posted by Kris .
Is "sharpening a pencil" more of a physical change than souring of milk, fermenting of grape juice or rusting of iron?
definitely. Sharpening a pencil leaves the lead, the wood, and the eraser in their original formulations. The others you mention changes the composition of the milk, grape juice or iron into something else.
Thanks, Dr. Bob. So are new substances formed in a chemical reaction?