  The renowned van der Waals (VDW) state equation quantifies the equilibrium
relationship between pressure $P$, volume $V$ and temperature $k_{B}T$ of a
real gas. We assign new variable interpretations adapted to the economic
context: $P \rightarrow Y$, representing price; $V \rightarrow X$, representing
demand; and $k_{B}T \rightarrow \kappa$, representing income, to describe an
economic state equilibrium. With this reinterpretation, the price elasticity of
demand (PED) and the income elasticity of demand (YED) are non-constant factors
and may exhibit a singularity of the cusp-catastrophe type. Within this
economic framework, the counterpart of VDW liquid-gas phase transition
illustrates a substitution mechanism where one product or service is replaced
by an alternative substitute. The conceptual relevance of this reinterpretation
is discussed qualitatively and quantitatively via several illustrations ranging
from transport (carpooling), medical context (generic versus original
medication) and empirical data drawn from the electricity market in Germany.
