What is the primary difference between a physical change and a chemical change?

The primary difference between a physical change and a chemical change lies in how they affect substances at the molecular level. A physical change involves alterations that affect the physical properties of a substance, such as its shape, size, or state of matter (like melting or freezing), without altering its chemical structure or composition. For example, when water freezes into ice, it changes from a liquid to a solid, but the molecular composition of water (H₂O) remains the same.

This understanding is crucial in distinguishing between changes that can be reversed, like dissolving sugar in water or melting ice, and those that create new substances, which would be indicative of a chemical change. In a chemical change, the original substances undergo a transformation that results in new substances with different properties, such as using heat to convert hydrogen and oxygen into water, which cannot be reversed to obtain the original gases without another chemical reaction.

Recognizing that a physical change affects the form without altering the chemical identity clarifies this distinction and reinforces how materials interact under various conditions.