How does a physical change differ from a chemical change?

A physical change doesn't change what the substance is but for chemical change a new substance is formed and energy is either given off or absorbed.

The term physical change refers to transformations that may change the state of aggregation or the appearance of substances but do not change their chemical nature. In chemical changes the modifications involve the appearance of new substances by combination or breakage, so that their chemical composition is modified.

Physical change occurs when objects or substances undergo a change that does not affect their chemical nature, in contrast to the concept of chemical change. Many chemical changes are irreversible processes, while many physical changes are thermodynamically reversible.