* 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, it changes from a liquid to a gas (water vapor). The chemical composition of water (H₂O) remains the same, it's just the arrangement of the water molecules that changes.

Here's a simple way to think about it:

* Boiling water: You can condense the steam back into liquid water, proving that the water molecules are still intact.

* Burning wood: The wood reacts with oxygen, forming ash, smoke, and other substances. This is a chemical change because the wood is transformed into new substances.