Substances can undergo chemical or physical changes, which affect the substances in different ways. A physical change in a substance doesn't change what the substance is. In a chemical change where there is a chemical reaction, a new substance is formed and energy is either given off or absorbed. For example, if a piece of paper is cut up into small pieces it still is paper. This would be a physical change in the shape and size of the paper. If the same piece of paper is burned, it is broken up into different substances that are not paper.
Physical changes can be reversed; chemical changes cannot be reversed without extraordinary means, if at all. For example, a cup of water can be frozen when cooled and then can be returned to a liquid form when heated. If a person decided to mix sugar into water to make sugar water, this would be a physical change because the water could be left out to evaporate and the sugar crystals would remain, meaning you could change the substance back to water and sugar. However, if a person made a recipe for a cake with flour, water, sugar and other ingredients and baked them together, you could not separate the ingredients once it’s baked together. Baking the cake would be a chemical change since this can’t be reversed.
