With Oxidation Numbers you can see electrons move. In Predicting Single Replacements a zinc strip in copper sulfate solution grew a coat of copper; now we can say exactly what happened to the electrons.
Key idea
Oxidation is the loss of electrons (oxidation number goes up). Reduction is the gain of electrons (oxidation number goes down). They always happen together, and the electrons lost equal the electrons gained. The substance that is reduced is the oxidizing agent (it takes electrons from the other); the substance that is oxidized is the reducing agent.
Why it works
Electrons can't vanish or appear from nowhere, so every electron one substance loses, another must gain: there is no oxidation without reduction. The names of the agents describe what they do to the other substance. Copper(II) ions oxidize zinc by taking its electrons, so $\mathrm{Cu^{2+}}$ is the oxidizing agent, and in doing so it is itself reduced. A memory aid: OIL RIG, oxidation is loss, reduction is gain.
Worked example: Zinc and copper ions
In $\mathrm{Zn(s)} + \mathrm{Cu^{2+}(aq)} \to \mathrm{Zn^{2+}(aq)} + \mathrm{Cu(s)}$, identify what is oxidized, what is reduced, and the two agents.
- Zn: $0 \to +2$, loses 2 electrons: oxidized. So Zn is the reducing agent.
- Cu: $+2 \to 0$, gains 2 electrons: reduced. So $\mathrm{Cu^{2+}}$ is the oxidizing agent.
- 2 electrons transferred per zinc atom.
Answer: Zn oxidized (reducing agent); $\mathrm{Cu^{2+}}$ reduced (oxidizing agent).
Worked example: Burning magnesium
In $2\,\mathrm{Mg} + \mathrm{O_2} \to 2\,\mathrm{MgO}$, how many electrons are transferred as written?
- Mg: $0 \to +2$, 2 electrons lost per atom; 2 atoms: 4 electrons.
- O: $0 \to -2$, 2 gained per atom; 2 atoms: 4 electrons. They match.
Answer: 4 electrons; Mg is oxidized and $\mathrm{O_2}$ is reduced (the oxidizing agent).
Worked example: Is it redox?
Is $\mathrm{AgNO_3} + \mathrm{NaCl} \to \mathrm{AgCl} + \mathrm{NaNO_3}$ a redox reaction?
- Ag stays $+1$, Cl stays $-1$, Na stays $+1$, N stays $+5$ and O stays $-2$.
- No oxidation number changes.
Answer: No: it is a precipitation, not a redox reaction.
Common mistake
Calling the substance that gets oxidized the oxidizing agent. The agent is named for what it does to its partner: the oxidizing agent causes oxidation in something else, by taking its electrons, so it is the one reduced. Check: the oxidizing agent's oxidation number goes down.