In chemistry, OIL RIG is a mnemonic device to help students remember the difference between oxidation and reduction. It stands for "Oxidation is loss, reduction is gain." It refers to the transfer of oxygen, hydrogen and electrons during a reduction-oxidation reaction between two molecules.
Oxidation and reduction always occur together in one of these reactions, since what is lost by one molecule is gained by the other. For instance, when zinc is placed into hydrochloric acid, the zinc oxidizes and loses electrons. These electrons reduce the hydrogen atoms, allowing them to bond and form dihydrogen gas while the zinc bonds with the free chlorine to create zinc chloride.