You leave your bicycle out in the rain and it rust is a physical or chemical change

It's a chemical change

The process when the iron rusts in the rain is a chemical change. Since the iron is exposed to oxygen and water. Generally, in my opinion it's not a good idea to leave your bicycle in the rain, it'll start with chain, it can spread to other parts of a bicycle, so dont do that.

Good luck!

Intelligent Muslim,

From Uzbekistan.