Oxygen is not always involved, so chemists also define oxidation and reduction by electrons:

Oxidation Is Loss of electrons
Reduction Is Gain of electrons

In a displacement the more reactive metal atom loses electrons and the ion of the less reactive metal gains them:

1. Zn + Cu²⁺ → Zn²⁺ + Cu
2. Zn → Zn²⁺: zinc loses 2 electrons, so zinc is oxidised
3. Cu²⁺ → Cu: copper ions gain 2 electrons, so they are reduced

A reaction in which one substance is oxidised and another is reduced is a redox reaction.

In zinc + copper sulfate solution, the sulfate ions are there before and after, unchanged. They are spectator ions. An ionic equation leaves them out and shows only the particles that change.

1. Full equation: Zn + CuSO₄ → ZnSO₄ + Cu
2. SO₄²⁻ is a spectator ion: cross it out on both sides
3. Ionic equation: Zn + Cu²⁺ → Zn²⁺ + Cu

Check an ionic equation two ways: the atoms balance, and the total charge is the same on each side. In Cu + 2Ag⁺ → Cu²⁺ + 2Ag, the charge is 2+ on both sides.

An ionic equation never has spare electrons in it: the electrons lost by one particle are gained by another.

Metals above hydrogen react with dilute acids to give a salt and hydrogen:

metal + acid → salt + hydrogen
Mg + 2HCl → MgCl₂ + H₂

Hydrochloric acid makes chlorides; sulfuric acid makes sulfates. Test for hydrogen: hold a lit splint at the mouth of the tube: a squeaky pop.

This is a redox reaction too (Higher): Mg + 2H⁺ → Mg²⁺ + H₂. Magnesium atoms lose electrons (oxidised); hydrogen ions gain them (reduced). The chloride ions are spectators.