(P + an^2/V^2)(V - nb) = nRT. For 1 mole: (P + a/V^2)(V - b) = RT. 'a' = intermolecular attraction correction (units: L^2.atm/mol^2). Higher a = stronger attraction = easier liquefaction = larger deviation from ideality at moderate pressures. 'b' = excluded volume correction (units: L/mol). b = 4 x actual molecular volume per mole. Higher b = larger molecules. Compressibility factor Z = PV/(nRT). Ideal gas: Z = 1 at all conditions. Real gas: Z < 1 at moderate P (attraction dominates, gas more compressible). Z > 1 at very high P (volume exclusion dominates, gas less compressible). Z = 1 at Boyle temperature T_B = a/(Rb).