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

* Physical Change: A physical change alters the appearance or form of a substance but does not change its chemical composition.

The Reaction:

When hydrogen (H₂) reacts with chlorine (Cl₂), they combine to form hydrogen chloride (HCl), a completely different substance with different properties.

H₂ + Cl₂ → 2 HCl

* Hydrogen (H₂) and Chlorine (Cl₂): These are gases at room temperature.

* Hydrogen Chloride (HCl): This is a colorless gas that dissolves in water to form hydrochloric acid.

Key Indicators of a Chemical Change:

* Formation of a new substance: Hydrogen chloride (HCl) is a completely different substance than hydrogen or chlorine.

* Release of energy: This reaction releases heat and light, indicating a chemical change.

* Irreversible process: While HCl can be broken down back into hydrogen and chlorine, this requires further chemical processes.

Therefore, the burning of hydrogen in chlorine gas is a chemical change due to the formation of a new substance with different properties.