A physical reaction involves a change in the form or state of a substance without altering its chemical composition. Examples include melting, freezing, and dissolving, where the substance remains the same but changes its physical state or appearance. These changes are typically reversible.

In contrast, a chemical reaction involves a transformation that changes the chemical composition of the substances involved. During a chemical reaction, bonds between atoms are broken and reformed, resulting in the creation of new substances with different properties. Examples include rusting, combustion, and baking, and these changes are usually irreversible.

In summary, physical reactions change only physical properties, while chemical reactions change the substance's chemical identity.