A physical change alters some properties of a sample of matter, such as its state or shape, but the identity of the matter stays the same. A chemical change, also called a chemical reaction, turns one or more substances into one or more new substances with different properties. Four visual clues suggest a chemical reaction: a color change, a gas produced, a new solid (a precipitate), or a transfer of energy. Physical changes can transfer energy too, so the only sure test is to compare the properties of the substances before and after the change.