Final answer:
The rusting of iron represents a chemical change where iron reacts with oxygen to form a new substance with different properties, whereas the other options listed are physical changes that do not alter the chemical identity of the substances.
Step-by-step explanation:
The rusting of iron represents a chemical change. In this process, the iron (Fe) reacts with oxygen (O₂) in the presence of moisture to form iron oxide (Fe₂O₃), commonly known as rust. This process results in a new substance with different properties from the original iron, which is a key characteristic of a chemical change. The other options, such as melting of ice, boiling of water, and dissolving sugar in water, are examples of physical changes where the substances do not undergo any chemical transformation but change their physical state.
Chemical changes are often characterized by a change in color, the production of a gas, the formation of a precipitate, or other indicators, such as heat or light being produced or absorbed. However, these indicators are not present in physical changes. Physical changes involve a change in the form of the substance but not its chemical identity.