# Volume change

 Left: The expansion phase (volume increase) of a standard heat cycle. Right: the contraction phase (volume decrease) of a standard heat cycle.
In thermodynamics, volume change, in terms of either a small differential volume change dV or large volume change ΔV, refers to a change in the three-dimensional structure of a body, delineated by a boundary (or boundary surface), on going from in initial to a final volume V during a reaction or process. This is expressed mathematically as:

$\Delta V = V_f - V_i \,$

where Vf is the final volume of the system and Vi is the initial volume.

Social volumes
The measure of volume changes in human chemical processes and reactions can be understood in several ways: (a) in terms of daily heat cycle expansions of society due to expansion phase daily heat input from the hot body of the sun, followed by contraction during contact with the cold body of the night sky, (b) in terms of interpersonal chemical reactions, and the volume changes accruing therein, and (c) in terms of large scale territorial changes of societies over decades and centuries.
 A depiction of volume decrease resulting from a change in the number of chemical entities in during a chemical reaction, such as would be the case when two atoms unite to form a diatomic molecule.

This reaction volume change aspect is illustrated adjacent, where in the initial state volume Vi two single men and two single women are shown each having a set amount of personal space, which he or she would occupy in a large city in terms of costly square foot living space, and in the final state volume, one man and one women have chemically reacted to form a dihumanide molecule (two human molecules bound as a single entity) or married pair at which point their previously dual volumes become united to into one shared space of existence, and hence a volume decrease accrued in the process, quantified by a specific amount of PΔV work energy.