Burning of paper is a chemical change because a new substance (ash and gases) is formed, and the change is generally irreversible. In contrast, melting of ice, boiling of water, and dissolving sugar in water are all physical changes. In physical changes, no new chemical substance is formed, and the original substance can often be recovered by simple physical means (e.g., ice can be refrozen, water vapor can be condensed, sugar can be recovered by evaporating water).