Final answer:
In the given reaction, magnesium (Mg) is oxidized as it goes from an oxidation number of 0 to +2, while copper (Cu) is the element that gets reduced going from +2 to 0.
Step-by-step explanation:
In the chemical reaction Mg(s) + Cu(NO3)2(aq) → Mg(NO3)2(aq) + Cu(s), we are asked to identify what substance is oxidized. Oxidation involves the loss of electrons by a substance. In this metal replacement reaction, magnesium (Mg) starts in its elemental form with an oxidation number of 0 and ends up in a compound, magnesium nitrate, where it has an oxidation number of +2. This increase in oxidation number indicates that magnesium is the element that gets oxidized.
Using oxidation numbers, we can also determine what is reduced. Copper (Cu) is reduced as it goes from an oxidation state of +2 in copper(II) nitrate to 0 in its elemental form. Therefore, in this reaction, Mg is oxidized and Cu is reduced following the oil rig mnemonic: 'oxidation is loss, the reduction is gain'.