Explain how changing the volume of a fixed mass of gas affects the frequency of particle collisions with the walls of its container.

When the volume of a fixed mass of gas decreases, the gas particles have less space to move. This leads to more frequent collisions with the walls of the container, resulting in increased pressure.

Exam lens: Write the relationship, substitute values only when needed, and finish by interpreting the result. This question asks: Explain how changing the volume of a fixed mass of gas affects the frequency of particle collisions with the walls of its container. The correct response is When the volume of a fixed mass of gas decreases, the gas particles have less space to move. This leads to more frequent collisions with the walls of the container, resulting in increased pressure., 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 MS 3b, MS 3c and MS 4a skills when calculating or interpreting pressure-volume relationships. 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 575 is distinct because it uses this exact question context and the exam lens rather than a generic particle-model sentence.