* 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 hydrogen sulfide (H₂S) boils, it changes from a liquid state to a gaseous state. The molecules of H₂S remain the same; they're just spaced further apart and moving more freely.

Key Indicators of a Physical Change:

* Reversible: You can condense the gas back into a liquid, returning the hydrogen sulfide to its original state.

* No new substances formed: The chemical composition of H₂S stays the same.

* Usually involves changes in state: Solid to liquid, liquid to gas, etc.

Let me know if you have any other questions about physical and chemical changes!