Which type of change affects only the form of a substance and not its chemical composition?

The correct choice is a physical change. A physical change refers to any change that alters the form or appearance of a substance but does not affect its chemical composition. This means that during a physical change, the actual molecules of the substance remain intact and retain their original chemical properties. Common examples of physical changes include melting ice into water, dissolving sugar in water, or breaking a glass. In all of these instances, the substance maintains its chemical identity regardless of changes in its physical state or appearance.

In contrast, a chemical change involves a transformation that alters the chemical composition of a substance, leading to the formation of new substances. Biological changes also involve chemical processes that lead to new products, while phase changes, although related to physical changes, specifically refer to transitions between solid, liquid, and gas states and do not encompass all types of physical changes. Thus, physical change is the most accurate answer in this context.