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absolute t & p
Therefore, if we want to find the density of some air, both the pressure and the temperature needs to be specified (it is possible to substitute one or both with something else, but we shall not investigate this complexity). For example, atmospheric air and the air in a tyre are at the same temperature, but since the air in the tyre has a higher pressure (if you remember pumping it in), it has a higher density than atmospheric air.
In this equation, the temperature and pressure must be absolute quantities. In other words, the temperature (with the unit of Kelvins) needs to be measured against absolute zero (where the entropy of any perfect crystal is zero), and the pressure is measured against absolute vacuum (the pressure to be found in a finite volume in which there is no substance). You may have come across the term "gauge pressure" where the pressure is measured against something else: these relative quantities are incompatible with thermodynamics.
== Pressure ==
