4.2 Net Ionic Equations

Strong electrolytes split apart

In water, strong electrolytes (soluble ionic compounds, strong acids, strong bases) fully dissociate into ions. When writing a net ionic equation, these species are shown as free ions, not intact formulas. Weak electrolytes, molecular compounds, gases, and solids stay written as whole formulas since they don't fully break apart.

Building the net ionic equation

Start with the balanced molecular equation, convert soluble strong electrolytes into ions (the complete ionic equation), then cancel any ions that appear unchanged on both sides — the spectator ions. What remains is the net ionic equation, showing only the species that actually participate in the chemical change.

Why spectator ions matter

Spectator ions don't react; they're present in solution but end up unchanged. Removing them reveals the true chemistry — like precipitate formation, gas evolution, or neutralization. Net ionic equations must be balanced for both mass and charge, since ions carry charge that must match on both sides.

Key terms

Net ionic equation

An equation showing only the species that actually undergo chemical change, with spectator ions removed.

Complete ionic equation

An equation where all soluble strong electrolytes are written as dissociated ions, before removing spectators.

Spectator ion

An ion present in solution that does not participate in the reaction and appears unchanged on both sides.

Strong electrolyte

A substance that fully dissociates into ions in water (soluble ionic compounds, strong acids, strong bases).

Balancing net ionic equations

Must balance both atoms (mass) and overall charge on each side of the equation.

Try an AP-style question

What is the net ionic equation for AgNO₃(aq) + NaCl(aq)?

1. Ag⁺ + NO₃⁻ → AgNO₃(s)
2. Na⁺ + NO₃⁻ → NaNO₃(aq)
3. Ag⁺ + Cl⁻ → AgCl(s)
4. AgNO₃ + NaCl → AgCl + NaNO₃