* 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 liquid ammonia boils, it changes from a liquid state to a gaseous state (ammonia vapor). The chemical composition of the ammonia (NH₃) remains the same. It's just a change in the arrangement and movement of the ammonia molecules.

Here's an analogy: Think of ice melting into water. The water molecules themselves haven't changed, just their arrangement and energy levels.