Reduction is the gain of electrons or the decrease of an oxidation number. Oxidation is the loss of electrons or the increase of the oxidation number. A particle gains electrons when it is reduced and a particle loses electrons when it is oxidised. Redox half-equations provide information about the change in particles and the loss or gain of electrons. Many half-equations can be balanced by adding electrons, water molecules and hydrogen ions.

Fe³⁺ + e⁻ → Fe²⁺ (reduction)
Mg → Mg²⁺ + 2e⁻ (oxidation)

The reactivity of halogens decreases going down the group 7 of the periodic table. A more reactive halogen displaces a less reactive halogen in a halide salt solution.

A more reactive halogen displaces a less reactive halogen in halide salt solutions. The reactivity of halogens decreases going down the group 7 of the periodic table. Since iodine is lower down the group 7 than chlorine, it it is less reactive and does not displace chlorine in its salt.

Iodine is less reactive than chlorine.

A particle gains electrons when it is reduced. In these half-equations Mn⁵⁺ gains electrons.

Mn⁵⁺

A more reactive halogen displaces a less reactive halogen in halide salt solutions. Chlorine is more reactive than iodine, thus chlorine displaces iodine in the salt.

CaI₂ + Cl₂ → CaCl₂ + I₂

A more reactive halogen displaces a less reactive halogen in halide salt solutions. Fluorine is more reactive than bromine, thus fluorine displaces bromine in the salt.

2LiBr + F₂ → 2LiF + Br₂

MnO₄⁻ + H⁺ + e⁻ → Mn²⁺ + 4H₂O
MnO₄⁻ + 8H⁺ + e⁻ → Mn²⁺ + 4H₂O
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O