Which type of change does NOT alter the chemical composition of a substance?

The correct answer is that physical changes do not alter the chemical composition of a substance. During a physical change, the form of the substance may change, but its chemical identity remains the same. For example, when ice melts into water, it transitions from a solid to a liquid, but the chemical formula H2O remains unchanged.

In contrast, during a chemical change, the substances involved undergo a reaction that transforms them into different substances with distinct chemical properties. An example of this is the rusting of iron, where iron reacts with oxygen to form iron oxide, resulting in a new chemical composition.

Biological changes often involve chemical processes, such as digestion or fermentation, where substances are broken down or transformed into new chemicals. Nuclear changes involve alterations in the nucleus of an atom, typically resulting in the formation of different elements or isotopes, thereby changing the substance's chemical identity.

Thus, physical changes are characterized by alterations in physical state or appearance without any change to the underlying chemical structure.