Physical change is a concept introduced to contrast with the concept of chemical change. A change in some quantity of matter is said to be a physical change if there is no change in the chemical make-up of the matter. Matter undergoes chemical change when the composition of the substances changes: one or more substances combine or break apart to form new substances.
Changes are sometimes hard to categorize as physical or chemical. Dissolving a salt in water involves the breaking of chemical bonds, yet is often described as a physical change. Some teachers hold that a chemical change is a rearrangement of atoms, but many physical changes also involve the rearrangement of atoms. Many chemical changes are irreversible, and many physical changes are reversible, but reversibility is not a certain criterion for classification. Although chemical changes are often recognized by an indication such as odor, color change, production of a precipitate, or production of a gas, every one of these indicators can result from physical change.
