Which of the following best represents a chemical change?

A chemical change involves a transformation that results in the formation of new substances with different properties from the original materials. Rusting of iron exemplifies this process, as it involves the reaction of iron with oxygen and moisture in the environment, resulting in the creation of iron oxide, commonly known as rust. This transformation is permanent and the original iron cannot be recovered simply by reversing the process.

In contrast, the melting of ice and the boiling of water are both physical changes. They involve changes in state without altering the chemical composition of the substance; ice and water are both composed of H2O, regardless of their state. Similarly, crushing a rock is a physical change as it involves breaking the rock into smaller pieces without changing the chemical identity of the minerals that make up the rock. Therefore, the rusting of iron is the only choice that represents a true chemical change where new substances are formed.