* Physical Changes: Alter the form or appearance of a substance but not its chemical composition.

* Chemical Changes: Result in the formation of new substances with different chemical properties.

When water boils:

* State change: It transitions from a liquid to a gas (water vapor).

* Chemical composition: The water molecules (H₂O) remain the same. They are simply moving more rapidly and spreading further apart.

Key point: You can get the water back to its liquid state by cooling the steam, demonstrating that no new substance was formed.