This other type, on the other hand, arises from the comparison between the density of the substance in question and some other that serves as a reference, so it is a dimensionless quantity (without units). For the liquids Y solid, the density of water is used as a reference (at 1atm and 4 ° C), while for gases the density of air (at 1atm and 0 ° C). It is calculated as follows:
where 𝛒r is the relative density, 𝛒 is the absolute density and 𝛒0 is the density of the reference substance. - Apparent density. It is applied to heterogeneous materials, as well as to porous ones, whose mixture It affects the density (being less than if each element were compacted separately). Hence, this type of density does not depend on the nature of matter, but on the way it is arranged. For example, if we have a hydrated material, its apparent density will be:
where 𝛒a is the apparent density of the material, mh is the mass of the dried material and V0 is the volume of the hydrated material, that is, without drying.
Examples of density
Some examples of the absolute density of different elements and substances (expressed in their appropriate units) can be:
- Magnesium (Mg). 1,738 g / cm3
- Calcium (Ca). 1.54 g / cm3
- Iron (Fe). 7.874 g / cm3
- Molybdenum (Mo). 10.22 g / cm3
- Silver (Ag). 10.5 g / cm3
- Gold (Au). 19.3 g / cm3
- Iridium (Go). 22.562 g / cm3
- Dubnio (Db). 29.3 g / cm3
- Bohrio (Bh). 37.1 g / cm3
- Water (H2O). 1 g / cm3
- Oil. 0.92 g / cm3
- Air. 1,225 kg / m3