Density of matter
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?
The density is a scalar magnitude, often used in physics and chemistry, which refers to the amount of mass present in a given body or substance. It is usually represented by the symbol .
Two bodies of the same exact size and proportions can have different densities, and this is measured through the average density: the relationship between the mass of a body and the volume it occupies, according to the following formula:
= m / V, where m is the mass and V the volume, so the unit of measurement of density in the International System will be the kilogram per cubic meter (kg / m 3 ) or similar measures. Temperature and pressure variations affect the measurement of the density of a substance.
The density of matter is often associated with the history of the Greek philosopher Archimedes, who was supposedly tasked to determine if the king's crown had been forged using pure gold or if there was has been diluted with 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 it or break it; and that knowing the density of gold, which is a constant, could then weigh the crown and determine, using the formula detailed above, whether it was pure gold or an alloy, since the Gold density would have varied when mixed with other metals.
See also: Specific Weight.
There are several types of density of matter:
- Absolute density In general, we speak of absolute density when we use the term density, and it is an intensive quantity calculated, as we said above, from volume and mass, and measured in the SI in kg / m 3 .
- Relative density This other type, however, 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 magnitude (without units). For liquids and solids, the density of water is used as a reference (at 1atm and 4 ° C), while for gases the air density is used (at 1atm and 0 ° C).
- Bulk density . It is applied to heterogeneous materials, as well as to porous materials, whose mixture affects density (being less than if each element is compacted separately). Hence, this type of density does not depend on the nature of the matter, but on the way in which it is arranged.
Some examples of the absolute density of different elements and substances (expressed in their appropriate units) can be:
- Magnesium (Mg) : 1, 738 gr.cm -3
- Calcium (Ca) : 1, 54 gr.cm -3
- Iron (Fe) : 7, 874 gr.cm -3
- Molybdenum (Mo) : 10.22 gr.cm -3
- Silver (Ag) : 10.5 gr.cm -3
- Gold (Au) : 19.3 gr.cm -3
- Iridium (Ir) : 22, 562 gr.cm -3
- Dubnium (Db) : 29.3 gr.cm -3
- Bohrio (Bh) : 37.1 gr.cm -3
- Water (H 2 O) : 1 g / cm 3
- Oil : 0.92 g / cm 3
- Air : 1, 225 kg / m 3