Water changing to ice is considered a physical change. This is because the process of freezing involves a change in the state of matter from liquid to solid without altering the chemical composition of water itself. The molecules in water remain H2O whether they are in liquid or solid form; they simply rearrange themselves into a more ordered structure when water freezes.
On the other hand, iron rusting is classified as a chemical change. Rusting occurs when iron reacts with oxygen in the presence of moisture to form iron oxide, commonly known as rust. This reaction results in a substance with entirely different properties than iron itself, indicating a fundamental change in its chemical composition.