In short
The signs of a chemical change are a new substance forming, a change of colour, a gas given off, a solid (precipitate) forming, or heat, light or a smell given out or taken in.
A chemical change makes a new substance with new properties, and it is usually hard to reverse. A physical change, such as melting ice, does not make a new substance. No single sign is proof by itself: boiling water gives off bubbles of vapour, but it is not a chemical change. The best evidence is a new substance with properties different from the starting materials.
Iron nails left in copper sulfate solution turn the blue solution paler and become coated in a reddish-brown layer. The colour change and the new coating show that a chemical change has happened.
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