Final answer:
POR in chemistry stands for Proof of Replacement and is used to show a substitution or exchange in a molecule, making the statement true.
Step-by-step explanation:
The statement 'POR is used to show a substitution or exchange' is true. In chemistry, POR typically stands for Proof of Replacement, which indeed involves showing how one element or group within a molecule is substituted or exchanged for another. This concept is foundational in understanding chemical reactions and the rearrangement of atoms to form new substances.