What happens when gas particles collide with the walls of their container (Particle motion in gases)

The correct answer is They exert a force on the walls..

Definition lens: Give the precise definition, then add a context sentence that shows how it is used. This question asks: What happens when gas particles collide with the walls of their container (Particle motion in gases). The correct response is They exert a force on the walls., because gas pressure comes from particle collisions with container walls. In Particle motion in gases, the marking point should connect directly to describe gas particles as moving in constant random motion. If the question includes values, the working must keep the appropriate unit and operation; if it is an explanation, it must name the relevant particle behaviour or energy change. This item belongs to Particle model and pressure, so avoid answers that switch to a different quantity, confuse heat with temperature, or describe gas pressure without collisions when collisions are the reason. Checkpoint 450 is distinct because it uses this exact question context and the definition lens rather than a generic particle-model sentence.