Three Levels of the Same Equation
Most reactions in this unit happen in water, between species that are already dissolved. One reaction can be written at three levels of detail, differing in how much of what is present they show.
The molecular equation writes every substance as a complete formula: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq). It records what was mixed and what formed, but it hides the fact that the dissolved compounds exist as separated ions.
The complete ionic equation splits every soluble ionic compound into its ions: Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq). Only strong electrolytes marked (aq) split; AgCl is a solid, so it stays a formula.
The net ionic equation deletes the ions that appear unchanged on both sides: Ag⁺(aq) + Cl⁻(aq) → AgCl(s). What survives is the actual chemical change, the formation of the solid from two dissolved ions.
What Splits Into Ions and What Stays Whole
A strong electrolyte dissociates completely in water, so in a complete ionic equation it is written as its separate ions. Three groups qualify: soluble ionic salts such as NaCl and KNO₃, strong acids, and strong bases.
The strong acids to know are HCl, HBr, HI, HNO₃, H₂SO₄ and HClO₄; the strong bases are the Group 1 hydroxides, such as NaOH and KOH, together with Ba(OH)₂. Each exists entirely as ions.
A weak electrolyte ionizes only slightly, so it is written as an intact formula. Acetic acid stays CH₃COOH(aq) and ammonia stays NH₃(aq): most of their particles never become ions.
A nonelectrolyte forms no ions at all. Glucose dissolves as whole C₆H₁₂O₆ molecules, never split.
Solids, pure liquids and gases keep their formulas, not being separated into ions: AgCl(s), H₂O(l), CO₂(g) and Zn(s) are all written whole. Water is always H₂O(l), never H⁺ and OH⁻.
Polyatomic ions travel as single units. Sulfate stays SO₄²⁻ and nitrate stays NO₃⁻; they are never broken into atoms of sulfur, nitrogen and oxygen.
Predicting Precipitates: Solubility Anchors in Action
When two solutions mix, the ions can pair up in new combinations. Whether anything happens depends on whether one new pairing is insoluble: if it is, those ions lock together and leave the solution as a precipitate.
The anchor rules from Topic 3.10 are applied, not re-derived. All nitrates are soluble, and all Group 1 and ammonium salts are soluble, so Na⁺, K⁺, NH₄⁺ and NO₃⁻ almost always end up as spectators.
Chlorides are soluble except AgCl and PbCl₂. Sulfates are soluble except BaSO₄ and PbSO₄. Carbonates and sulfides are insoluble except those of Group 1 and NH₄⁺, so CaCO₃ and CdS precipitate while Na₂CO₃ and (NH₄)₂S dissolve.
If both possible products are soluble, every ion stays dissolved and no reaction occurs, and there is no net ionic equation to write.
Spectator Ions and the Route to the Net Equation
A spectator ion is an ion that appears identically on both sides of the complete ionic equation. It is dissolved before the reaction and dissolved after it, chemically untouched.
Four steps lead to the net ionic equation. First, write the balanced molecular equation with state symbols. Second, split only the strong electrolytes marked (aq), keeping everything else whole. Third, cancel every species appearing unchanged on both sides. Fourth, check what remains: atoms of every element must balance, and so must charge.
Charge balance is the check students skip: in Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) the left side sums to zero, +2 with -2, matching the neutral solid. A net equation whose charges do not match is wrong, however tidy it looks.
One Net Equation Behind Many Reactions
Strong acid with strong base always reduces to the same net equation: H⁺(aq) + OH⁻(aq) → H₂O(l). HCl with NaOH and HNO₃ with KOH have different spectators, but the change itself, hydrogen ions and hydroxide ions forming water, is identical.
If the acid is weak, it stays whole and appears in the net equation itself: CH₃COOH(aq) + OH⁻(aq) → CH₃COO⁻(aq) + H₂O(l). Splitting the acetic acid would invent ions that barely exist.
Acid with a dissolved carbonate gives 2H⁺(aq) + CO₃²⁻(aq) → H₂O(l) + CO₂(g). The gas escapes, and its leaving drives the reaction to completion.
A reactive metal in acid gives Zn(s) + 2H⁺(aq) → Zn²⁺(aq) + H₂(g), with the acid’s anion, such as Cl⁻, as spectator. Zinc atoms hand electrons to the hydrogen ions, which is why the metal dissolves and hydrogen bubbles off; the bookkeeping of those electrons belongs to Topic 4.9.
These families share a pattern: a reaction in solution goes ahead when it removes ions from solution, by forming a precipitate, a gas, or water. If none of these forms, the ions simply stay mixed.
Worked Examples
Example 1: barium chloride with sodium sulfate, at all three levels. Molecular: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq). Complete ionic: Ba²⁺(aq) + 2Cl⁻(aq) + 2Na⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) + 2Na⁺(aq) + 2Cl⁻(aq), six ion units on the left. The 2Na⁺ and 2Cl⁻ appear unchanged on the right, so four of the six units are spectators and cancel. Net ionic: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s), its left side summing to zero charge, +2 with -2.
Example 2: lead(II) nitrate with potassium iodide. Molecular: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq), the yellow precipitate. The complete ionic equation holds seven ions on the left: one Pb²⁺, two NO₃⁻, two K⁺ and two I⁻. Potassium and nitrate cancel, four spectator ions, leaving Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s). The charge check is +2 + 2(-1) = 0, and the iodine atoms balance only because of the coefficient 2 in front of I⁻.
Example 3: a mixing that is not a reaction. Solutions of KNO₃ and NaCl are mixed. Swapping the partners would give KCl and NaNO₃, both soluble. Written out, the complete ionic equation has the same four ions, K⁺, Na⁺, NO₃⁻ and Cl⁻, on both sides, so everything cancels: no precipitate, no gas, no water formed, no reaction.
Traps That Cost Marks in Topic 4.2
Splitting whatever is written (aq). Only strong electrolytes split. CH₃COOH(aq) and NH₃(aq) are aqueous but weak, so they stay whole; splitting them is the most common error in this topic.
Splitting a solid, liquid or gas. AgCl(s), CaCO₃(s), H₂O(l) and CO₂(g) never appear as ions, even alongside dissolved ions that do split.
Cancelling a species that changes form. In the acid and carbonate reaction, CO₃²⁻ on the left and CO₂ on the right are not the same species, so nothing about them cancels; the carbonate is consumed, not spectating.
Forgetting the charge check. Pb²⁺ + I⁻ → PbI₂ fails twice: the iodine atoms do not balance, and the charges sum to +1 on the left against a neutral solid. Coefficients must fix both counts together.
Breaking up a polyatomic ion. Writing SO₄²⁻ as separate sulfur and oxygen particles invents species that do not exist. The whole ion moves, hydrates and precipitates as one unit.
Writing spectators as if they reacted. Na⁺(aq) + NO₃⁻(aq) → NaNO₃(aq) describes nothing happening. Ions that stay dissolved are deleted from the net equation, not paired up.
Assuming every mixture reacts. When all possible products are soluble, the correct conclusion is no reaction. An empty net ionic equation is an answer, not a failure.
Key Relationships
- Three levels, one reaction: the molecular equation shows formulas, the complete ionic equation shows every dissolved ion, and the net ionic equation shows only species that change
- Split only strong electrolytes in solution: soluble ionic salts, strong acids and strong bases; weak electrolytes, nonelectrolytes, solids, liquids and gases stay intact
- Polyatomic ions such as NO₃⁻, SO₄²⁻ and CO₃²⁻ move and react as single units and are never split into atoms
- A spectator ion appears unchanged on both sides of the complete ionic equation and is cancelled; what remains must balance for atoms and for charge
- Solubility anchors decide precipitation: nitrates and Group 1 and ammonium salts stay dissolved; AgCl, PbCl₂, BaSO₄, PbSO₄, most carbonates and most sulfides precipitate
- The same net equation covers a whole family: H⁺ + OH⁻ → H₂O for every strong acid and strong base pairing, and 2H⁺ + CO₃²⁻ → H₂O + CO₂ for acid with a dissolved carbonate
- Reactions in solution proceed when ions are removed as a precipitate, a gas, or water; if every product stays dissolved, no reaction occurs
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