* Chlorophyll Breakdown: The green pigment chlorophyll, which is responsible for photosynthesis, breaks down as the days get shorter and cooler. This breakdown is a chemical reaction.

* Other Pigments Revealed: As chlorophyll degrades, other pigments that were present all along become visible. These include carotenoids (yellows and oranges) and anthocyanins (reds and purples). The change in visibility is due to the chemical change in chlorophyll.

* Chemical Reactions: The production of anthocyanins, especially, is directly tied to chemical reactions within the leaves, influenced by factors like sunlight and sugar content.

In summary: While the visual change of leaves turning color seems like a physical change, the process is driven by chemical reactions within the leaves, making it a chemical change.