Q.List the basic assumptions of the kinetic theory of gases. Which of these assumptions is directly responsible for a gas exerting pressure on the walls of its container?

The kinetic-theory assumptions are: (1) very large number of molecules in ceaseless random motion; (2) molecular size negligible compared to separation; (3) no intermolecular force except during collision; (4) collisions perfectly elastic and of negligible duration; (5) Newton's laws obeyed throughout; (6) gravity neglected on the molecular scale.

Of these, assumption 4 -- that a molecule strikes a wall and rebounds elastically, delivering a definite impulse ($2mv_x$ per collision, Section 9.3) -- is what directly produces pressure: every collision transfers momentum to the wall, and summing this transfer over the enormous number of molecules striking the wall every instant, from every direction (assumption 1), gives the steady, continuous force per unit area measured as pressure. Without elastic collisions delivering momentum, there would be no mechanism at all for the gas to push on its container.

List all six assumptions, then identify which one describes the actual momentum-transfer mechanism behind pressure.

• Attributing pressure to random motion alone (assumption 1) without recognising that it is specifically the ELASTIC COLLISION with the wall (assumption 4) that transfers the momentum producing a force.
• Forgetting that pressure requires summing an enormous NUMBER of individual collisions, not just describing one.