Chemistry: Why Burning Is Chemical, Melting Is Physical

Physical and chemical changes are distinguished not by how dramatic a reaction looks, but by whether the substances present at the end are chemically identical to those at the start. A physical change, such as melting or dissolving, alters form or state while the particles themselves remain the same. A chemical change, by contrast, produces one or more new substances with different chemical identities, formed through the rearrangement of atoms into new combinations. Burning magnesium illustrates this clearly. The magnesium ribbon (Mg) reacts vigorously with oxygen gas (O₂) in the air, and the intense white flame leaves behind a white powder with the formula MgO — a compound chemically unlike either reactant. Because new bonds form and a genuinely new substance appears, the identities before and after the reaction differ, making this a chemical rather than a physical change.