A chemical change happens when the substance changes its physical state and composition resulting from a chemical reaction. This chemical reaction may either give off or absorb energy. A change in color is a chemical change. For example, litmus paper turns to red when placed in an acid solution. Production of gas is a chemical change. For example, adding hydrochloric acid to the tiles of the bathroom gives a hissing sound with gases given off. Formation of a precipitate is also a chemical change. Change in shape is not a chemical change.