A chemical substance which maintains its molecular structure and amount of substance during the chemical and physical processes.
In petroleum industry the most frequent pure substances are Alkanes and water (H2O).
In state of Thermodynamic equilibrium the various phases of a pure substance are always
A pure substance in a state of Thermodynamic equilibrium is always resolved in one of the state of matter: solid, liquid, gas or supercritical fluid except phase equilibrium curves on PT Diagram.
This is not the case for mixtures where various phases may occupy a continuous area in PT Diagram (see for example Vapour Liquid Equilibrium).
See Also
Natural Science / Chemistry / Chemical Substance
[ Mixtures ][ Fluid Mixtures ]
[ Thermodynamic equilibrium ][ Vapour Liquid Equilibrium (VLE) ]