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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).
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.
This is not the case for fluid mixtures where liquid and gas phases can coexist in Vapour Liquid Equilibrium (VLE).
See Also
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Natural Science / Chemistry / Chemical Substance
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