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Precipitation Reactions and Net Ionic Equations

How do you predict a precipitate and write the net ionic equation?

IntermediateReactions & StoichiometryLast reviewed 5 October 2026
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What is it?

When two solutions of ionic compounds are mixed, the ions move around freely. If a positive ion and a negative ion can form an insoluble compound, they come together as a solid that settles out: a precipitate. This is a precipitation reaction.

Mixing silver nitrate and sodium chloride solutions gives a white precipitate of silver chloride:

AgNOX3(aq)+NaCl(aq)→AgCl(s)+NaNOX3(aq)\begin{aligned} &\ce{AgNO3(aq) + NaCl(aq)} \\[4pt] &\ce{-> AgCl(s) + NaNO3(aq)} \end{aligned}

Only the AgX+\ce{Ag+} and ClX−\ce{Cl-} ions actually react. The NaX+\ce{Na+} and NOX3X−\ce{NO3-} ions stay dissolved, unchanged: they are spectator ions. The net ionic equation shows only the change that happens:

AgX+(aq)+ClX−(aq)→AgCl(s)\ce{Ag+(aq) + Cl-(aq) -> AgCl(s)}

Key idea

Soluble ionic compounds exist in water as separate ions. A net ionic equation removes the spectator ions to show the real reaction. The same net ionic equation applies to any soluble silver compound mixed with any soluble chloride.

Why does it matter?

  • Testing for ions. Silver nitrate tests for halides; barium chloride tests for sulfate. The colour of the precipitate identifies the ion.
  • Water treatment and analysis. Precipitation removes phosphates and heavy metals from water, and gravimetric analysis finds how much of an ion is present by weighing the precipitate.
  • Health and nature. Kidney stones (calcium oxalate), limescale (calcium carbonate) and stalactites are precipitates.

How does it work?

1. Solubility rules

Usually solubleExceptions (insoluble)
All compounds of Group 1 ions (NaX+\ce{Na+}, KX+\ce{K+}, …) and NHX4X+\ce{NH4+}none
All nitrates (NOX3X−\ce{NO3-}) and ethanoates (CHX3COOX−\ce{CH3COO-})none
Chlorides, bromides, iodideswith AgX+\ce{Ag+} and PbX2+\ce{Pb^2+}
Sulfates (SOX4X2−\ce{SO4^2-})with BaX2+\ce{Ba^2+}, PbX2+\ce{Pb^2+}; CaX2+\ce{Ca^2+} only slightly soluble
Usually insolubleExceptions (soluble)
Carbonates, phosphateswith Group 1 ions and NHX4X+\ce{NH4+}
Hydroxideswith Group 1 ions; Ba(OH)X2\ce{Ba(OH)2} soluble, Ca(OH)X2\ce{Ca(OH)2} slightly
Most oxides and sulfideswith Group 1 ions and NHX4X+\ce{NH4+}

2. Predicting a precipitate

  1. Write the ions present in each solution.
  2. Swap partners to find the two possible new compounds.
  3. Check each with the solubility rules. If one is insoluble, it precipitates. If both are soluble, no reaction.

3. Writing the three equations

  1. Molecular equation: balanced, with all compounds written as formulas and state symbols.
  2. Complete ionic equation: split every soluble (aq) ionic compound into its ions. Keep solids, liquids, gases and weak electrolytes (such as HX2O\ce{H2O}) as formulas.
  3. Net ionic equation: cross out ions that appear unchanged on both sides (spectators). Check that atoms and charge balance.

Think of it like this

Think of a dance floor. When two groups of dancers mix, most keep dancing alone (spectator ions), but some pairs lock together and leave the floor (the precipitate). The net ionic equation only reports the pairs that left.

More precisely

“Insoluble” really means very slightly soluble: a tiny amount always dissolves, described quantitatively by the solubility product, KspK_\text{sp}. Solubility also depends on temperature. Strong acids, strong bases and soluble salts are written as ions because they are fully dissociated; weak acids, such as ethanoic acid, are written as molecules because they are mostly undissociated. The neutralization of any strong acid by any strong base has the same net ionic equation: HX+(aq)+OHX−(aq)→HX2O(l)\ce{H+(aq) + OH-(aq) -> H2O(l)}.

Visualise it

Two beakers are mixed. The first contains silver ions and nitrate ions, the second sodium ions and chloride ions. In the mixture, silver and chloride ions have joined as a white solid at the bottom, while sodium and nitrate ions remain dissolved as spectator ions.
Only Ag⁺ and Cl⁻ react; Na⁺ and NO₃⁻ are spectators.

Worked example

Worked example: Does a precipitate form?

Question: Predict whether a precipitate forms when solutions of (a) barium chloride and sodium sulfate, (b) potassium nitrate and ammonium chloride are mixed.

  1. (a) Possible new compounds: BaSOX4\ce{BaSO4} and NaCl\ce{NaCl}. Barium sulfate is insoluble: a white precipitate forms.
  2. (b) Possible new compounds: KCl\ce{KCl} and NHX4NOX3\ce{NH4NO3}. Both are soluble: no reaction.

Worked example: Molecular, complete ionic and net ionic equations

Question: Write the three equations for the reaction of lead(II) nitrate with potassium iodide.

  1. Molecular:

    Pb(NOX3)X2(aq)+2 KI(aq)→PbIX2(s)+2 KNOX3(aq)\small\begin{aligned} &\ce{Pb(NO3)2(aq) + 2KI(aq)} \\[4pt] &\ce{-> PbI2(s) + 2KNO3(aq)} \end{aligned}

  2. Complete ionic:

    PbX2+(aq)+2 NOX3X−(aq)+2 KX+(aq)+2 IX−(aq)→PbIX2(s)+2 KX+(aq)+2 NOX3X−(aq)\begin{aligned} &\ce{Pb^2+(aq) + 2NO3-(aq)} \\[4pt] &\quad \ce{+ 2K+(aq) + 2I-(aq)} \\[4pt] &\ce{-> PbI2(s) + 2K+(aq)} \\[4pt] &\quad \ce{+ 2NO3-(aq)} \end{aligned}

  3. Cancel the spectators (KX+\ce{K+} and NOX3X−\ce{NO3-}). Net ionic:

    PbX2+(aq)+2 IX−(aq)→PbIX2(s)\begin{aligned} &\ce{Pb^2+(aq) + 2I-(aq)} \\[4pt] &\ce{-> PbI2(s)} \end{aligned}

  4. Check: 1 Pb and 2 I on each side; charge (+2)+2(−1)=0(+2) + 2(-1) = 0 on the left and 0 on the right. Lead(II) iodide is a bright yellow precipitate.

Worked example: A gravimetric calculation

Question: Excess silver nitrate is added to 25.0 mL of 0.100 mol/L sodium chloride. What mass of silver chloride precipitates? (AgCl\ce{AgCl}: 143.32 g/mol)

  1. Moles of chloride:

    n=0.100 molL×0.0250 L=2.50×10−3 mol Cl−\begin{aligned} &n = 0.100\ \frac{\text{mol}}{\cancel{\text{L}}} \\[4pt] &\quad \times 0.0250\ \cancel{\text{L}} \\[4pt] &= 2.50 \times 10^{-3}\ \text{mol Cl}^- \end{aligned}

  2. AgX++ClX−→AgCl\ce{Ag+ + Cl- -> AgCl}: 1 mol ClX−\ce{Cl-} gives 1 mol AgCl.

    2.50×10−3 mol×143.32 gmol=0.358 g AgCl\begin{aligned} &2.50 \times 10^{-3}\ \cancel{\text{mol}} \\[4pt] &\quad \times 143.32\ \frac{\text{g}}{\cancel{\text{mol}}} \\[4pt] &= 0.358\ \text{g AgCl} \end{aligned}

Common mistake

Common mistake: Splitting a precipitate into ions

A solid (s) stays as a formula in the ionic equations. Only dissolved (aq) ionic compounds are split into ions.

Common mistake: Forgetting coefficients when splitting

2 KI(aq)\ce{2KI(aq)} gives 2 KX+(aq)+2 IX−(aq)\ce{2K+(aq) + 2I-(aq)}, and Pb(NOX3)X2(aq)\ce{Pb(NO3)2(aq)} gives two nitrate ions. Polyatomic ions stay together: nitrate is NOX3X−\ce{NO3-}, not N and O.

Common mistake: An unbalanced net equation

A net ionic equation must balance in charge as well as atoms. BaX2++SOX4X2−→BaSOX4\ce{Ba^2+ + SO4^2- -> BaSO4} is balanced: (+2) + (−2) = 0.

Notation note

  • (aq) means dissolved in water; (s) marks the precipitate.
  • Spectator ions can be shown struck through in the complete ionic equation before they are removed.

Remember this

Remember this

  • Group 1, NHX4X+\ce{NH4+} and nitrate compounds are always soluble.
  • Chlorides are soluble except with AgX+\ce{Ag+}, PbX2+\ce{Pb^2+}; sulfates except with BaX2+\ce{Ba^2+}, PbX2+\ce{Pb^2+} (and CaX2+\ce{Ca^2+}, slightly).
  • Carbonates, phosphates and hydroxides are mostly insoluble.
  • Molecular → complete ionic (split aq ionic compounds) → net ionic (remove spectators).
  • Check atoms and charge balance in the net equation.

Test yourself

Check your understanding before moving on.

Flashcards

10 cards

  1. Question
    What is a precipitate?
    Answer

    An insoluble solid that forms when two solutions are mixed.

  2. Question
    Which ionic compounds are always soluble?
    Answer

    Compounds of Group 1 ions and NH₄⁺, and all nitrates (and ethanoates).

  3. Question
    Which chlorides are insoluble?
    Answer

    AgCl and PbCl₂.

  4. Question
    Which sulfates are insoluble?
    Answer

    BaSO₄ and PbSO₄ (CaSO₄ only slightly soluble).

  5. Question
    Are carbonates and hydroxides usually soluble?
    Answer

    No: mostly insoluble, except with Group 1 ions and NH₄⁺ (Ba(OH)₂ is soluble).

  6. Question
    What is a spectator ion?
    Answer

    An ion present on both sides of the equation, unchanged: it does not take part in the reaction.

  7. Question
    Write the net ionic equation for AgNO₃(aq) + NaCl(aq).
    Answer

    AgX+(aq)+ClX−(aq)→AgCl(s)\ce{Ag+(aq) + Cl-(aq) -> AgCl(s)}

  8. Question
    Which compounds are split into ions in a complete ionic equation?
    Answer

    Only soluble (aq) ionic compounds, strong acids and strong bases. Solids, liquids and gases stay as formulas.

  9. Question
    What is the net ionic equation for a strong acid reacting with a strong base?
    Answer

    HX+(aq)+OHX−(aq)→HX2O(l)\ce{H+(aq) + OH-(aq) -> H2O(l)}

  10. Question
    What two things must balance in a net ionic equation?
    Answer

    Atoms and charge.

Quiz

7 questions

  1. Question 1EasyWhich compound is insoluble in water?
    Show answer

    Answer: BaSO₄

    Sulfates are usually soluble, but barium sulfate is an exception. Sodium, potassium and ammonium compounds, and all nitrates, are soluble.

  2. Question 2EasyWhich mixture of solutions gives a precipitate?
    Show answer

    Answer: AgNO₃ + KBr

    Silver bromide (AgBr) is insoluble and precipitates (cream colour). In the other mixtures all possible products are soluble.

  3. Question 3MediumIn the reaction of BaCl₂(aq) with Na₂SO₄(aq), which ions are spectators?
    Show answer

    Answer: Na⁺ and Cl⁻

    Ba²⁺ and SO₄²⁻ form the BaSO₄ precipitate. Na⁺ and Cl⁻ stay dissolved, unchanged: they are spectator ions.

  4. Question 4MediumWhat is the net ionic equation for Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)?
    Show answer

    Answer: Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s)

    K⁺ and NO₃⁻ are spectators. The net reaction is lead(II) ions combining with two iodide ions; atoms and charge both balance.

  5. Question 5MediumWhich substance should NOT be split into ions in a complete ionic equation?
    Show answer

    Answer: AgCl(s)

    AgCl is a solid precipitate, so it is written as a formula. Soluble salts and strong acids in solution are written as ions.

  6. Question 6MediumWhich hydroxide is soluble in water?
    Show answer

    Answer: KOH

    Hydroxides are mostly insoluble, except those of Group 1 metals (and Ba(OH)₂). Potassium hydroxide is soluble.

  7. Question 7HardExcess AgNO₃ is added to 25.0 mL of 0.100 mol/L NaCl. What mass of AgCl (143.32 g/mol) precipitates?
    Show answer

    Answer: 0.358 g

    n(Cl⁻) = 0.100 mol/L × 0.0250 L = 2.50 × 10⁻³ mol; mass = 2.50 × 10⁻³ mol × 143.32 g/mol = 0.358 g. 358 g comes from using 25.0 L.

Notes and downloads

References

  1. Brown, T. L.; LeMay, H. E., Jr.; Bursten, B. E.; Murphy, C. J.; Woodward, P. M.; Stoltzfus, M. W. Chemistry: The Central Science, 15th ed.; Pearson, 2022.

Practise this topic with flashcards and a quiz at chemistryclarity.com/chemistry/precipitation-and-net-ionic-equations/

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