When a substance changes from a solid to a liquid, what happens to its mass (Changes of state) (Changes of state; definition focus: Explain that mass is conserved when a substance changes state) (Changes of state; definition focus: Explain that mass is conserved when a substance changes state)

The correct answer is It stays the same.

Unit lens: Check the units before giving the final statement so the physics quantity is not swapped. This question asks: When a substance changes from a solid to a liquid, what happens to its mass (Changes of state) (Changes of state; definition focus: Explain that mass is conserved when a substance changes state) (Changes of state; definition focus: Explain that mass is conserved when a substance changes state). The correct response is It stays the same, 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 181 is distinct because it uses this exact question context and the unit lens rather than a generic particle-model sentence.