Sawing the piece of wood in half is a physical change,
because no new substances are formed that weren't there
before the sawing.
Burning wood produces carbon monoxide and carbon dioxide
that weren't there before the burning. That's a chemical change.
Rust forming on iron is iron oxide that wasn't there before the
rusting. That's a chemical change.
The green that appears on copper is copper oxide that wasn't
there before the green appeared. That's a chemical change.
