Which postulate of kinetic theory is violated when a gas is compressed to very high pressure, and what is the observable consequence? Solution: Postulate 1 (negligible molecular volume) and postulate 4 (no intermolecular forces) both break down: molecules are crowded so their own volume matters, and attractions become significant. Observable consequence: the gas no longer obeys PV = nRT exactly - measured pressure deviates, as described by the van der Waals equation with its a and b corrections.