ans ; Yes, it is a chemical reaction called oxidation, similar to metal rusting. For details about it and ways to stop it happening read the following article:
Keep Apple Slices From Turning Brown Naturally
Why does a freshly cut apple change colour after a while?
Oxidation - to prevent this happening place fresh cut apples and pears into a bath of 2 cups ice water and 1 teaspoon lemon, lime or orange juice to prevent browning for up to 12 hours this is also advisable if baking the same.
p/s; i found this article while searching idea for food chemistry subject...
enjoy! its a great info for you to read...perhaps..~