Learning focus
Use chemical formulae and equations accurately, convert measured quantities into moles, apply balanced-equation ratios and present multi-step calculations with units and checks.
Dissociating aqueous ionic compounds
Strong soluble ionic substances are written as separate aqueous ions in a full ionic equation. Solids, liquids, gases and weakly ionised substances are normally kept intact at this level.
Spectator ions
A spectator ion is present in solution but does not undergo chemical change. It appears in the same form on both sides and is cancelled. Cancellation must preserve coefficients and charge.
Net ionic equation
The remaining species form the net ionic equation. This equation shows the essential chemical change shared by many molecular equations.
Checks
Count atoms and calculate total charge on each side. A correct ionic equation conserves both. Do not cancel ions that change state, combine into a precipitate or form a molecule.
Worked example
AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq). Full ionic: Ag+ + NO3- + Na+ + Cl- -> AgCl + Na+ + NO3-. Cancel Na+ and NO3-.
Practical or data skill
Use colour coding to mark cations and anions in full ionic equations, then cross out identical spectators.
Examination tip
Only split substances labelled (aq), and only when they are ionic electrolytes.
Review questions and suggested answers
Question 1
What is a spectator ion?
Suggested answer
An ion present but unchanged in the reaction.
Question 2
Which ions are spectators in AgNO3 + NaCl?
Suggested answer
Na+ and NO3-.
Question 3
What two quantities must an ionic equation conserve?
Suggested answer
Atoms and total charge.
