Theorem of corresponding states
According to van der Waals, the theorem of corresponding states (or principle/law of corresponding states) indicates that all fluids, when compared at the same reduced temperature and reduced pressure, have approximately the same compressibility factor and all deviate from ideal gas behavior to about the same degree. Material constants that vary for each type of material are eliminated, in a recast reduced form of a constitutive equation. The reduced variables are defined in terms of critical variables. The principle originated with the work of Johannes Diderik van der Waals in about 1873 when he used  the critical temperature and critical pressure to characterize a fluid. The most prominent example is the van der Waals equation of state, the reduced form of which applies to all fluids. Compressibility factor at the critical point. The compressibility factor at the critical point, which is defined as formula_1, where the subscript formula_2 indicates the critical point, is predicted to be a constant independent of substance by many equations of state; the Van der Waals equation e.g. predicts a value of formula_3. Where: For example: External links.