Many reactions in water take place between ions, and a molecular equation such as Pb(NO₃)₂ + 2 KI → PbI₂ + 2 KNO₃ hides what really happens. In solution, lead nitrate and potassium iodide exist as separate Pb²⁺, NO₃⁻, K⁺ and I⁻ ions. When they meet, Pb²⁺ and I⁻ combine into a yellow precipitate, while K⁺ and NO₃⁻ stay in solution exactly as they were – they are spectator ions.
Writing the complete ionic equation means splitting every strong electrolyte into its ions: strong acids (HCl, HBr, HI, HNO₃, HClO₄, HClO₃, H₂SO₄), strong bases (Group 1 hydroxides and Ca, Sr, Ba hydroxides) and soluble ionic compounds. Weak acids such as acetic acid or HF, weak bases such as ammonia, water, gases, molecular substances and precipitates are written whole. Crossing out ions that appear unchanged on both sides leaves the net ionic equation – Pb²⁺(aq) + 2 I⁻(aq) → PbI₂(s) – which applies to any soluble lead salt and any soluble iodide.
Type the reactants and products, with or without state symbols. Without them, the calculator decides solubility from the standard rules; with them – (aq), (s), (l), (g) – it follows what you wrote.
How to use the Net Ionic Equation Calculator
- Reactants (left-hand side) — Optional state symbols: AgNO3(aq) + NaCl(aq)
- Products (right-hand side) — Balancing is done for you
Everything is calculated in your browser, so results appear instantly and nothing you type is sent to a server.
How it works
The molecular equation is balanced first. Each compound is classified as a strong acid, strong base, soluble or insoluble ionic compound (common solubility rules), weak electrolyte, molecular substance or free element; strong electrolytes in solution are split into ions, identical species on both sides are cancelled and the remaining coefficients are reduced by their greatest common divisor.
Worked example
Lead(II) nitrate + potassium iodide: Pb(NO3)2 + KI → PbI2 + KNO3 balances to Pb(NO₃)₂ + 2 KI → PbI₂ + 2 KNO₃. Lead iodide is insoluble (halides of Pb²⁺ are an exception to the halide rule), the rest dissolve.
Complete ionic: Pb²⁺(aq) + 2 NO₃⁻(aq) + 2 K⁺(aq) + 2 I⁻(aq) → PbI₂(s) + 2 K⁺(aq) + 2 NO₃⁻(aq). Cancelling the spectators K⁺ and NO₃⁻ leaves Pb²⁺(aq) + 2 I⁻(aq) → PbI₂(s); charge 0 on both sides.
Typical net ionic equations
| Molecular equation | Net ionic equation | Type |
|---|---|---|
| HCl + NaOH → NaCl + H₂O | H⁺ + OH⁻ → H₂O | Neutralisation |
| AgNO₃ + NaCl → AgCl + NaNO₃ | Ag⁺ + Cl⁻ → AgCl(s) | Precipitation |
| Na₂CO₃ + 2 HCl → 2 NaCl + H₂O + CO₂ | CO₃²⁻ + 2 H⁺ → H₂O + CO₂(g) | Gas-forming |
| Zn + CuSO₄ → ZnSO₄ + Cu | Zn(s) + Cu²⁺ → Zn²⁺ + Cu(s) | Redox |
| HNO₂ + KOH → KNO₂ + H₂O | HNO₂ + OH⁻ → NO₂⁻ + H₂O | Weak acid + strong base |
Solubility rules used
- Always soluble: compounds of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and NH₄⁺; nitrates, acetates, chlorates, perchlorates, permanganates and hydrogen carbonates.
- Chlorides, bromides and iodides are soluble except with Ag⁺, Pb²⁺, Hg₂²⁺ and Cu⁺.
- Sulfates are soluble except with Ba²⁺, Sr²⁺, Pb²⁺ and Hg₂²⁺; CaSO₄ and Ag₂SO₄ are only slightly soluble and are treated as precipitates.
- Fluorides of Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺ and Pb²⁺ are insoluble.
- Hydroxides and sulfides are insoluble except with Group 1, NH₄⁺, Ca²⁺, Sr²⁺ and Ba²⁺.
- Carbonates, phosphates, sulfites, oxalates and silicates are insoluble except with Group 1 and NH₄⁺.
Tips
- Check the equation is balanced before using coefficients.
- Find the limiting reagent before calculating a yield.
- Keep temperatures in kelvin for rate and equilibrium equations.
- Everything is calculated in your browser.
Common mistakes to avoid
- Using the excess reagent to calculate theoretical yield.
- Including pure solids and liquids in equilibrium expressions.
- Entering temperatures in °C into the Arrhenius equation without converting.
Formula
Molecular equation → complete ionic equation (strong electrolytes as ions) → cancel ions identical on both sides → net ionic equationWorked example
Inputs — Reactants (left-hand side): Pb(NO3)2 + KI; Products (right-hand side): PbI2 + KNO3.
Results — Net ionic equation = Pb²⁺(aq) + 2 I⁻(aq) → PbI₂(s); Balanced molecular equation = Pb(NO₃)₂(aq) + 2 KI(aq) → PbI₂(s) + 2 KNO₃(aq); Complete (total) ionic equation = Pb²⁺(aq) + 2 NO₃⁻(aq) + 2 K⁺(aq) + 2 I⁻(aq) → PbI₂(s) + 2 K⁺(aq) + 2 NO₃⁻(aq); Spectator ions = NO₃⁻, K⁺; Reaction type = Precipitation; Charge check = Net charge 0 on the left and 0 on the right – balanced.
Key terms
- Strong electrolyte
- A substance that is fully dissociated into ions in solution (strong acids, strong bases, soluble salts).
- Weak electrolyte
- A substance only partly ionised in solution, such as acetic acid or ammonia.
- Spectator ion
- An ion that is present but unchanged during the reaction.
- Complete ionic equation
- The equation with every dissolved strong electrolyte written as separate ions.
- Precipitate
- An insoluble solid formed when two solutions are mixed.
Frequently asked questions
What is a net ionic equation?
The equation that keeps only the species that actually change in a reaction in solution, after the spectator ions are removed. For any strong acid plus strong base it is H⁺(aq) + OH⁻(aq) → H₂O(l).
How do you write a net ionic equation?
Balance the molecular equation, split all soluble strong electrolytes into ions to get the complete ionic equation, cancel ions that appear unchanged on both sides, and check that atoms and charge balance.
What are spectator ions?
Ions present on both sides of the complete ionic equation in the same form, such as Na⁺ and NO₃⁻ in AgNO₃ + NaCl → AgCl + NaNO₃.
Which compounds are split into ions?
Strong acids, strong bases and soluble ionic compounds in solution. Weak acids and bases, water, gases, molecular compounds and solids stay whole.
Why is acetic acid not split into ions?
It is a weak acid – in a 0.1 M solution only about 1% of the molecules are ionised – so it is written as CH₃COOH(aq): CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O.
What if every ion cancels?
Then no reaction occurs – for example NaCl(aq) + KNO₃(aq) just gives a solution of four ions, with no precipitate, gas or water formed.
How does it know what is soluble?
It applies the standard solubility rules: Group 1, NH₄⁺, nitrate, acetate and perchlorate salts are soluble; chlorides, bromides and iodides except Ag⁺, Pb²⁺, Hg₂²⁺; sulfates except Ba²⁺, Sr²⁺, Pb²⁺ (Ca²⁺ and Ag⁺ slightly); carbonates, phosphates, sulfides and most hydroxides are insoluble.
What formula does the Net Ionic Equation Calculator use?
Molecular equation → complete ionic equation (strong electrolytes as ions) → cancel ions identical on both sides → net ionic equation.