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How To Write Ionic Equations

Ionic equations: split aqueous ions, cancel spectator ions, and balance atoms and charge for common precipitation, neutralisation, and gas reactions.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • construct chemical equations, with state symbols, including ionic equations.

Ionic equation questions are “cancel spectator ions” questions. If you do not split (aq) correctly, you cannot get the final line.

1. Definition

An ionic equation (net ionic equation) shows only the ions/particles that actually react. Spectator ions are ions that remain unchanged and are cancelled out.

2. Key Ideas

  • Split aqueous ionic compounds into ions; do not split (s), (l), or (g).
  • Cancel ions that appear identical on both sides (including the same charge and state).
  • The net ionic equation must still be balanced (atoms and total charge).
  • Many standard net ionic equations repeat in exams (neutralisation, precipitation, carbonate + acid).

3. Detailed Explanations

Quick Recall (ionic equations)
  • Split aqueous ionic compounds and the acids/alkalis represented as ions in the question; do not split (s), (l), or (g).
  • Cancel spectator ions only if they are identical (same charge and state) on both sides.
  • Final line must be balanced for atoms and total charge.

A. Core Terms

  • Dissociation: when an ionic compound dissolves in water, its ions separate and move freely.
  • Spectator ions: ions present in solution that do not change; they cancel out.

B. Method: Split → Cancel → Write

  1. Write the balanced molecular equation with state symbols.
  2. Split aqueous ionic compounds and the acids/alkalis represented as ions.
  3. Cancel spectator ions (exactly the same on both sides).
  4. Write the net ionic equation.
Rule that saves marks

Split the aqueous species shown as ions. Never split a precipitate (s), water (l), or a gas (g).

C. Quick Reference: What to Split

Substance typeExampleSplit into ions?
Aqueous ionic compoundNaCl(aq)Yes
Aqueous acidHCl(aq)Yes
Alkali (aqueous base)NaOH(aq)Yes
Solid ionic compoundAgCl(s)No
WaterH₂O(l)No
GasCO₂(g)No

If a question includes a weak acid such as CH₃COOH(aq), follow the species given in the equation: it is not represented as fully dissociated ions.

Deriving a net ionic equationThe balanced equation between silver nitrate and sodium chloride is split into aqueous ions. Sodium and nitrate ions are unchanged spectator ions and are cancelled. The net equation is Ag plus aqueous plus chloride aqueous forms silver chloride solid.1 · Balanced equation with state symbolsAgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)2 · Split aqueous ions; keep the solid intactAg⁺ + NO₃⁻ + Na⁺ + Cl⁻ → AgCl(s) + Na⁺ + NO₃⁻spectators: Na⁺ and NO₃⁻ appear unchanged on both sides3 · Cancel spectators and check atoms + total chargeAg⁺(aq) + Cl⁻(aq) → AgCl(s)left charge: +1 + (−1) = 0 · right charge: 0
Split the aqueous ions, cancel unchanged spectator ions, then check both atoms and total charge.

4. Common Mistakes

  • Splitting solids (e.g., writing AgCl(s) → Ag⁺ + Cl⁻). Wrong.
  • Cancelling ions that are not identical (different charge or different state).
  • Forgetting to check charge balance at the end.
  • Writing H⁺ + OH⁻ → H₂O without state symbols when asked.
  • Mixing molecular and ionic forms on the same side (either split properly or don’t).

5. Exam Tips

Final line checklist
  1. Same atoms on both sides 2) Same total charge on both sides 3) Simplest form (no spectators left)
Where ionic equations show up

Neutralisation, precipitation, displacement, and gas-forming reactions (e.g., carbonate + acid).

6. Worked Examples

Modelled example 1

Neutralisation

Core

Problem

Derive the net ionic equation for NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l).
Study the worked solution

  1. Split aqueous electrolytes

    Method

    Represent sodium hydroxide, hydrochloric acid and sodium chloride as aqueous ions.

    Reason

    Aqueous ionic species dissociate; liquid water remains intact.

    Working

    Na + (aq) + OH-(aq) + H + (aq) + Cl-(aq) → Na + (aq) + Cl-(aq) + H₂O(l)

  2. Cancel spectator ions

    Method

    Remove identical Na⁺ and Cl⁻ ions from both sides.

    Reason

    Their formula, charge and aqueous state are unchanged.

    Working

    Spectators: Na + (aq), Cl-(aq).

  3. Write and check the net equation

    Method

    Retain only reacting particles.

    Reason

    Atoms and total charge must remain balanced.

    Working

    H + (aq) + OH-(aq) → H₂O(l)

Guided practice 2

Precipitation (Silver Chloride)

About 7 min

Problem

Write the net ionic equation for aqueous silver nitrate reacting with aqueous sodium chloride to form silver chloride precipitate.

Choose what splits and what remains intact

AgCl(s) treatment
Spectator ions

Hints

Hint 1: write the molecular equation
Start with AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq).
Hint 2: apply state rules
Split every aqueous ionic compound, but not AgCl(s).
View solution step by step

  1. Write and split the equation

    Method

    Split the three aqueous compounds into ions.

    Reason

    The solid silver chloride precipitate remains intact.

    Working

    Ag + (aq) + NO₃-(aq) + Na + (aq) + Cl-(aq) → AgCl(s) + Na + (aq) + NO₃-(aq)

  2. Cancel spectators

    Method

    Cancel sodium and nitrate ions.

    Reason

    They appear in identical aqueous forms on both sides.

    Working

    Cancel Na + (aq) and NO₃-(aq).

  3. Write the net equation

    Working

    Ag + (aq) + Cl-(aq) → AgCl(s)

Common misconception 3

Error Analysis (Spot the Wrong Split)

Find and correct the mistake

Learner response

A student writes AgCl(s) → Ag + (aq) + Cl-(aq) while deriving a precipitation equation. Locate the state-rule error and correct the representation.

Use the state symbol as the decision

Correct treatment
Species normally split

View solution step by step

  1. Locate the state-rule error

    Method

    Read the (s) state symbol.

    Reason

    It identifies an insoluble solid precipitate, not freely separated aqueous ions.

    Working

    AgCl(s) must not become Ag + (aq) + Cl-(aq).

  2. Apply the correct split rule

    Method

    Split aqueous ionic compounds only.

    Reason

    Solids, liquids and gases remain as complete species in net ionic equations.

    Working

    Correct representation: AgCl(s).

Examiner practice 4

Displacement (Zinc and Copper(II) Ions)

3 marks

Examination question

Derive the net ionic equation for Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). [3 marks]

Split, cancel and check charge

View solution step by step

  1. Split aqueous sulfates

    1 mark

    Method

    Separate CuSO₄(aq) and ZnSO₄(aq) into ions.

    Reason

    Solid zinc and copper remain intact.

    Working

    Zn(s) + Cu²⁺(aq) + SO₄²⁻(aq) → Zn²⁺(aq) + SO₄²⁻(aq) + Cu(s)

  2. Cancel sulfate

    1 mark

    Method

    Remove SO₄²⁻(aq) from both sides.

    Reason

    It is unchanged in charge and state and is therefore a spectator.

    Working

    Spectator: SO₄²⁻(aq).

  3. Write the net equation

    1 mark

    Method

    Retain zinc, copper(II) ions, zinc ions and copper.

    Reason

    These are the particles whose oxidation states change.

    Working

    Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

Challenge 5

Gas-Forming Reaction (Carbonate + Acid)

Minimal support

Words-to-ions transfer

Carbonate ions react with dilute acid to produce carbon dioxide and water. Construct the balanced net ionic equation with state symbols.

Balance atoms and total charge

Product states

Hints

Hint 1: balance charge first
The carbonate ion contributes total charge 2-; products are neutral.
Hint 2: then check hydrogen
Two hydrogen ions supply the two H atoms needed for one water molecule.
View solution step by step

  1. Write the reacting species

    Method

    Use carbonate ions and hydrogen ions.

    Reason

    Spectator cations and acid anions are not part of the stated net change.

    Working

    CO₃²⁻(aq) + H + (aq) → CO₂(g) + H₂O(l).

  2. Balance charge and hydrogen

    Method

    Place coefficient 2 before H⁺.

    Reason

    -2 + 2(+1) = 0, matching neutral products, and two H atoms form water.

    Working

    CO₃²⁻(aq) + 2H + (aq) → CO₂(g) + H₂O(l).

  3. Check the final equation

    Method

    Count C, O, H and total charge.

    Reason

    A valid net ionic equation conserves both atoms and charge.

    Working

    CO₃²⁻(aq) + 2H + (aq) → CO₂(g) + H₂O(l)

7. Mind Stretchers

Mind stretcher 1: Cancel CorrectlyExtension

Question: Find the net ionic equation for: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq).

Show Answer

Split aqueous substances:

Ba²⁺(aq) + 2Cl-(aq) + 2Na + (aq) + SO₄²⁻(aq) → BaSO₄(s) + 2Na + (aq) + 2Cl-(aq)

Cancel Na⁺ and Cl⁻:

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Mind stretcher 2: Charge Check TrapExtension

Question: A student writes H + (aq) + CO₃²⁻(aq) → CO₂(g) + H₂O(l). Explain what is wrong and fix it.

Show Answer

Charge is not balanced: left side total charge is + 1 + (-2) = -1, right side is 0.

You need 2 hydrogen ions:

CO₃²⁻(aq) + 2H + (aq) → CO₂(g) + H₂O(l)

8. Quiz

Quiz time

Ready to check your understanding? Try the interactive quiz, then review any questions you missed.

Go to quiz