Here's why:

* Physical changes alter the form or appearance of a substance but don't change its chemical composition.

* Chemical changes result in the formation of new substances with different chemical properties.

When a solid changes to a liquid (melting) and then to a gas (boiling/evaporation), the molecules of the substance remain the same. They are simply moving and interacting differently due to changes in temperature and pressure.

Example: Ice melting into water and then evaporating into steam is a physical change. The water molecules are still H₂O in all three states.