Question

bike rusting; physical; chemical

Explanation:

Bike rusting involves a chemical reaction where iron (Fe) reacts with oxygen and water to form iron oxide (rust, $\text{Fe}_2\text{O}_3\cdot n\text{H}_2\text{O}$). This creates a new substance, which defines a chemical change, not a physical change that only alters form without creating new matter.

Answer:

Chemical