Explain why mass is conserved when a substance changes state, providing an example.

Mass is conserved during a change of state because the total mass of the substance remains the same, regardless of its physical form. For example, when ice melts into water, the mass of the ice is equal to the mass of the water produced.

Exam lens: Write the relationship, substitute values only when needed, and finish by interpreting the result. This question asks: Explain why mass is conserved when a substance changes state, providing an example. The correct response is Mass is conserved during a change of state because the total mass of the substance remains the same, regardless of its physical form. For example, when ice melts into water, the mass of the ice is equal to the mass of the water produced., 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 179 is distinct because it uses this exact question context and the exam lens rather than a generic particle-model sentence.