A molecular chemical equation writes dissolved substances using their compound formulas. In an ionic equation, soluble strong electrolytes are instead represented as the ions that are actually dispersed in aqueous solution.

Consider the reaction

$$\mathrm{AgNO_3(aq)+NaCl(aq)\rightarrow AgCl(s)+NaNO_3(aq)}.$$

Silver nitrate, sodium chloride and sodium nitrate are soluble strong electrolytes, so their aqueous formulas are expanded into ions. Solid silver chloride remains written as a solid compound:

$$\mathrm{Ag^+(aq)+NO_3^-(aq)+Na^+(aq)+Cl^-(aq)}$$ $$\mathrm{\rightarrow AgCl(s)+Na^+(aq)+NO_3^-(aq)}.$$

This is the complete ionic equation.

The conversion follows the physical state of each species:

• soluble strong electrolytes in aqueous solution are written as separated ions;
• solids, pure liquids and gases are not split into aqueous ions;
• weak electrolytes are normally kept mainly in molecular form because they are not completely ionized.

The coefficients and ion charges must preserve both atom counts and total charge.

In the example, $\mathrm{Na^+}$ and $\mathrm{NO_3^-}$ appear unchanged on both sides. Such ions are called spectator ions: they are present in the solution and maintain charge balance, but they do not undergo the chemical or physical change represented by the reaction.

A complete ionic equation is therefore a more microscopic representation than the molecular equation. It exposes which dissolved species are actually present and makes it possible to distinguish reacting ions from spectators.