Explain how the pressure of a fixed mass of gas changes when the volume of its container is halved while the temperature remains constant.

When the volume is halved, the gas particles collide more frequently with the walls, so the pressure doubles. This follows from the particle model where pressure is proportional to collision frequency and force, and at constant temperature the kinetic energy of the particles remains unchanged.

Exam lens: Write the relationship, substitute values only when needed, and finish by interpreting the result. This question asks: Explain how the pressure of a fixed mass of gas changes when the volume of its container is halved while the temperature remains constant. The correct response is When the volume is halved, the gas particles collide more frequently with the walls, so the pressure doubles. This follows from the particle model where pressure is proportional to collision frequency and force, and at constant temperature the kinetic energy of the particles remains unchanged., 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) Calculate pressure when volume changes 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 551 is distinct because it uses this exact question context and the exam lens rather than a generic particle-model sentence.