Answer:
Leaving a Copper penny in vinegar until it turns green
Step-by-step explanation:
the others are physical changes. when copper penny is put in vinegar, a reactive oixde layer is formed which is green in colour. namely, Cu(II) acetate.
The green growing on the penny of copper and the rust forming on the nail of iron are chemical changes. Boiling away salt water, scraping iron filings from a mixture of sand with a magnet, and breaking a rock with a hammer, are physical changes.