What principle explains that mass is conserved when a substance changes state?

The correct answer is Law of conservation of mass.

Method lens: First identify the measured quantity, then match the equation or particle idea to the command word. This question asks: What principle explains that mass is conserved when a substance changes state. The correct response is Law of conservation of mass, because density links mass and volume, so the answer must preserve which quantity is being calculated. In Changes of state, the marking point should connect directly to explain that mass is conserved when a substance changes state. 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 176 is distinct because it uses this exact question context and the method lens rather than a generic particle-model sentence.