* Physical Change: A change in the form or appearance of a substance, but not its chemical composition.

* Chemical Change: A change where a new substance is formed with different chemical properties.

When you heat potassium chloride (KCl) crystals, the main thing happening is:

* Phase Change: The crystals are changing from a solid state to a liquid state (melting).

* Increased Kinetic Energy: The particles within the crystal are moving faster and vibrating more due to the added heat.

Important Note: While heating potassium chloride crystals to a high enough temperature *could* cause a chemical change (like decomposition), simply heating them to melt is considered a physical change.