A chemical reaction—or chemical change—is the process by which some substances transform into others, changing their chemical composition and their chemical bonds.

Chemical changes can be detected because the physical and chemical properties of the substances involved in the reaction or reactants are different from the ones formed through the reaction or products.

A chemical reaction can be easily identified when it causes a change in a property of the substances involved that can be easily measured or detected: for example in the presence of caustic soda (NaOH) and water (H2O), potassium permanganate (KMnO4)—which is purple in color—reacts with sugar (C6H12O6) to form potassium manganate (K2MnO4)—which is green in color. As the reaction progresses, this new substance keeps reacting with sugar, forming manganese(IV) oxide (MnO2)—which is brown in color.