State the relationship between pressure and volume for a fixed mass of gas at constant temperature.

The relationship is that pressure multiplied by volume is constant (P × V = constant) for a fixed mass of gas at constant temperature.

Graph lens: Read the trend, flat section, gradient, or axis labels before explaining the physical meaning. This question asks: State the relationship between pressure and volume for a fixed mass of gas at constant temperature. The correct response is The relationship is that pressure multiplied by volume is constant (P × V = constant) for a fixed mass of gas at constant temperature., 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 579 is distinct because it uses this exact question context and the graph lens rather than a generic particle-model sentence.