What ionic equations show

An ordinary balanced equation shows all the reactants and products. An ionic equation is more focused: it shows the reacting ions and any product formed, but removes ions that are unchanged.

For example, when aqueous silver nitrate reacts with aqueous sodium chloride, a white solid silver chloride forms:

$\ce{AgNO3 (aq) + NaCl (aq) -> AgCl (s) + NaNO3 (aq)}$

The sodium ions and nitrate ions are present before and after the reaction. They are not changed into a new substance, so they are spectator ions.

In the ionic equation, only the ions that form silver chloride remain:

$\ce{Ag^+ (aq) + Cl^- (aq) -> AgCl (s)}$

Notice the state symbols. Aqueous ionic compounds can be shown as separate ions. A solid precipitate (an insoluble solid formed in solution), a liquid, or a gas is not split into ions. The symbol (aq) alone does not mean a substance must be split: dissolved molecular substances such as sugar stay as molecules. Strong acids such as hydrochloric acid are represented by their ions in aqueous solution.

Writing and simplifying the equation

Use the same reliable method each time. The aim is not to remove random substances; it is to remove only ions that are unchanged on both sides.

1. Start with the balanced symbol equation, including state symbols where they are given or required.
2. Split dissolved ionic compounds into their ions; also show strong acids such as hydrochloric acid as ions.
3. Do not split solids, liquids, gases, or substances such as water that are kept as molecules in the equation.
4. Cancel ions that are exactly the same on both sides.
5. Check that atoms and charge are balanced in the final ionic equation.

Here is the full method applied to silver nitrate and sodium chloride.

Balanced symbol equation:

$\ce{AgNO3 (aq) + NaCl (aq) -> AgCl (s) + NaNO3 (aq)}$

Full ionic equation:

$\ce{Ag^+ (aq) + NO3^- (aq) + Na^+ (aq) + Cl^- (aq) -> AgCl (s) + Na^+ (aq) + NO3^- (aq)}$

Cancel the spectator ions, $\ce{Na^+ (aq)}$ and $\ce{NO3^- (aq)}$:

$\ce{Ag^+ (aq) + Cl^- (aq) -> AgCl (s)}$

The final equation is balanced. There is one silver atom and one chlorine atom (represented in ions on the left and in the solid on the right) on each side. The total charge on the left is zero because +1 and -1 cancel; the solid product is neutral.

Checking atoms and charge

Balanced ionic equations must obey two checks:

• the number of each atom is the same on both sides
• the total charge is the same on both sides

The charge check is the part students often miss. For example:

$\ce{Mg^2+ (aq) + OH^- (aq) -> Mg(OH)2 (s)}$

This is not balanced because there are two hydroxide groups in $\ce{Mg(OH)2}$ but only one $\ce{OH^-}$ on the left. It is also not charge-balanced: +2 and -1 leave a total charge of +1 on the left, but the solid product is neutral.

The balanced ionic equation is:

$\ce{Mg^2+ (aq) + 2OH^- (aq) -> Mg(OH)2 (s)}$

Now there are two hydroxide groups on both sides. The charge on the left is $+2 + 2(-1) = 0$, which matches the neutral solid on the right.

Atoms of each element and total charge are conserved; the total number of particles need not stay the same.

Forming water in neutralisation

Ionic equations also describe reactions that make a molecular substance rather than a precipitate. In neutralisation, hydrogen ions from an acid react with hydroxide ions from an alkali to form water.

For aqueous hydrochloric acid and aqueous sodium hydroxide:

$\ce{HCl (aq) + NaOH (aq) -> NaCl (aq) + H2O (l)}$

The full ionic equation is:

$\ce{H^+ (aq) + Cl^- (aq) + Na^+ (aq) + OH^- (aq) -> Na^+ (aq) + Cl^- (aq) + H2O (l)}$

Cancel sodium and chloride ions. Keep liquid water together:

$\ce{H^+ (aq) + OH^- (aq) -> H2O (l)}$

There are two H atoms and one O atom on each side. The left-hand charge is +1 − 1 = 0, matching neutral water. The same ionic equation applies when a different strong acid and alkali supply these reacting ions.