While hiking in the woods, you see a brown, rotting apple lying on the ground. what changes is the physical properties of the apple are signs that a chemical reaction has occurred?

The change in color.

Explanation:

The apple turn brown in color because of the oxidation process. When the oxygen and water molecules in air react with it, oxidation take place. The oxidation process is very efficient in ambient temperature.

For example, if the peal off apple is placed into the refrigerator it take a time to got oxidize and turn brown, but if it is placed in room temperature it quickly turn brown.

when oxygen is react with peel off apple , it trigger the polyphenol oxidase enzyme to oxidize the phenolic compound and quinones are formed which then react with amino acids and produced brown color.