Final answer:
The expansion of metal due to heat represents a physical change, occurring as the kinetic energy of the atoms increases, which is reversible without altering the metal's chemical properties.
Step-by-step explanation:
The expansion of a piece of metal due to heat is a physical change. This occurs because heating the metal increases the kinetic energy of its atoms, causing them to vibrate more and take up more space. The substance's internal structure is not altered in a way that changes its chemical composition or identity. Therefore, once the metal cools down, it contracts back to its original size without any alteration to its chemical properties.