When gas particles collide with the walls of their container, what do they exert on the walls?

The correct answer is A force.

Particle lens: Describe arrangement, motion, spacing, collisions, or energy changes only when they are relevant here. This question asks: When gas particles collide with the walls of their container, what do they exert on the walls. The correct response is A force, because density links mass and volume, so the answer must preserve which quantity is being calculated. In Pressure in gases (physics only), the marking point should connect directly to (Physics only) Apply the relationship pressure x volume = constant for a fixed mass of gas at constant temperature. 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 540 is distinct because it uses this exact question context and the particle lens rather than a generic particle-model sentence.