What is the definition of density?

The correct answer is Mass per unit volume.

Definition lens: Give the precise definition, then add a context sentence that shows how it is used. This question asks: What is the definition of density. The correct response is Mass per unit volume, because density links mass and volume, so the answer must preserve which quantity is being calculated. In Density of materials, the marking point should connect directly to define density as mass per unit volume. 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 Changes of state and the particle model, 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 6 is distinct because it uses this exact question context and the definition lens rather than a generic particle-model sentence.