The mass of a black hole has traditionally been identified with its energy.
We describe a new perspective on black hole thermodynamics, one that identifies
the mass of a black hole with chemical enthalpy, and the cosmological constant
as thermodynamic pressure. This leads to an understanding of black holes from
the viewpoint of chemistry, in terms of concepts such as Van der Waals fluids,
reentrant phase transitions, and triple points. Both charged and rotating black
holes exhibit novel chemical-type phase behaviour, hitherto unseen.Comment: 12 pages, Essay written for the Gravity Research Foundation 2014
Awards for Essays on Gravitatio