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Preparing Insoluble Salts

Prepare an insoluble salt by precipitation: choose soluble ion sources, collect the residue, wash away dissolved impurities and dry the product.

  • SEC G3 Pure Chemistry 2027

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An insoluble salt forms as a solid when suitable dissolved ions meet. Your product is the residue on the filter paper. Collecting the wrong fraction loses the product; skipping washing leaves soluble impurities on it.

Before starting, use the solubility rules to establish that the target salt is insoluble. For a soluble target, use the soluble-salt methods.

Choose soluble sources of the required ions

Choose solutions of two soluble compounds that supply the required cation and anion. When mixed, the required ions form an insoluble solid.

Example: barium sulfate

BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

  • Macroscopic observation: a white precipitate forms.
  • Particle model: Ba²⁺ and SO₄²⁻ ions join to form a solid lattice; Na⁺ and Cl⁻ remain aqueous spectator ions.
  • Symbolic representation: the ionic equation shows only the ions that form the precipitate and balances both atoms and charge.

Separate, wash and dry the precipitate

  1. Mix the two solutions and stir.
  2. Filter to collect the insoluble salt as the residue.
  3. Wash the residue with distilled water to remove soluble ions on its surface.
  4. Dry the product between filter papers.

Choose enough of each solution to produce the required precipitate. You do not need to say that “both reactants are in excess”: one reagent can be in excess relative to the other. Any dissolved excess is removed by filtration and washing.

Use a little cold distilled water for washing. It dissolves soluble material in the solution clinging to the precipitate. Even a salt described as insoluble has a small solubility, so very large volumes of wash water can lose some product. Filtering alone does not remove liquid held among the solid particles.

For calcium carbonate made from calcium nitrate and sodium carbonate, sodium and nitrate ions remain in solution. The collected solid also initially holds some of that solution; washing removes those soluble impurities before drying.

Choose the correct filter fraction for your saltIn the first panel, black unreacted copper(II) oxide remains in the filter paper while blue copper(II) sulfate solution passes into the receiving beaker: keep the filtrate. In the second panel, white calcium carbonate product remains in the filter paper while colourless sodium nitrate solution passes into the beaker: keep and wash the residue.Excess-solid reactionMaking soluble CuSO₄Excess CuODiscard the residueKeep the blue filtrateConcentrate, cool, collect crystalsPrecipitationMaking insoluble CaCO₃CaCO₃ productKeep and wash the residueNaNO₃ stays in the filtrateDry the washed CaCO₃
The same filtration operation has different purposes. After an excess-solid reaction, discard unreacted CuO and keep the copper(II) sulfate solution. After precipitation, keep and wash the calcium carbonate residue; soluble spectator ions pass into the filtrate. Apparatus is schematic; circles represent solid grains rather than individual ions. Black CuO, blue copper(II) sulfate solution and white CaCO₃ are scientific colour cues; colourless filtrate is shown without a fill.

Explain the reaction at three levels

The white solid is the observation. The required positive and negative ions join in an ionic lattice, which is the particle explanation. The net ionic equation represents that change and omits spectator ions. Do not treat the spectator salt as the precipitate just because its formula appears in the full equation.

For the barium sulfate example above, the ions that enter the solid are Ba²⁺ and SO₄²⁻. Sodium and chloride ions stay dissolved. Both charge and atoms balance in the net ionic equation.

Worked precipitation plan

Challenge 1

Plan a precipitation

Minimal support

Insoluble-product transfer

Describe how to prepare pure dry calcium carbonate from calcium nitrate and sodium carbonate solutions.

Keep the correct filter fraction

Fraction to keep
Finishing steps

Hints

Hint 1: form the insoluble pair
Mix soluble sources of Ca²⁺ and CO₃²⁻.
Hint 2: purify the solid
Filter, wash away soluble spectator ions, then dry the residue.
View solution step by step

  1. Form the precipitate

    Method

    Mix calcium nitrate and sodium carbonate solutions.

    Reason

    Mobile Ca²⁺ and CO₃²⁻ ions meet and form insoluble calcium carbonate.

    Working

    Ca(NO₃)₂(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2NaNO₃(aq).

  2. Separate the product

    Method

    Filter and retain the residue.

    Reason

    The required CaCO₃ is the insoluble solid; sodium nitrate stays in the filtrate.

    Working

    Residue: white CaCO₃(s).

  3. Purify and dry

    Method

    Wash with distilled water and dry between filter papers.

    Reason

    Washing removes soluble ions and drying removes water.

    Working

    Pure dry calcium carbonate is obtained.

Check your reasoning

Mind stretcher 1: Design suitable reactantsExtension

Question: A student wants lead(II) iodide. Why should both chosen reactants be soluble even though the product is insoluble?

Show answer

Soluble reactants release the required ions into solution, where Pb²⁺ and I⁻ can meet and form the insoluble precipitate. Soluble spectator ions remain in the filtrate and are washed from the product.

This is a planning exercise: practical work with toxic lead compounds requires the school laboratory’s controlled handling and disposal procedures.

Try independently: Choose two soluble starting salts to make lead(II) iodide. Give the net ionic equation and explain what washing removes.

Show answer and reasoning

Lead(II) nitrate and potassium iodide solutions are suitable: they supply Pb²⁺(aq) and I⁻(aq).

Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s)

Filter to collect the yellow precipitate. Washing with a little cold distilled water removes soluble potassium and nitrate ions, along with any dissolved excess reagent.

Try independently: A learner filters calcium carbonate, then dries the residue without washing it. Is a dry solid necessarily pure? Explain using the ions present after precipitation. Why would dilute hydrochloric acid be unsuitable for washing this calcium carbonate?

Show answer and reasoning

No. The residue holds solution containing soluble sodium and nitrate ions. Drying removes water, but those dissolved impurities can remain as solids on the calcium carbonate. Wash the residue with a little cold distilled water first, then dry it. Hydrochloric acid would react with calcium carbonate, consuming the required solid:

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Practise and check

Use the Acid–Base Chemistry topic check to practise choosing soluble reactants and purifying insoluble salts.

Syllabus and review details

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