Physical changes do not produce new substances, but chemical changes do produce new substances. A physical change is concerned with states of matter. Melting, vaporization, freezing, sublimation and condensation are physical changes. Melting an ice cube is a physical change because the substance remains as water. Most physical changes are reversible. Crushing a can, boiling water, breaking glass, chopping wood, mixing sand and water, and shredding paper are physical changes.
Chemical changes occur on the molecular level. The change produces a new substance. Chemical changes include combustion, synthesis, oxidation, neutralization and decomposition. For example, iron rusting is an instance of a chemical change. Food metabolism, burning wood, mixing and acid with a base, baking a cake and using a chemical battery are also examples of chemical changes.
A chemical change can be determined by evaluating if a reaction has occurred. A chemical reaction may have occurred if a change in light, heat, color, gas production, odor or sound is produced. If a precipitate develops, then a chemical change has occurred. An odor is a sign of a chemical change because it is evidence that decomposition is occurring. For example, a rotten egg produces an odor as it transforms into a new substance.