What clues indicate a rusty bike has gone through a chemical change

That the color isn't the same because it has rust, it will be now dark red or even green.

That bike is certainly made of iron. And when iron comes in contact with oxygen, it rusts. Well, a chemical change occurs! It is rusty, the red marks clearly show that - so, this is the evidence that the bike has gone through a chemical change. There is plenty of oxygen in the air so the bike has to rust.