Melting, freezing, boiling, condensing and subliming are physical changes of state. Particles remain the same, but their arrangement and energy change; a state change can be reversed. Mass is conserved in a closed system even when the volume and density change. Example: Explain why melting is physical. It changes arrangement, not the chemical identity of the substance. Distinguish the observation from the explanation, and support any judgement with evidence. Use clear labels. Explain the result.