Question

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

Answer 1

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

Answer 2

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.