When a substance changes state, what happens to its mass?

The correct answer is It remains the same.

Definition lens: Give the precise definition, then add a context sentence that shows how it is used. This question asks: When a substance changes state, what happens to its mass. The correct response is It remains the same, because changes of state are explained by particle energy and arrangement. In Changes of state, the marking point should connect directly to state that changes of state are physical changes. 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 174 is distinct because it uses this exact question context and the definition lens rather than a generic particle-model sentence.