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

* Chemical Change: A change that results in the formation of new substances with different chemical properties.

In the case of a thermometer:

* The liquid (usually mercury or alcohol) expands when heated because its molecules move faster and spread out further.

* This expansion is a change in volume and state (from a more compact to a less compact form) but doesn't alter the chemical composition of the liquid.

* If you cooled the thermometer, the liquid would contract back to its original state, demonstrating the reversibility of the physical change.

Key takeaway: The expansion of the liquid in a thermometer is a physical change because it only involves a change in the physical state of the substance, not its chemical makeup.