* Formation of gas: The bubbles are evidence of a gas being produced. This gas is usually hydrogen, which is formed when the metal reacts with the acid.
* New substances formed: The original metal and acid are transformed into new substances. The metal ions are released into the solution, and hydrogen gas is produced.
* Irreversible: This change is generally irreversible. You can't easily get the original metal back from the solution.
Example:
Think of a piece of zinc metal in hydrochloric acid. The zinc reacts with the acid, forming zinc chloride and hydrogen gas:
Zn (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂ (g)
The bubbles you see are the hydrogen gas being released. This is a clear indication of a chemical reaction, and thus a chemical change.
