* New substance formed: The reaction combines nitrogen (N₂) and hydrogen (H₂) to create a completely new substance, ammonia (NH₃). This is a fundamental characteristic of chemical changes.

* Bonds broken and formed: The strong triple bond in nitrogen gas (N₂) and the single bond in hydrogen gas (H₂) are broken, and new bonds are formed between nitrogen and hydrogen atoms to create ammonia molecules. This involves a rearrangement of atoms and the formation of new chemical bonds.

* Irreversible (under normal conditions): While the reaction is reversible, it requires specific conditions to shift back to nitrogen and hydrogen. Under normal conditions, ammonia remains stable and does not spontaneously decompose back into its constituent elements.

In contrast, physical changes involve:

* Changes in appearance, state (solid, liquid, gas), or shape without altering the chemical composition of the substance.

* No formation of new substances.

* Often reversible.

Example of a physical change: Water freezing into ice. The chemical composition (H₂O) remains the same, but the physical state changes.