Burning of paper is not a physical change. Ash is formed as result of burning of paper which is a new product. Chemical change a new substance must form. These changes are irreversible in nature. Air combines with the carbon and hydrogen in the paper (as paper is an organic material, obtained from woods), turning some of it into carbon dioxide and water. And energy is released into the surrounding as heat. Thus, when paper is burnt paper is changing chemically into completely different molecules.