Insoluble salts by precipitation
Select two soluble feeds, identify the solid product and explain filtration, washing and drying.
Section 7.3 · Preparation of salts
Extended only (Supplement) · Chemistry 0620, 2026–2028.
Before you begin
- Apply the chloride, sulfate and carbonate solubility rules.
- Distinguish residue, filtrate and dissolved spectator ions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which is the stated chloride exception useful as a precipitate?
What you will learn
- Describe preparation of an insoluble salt from suitable soluble reactants.
- Use full and ionic equations to explain precipitation and purification.
Choose soluble sources of the required ions
An insoluble salt can be prepared by mixing suitable aqueous solutions that supply its positive and negative ions. For silver chloride, aqueous silver nitrate and sodium chloride supply Ag⁺ and Cl⁻: AgNO₃(aq)+NaCl(aq)→AgCl(s)+NaNO₃(aq). The ionic equation is Ag⁺(aq)+Cl⁻(aq)→AgCl(s). Sodium and nitrate ions are spectators and remain in the liquid.
For barium sulfate, BaCl₂(aq)+Na₂SO₄(aq)→BaSO₄(s)+2NaCl(aq), with net ionic Ba²⁺(aq)+SO₄²⁻(aq)→BaSO₄(s). The specified soluble feeds make ions available throughout the liquid; simply choosing two insoluble solids containing the right elements does not give the same precipitation method. These reactions explain supervised observations; hazardous reagents require suitable laboratory handling.
Keep the solid and remove its surrounding solution
Filter the precipitated mixture and keep the solid residue, which is the intended insoluble product. Wash it with suitable distilled water to remove adhering soluble reactants and by-product salts, then dry it appropriately. Washing is needed because filtration alone leaves solution clinging to the solid; a wet residue is not automatically pure product.
This retained fraction is the opposite of the initial filter stage in an excess-insoluble-base preparation of a soluble salt, where the desired product was in the filtrate. In precipitation, the target's insolubility makes the filter residue useful. State product identity and solubility before deciding what to keep, rather than memorising that residue or filtrate is always the correct part.
Pause and explain
Which part is kept when preparing an insoluble salt by precipitation?
Worked example
Choose feeds for BaSO₄ and identify what to retain after mixing and filtration.
Show the worked solution
- Use soluble barium chloride and sodium sulfate solutions to supply Ba²⁺ and SO₄²⁻.
- Ba²⁺+SO₄²⁻→BaSO₄ forms the insoluble product as a solid.
- Keep the filter residue, wash away adhering soluble material and suitably dry the precipitate.
Answer Precipitate BaSO₄ from suitable soluble feeds; retain, wash and dry the solid residue.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write full and ionic equations for AgCl from AgNO₃(aq) and NaCl(aq), then describe purification.
Give me a hint
Only silver and chloride ions form the precipitate.
Compare my reasoning
- AgNO₃(aq)+NaCl(aq)→AgCl(s)+NaNO₃(aq).
- The net ionic equation is Ag⁺(aq)+Cl⁻(aq)→AgCl(s).
- Filter and retain AgCl residue, wash away soluble material and dry appropriately.
AgNO₃+NaCl→AgCl+NaNO₃; Ag⁺+Cl⁻→AgCl(s); keep, wash and dry residue.
Look for these in your work
- Full and ionic atom/charge accounting agree.
- The correct fraction and purification are stated.
Select soluble feeds for calcium carbonate and write its net ionic precipitation equation.
Give me a hint
Use a soluble calcium salt and a soluble carbonate exception.
Compare my reasoning
- Calcium chloride and sodium carbonate solutions are suitable ion sources in this rule set.
- Ca²⁺(aq)+CO₃²⁻(aq)→CaCO₃(s) is the net ionic equation.
- Filter, wash and suitably dry the insoluble calcium carbonate product.
CaCl₂(aq) and Na₂CO₃(aq); Ca²⁺+CO₃²⁻→CaCO₃(s); recover the washed residue.
Look for these in your work
- Both chosen feeds are soluble.
- The target is the insoluble carbonate rather than a spectator salt.
Compare the fraction retained after the first filtration in CuSO₄ preparation from excess CuO with the fraction retained in BaSO₄ precipitation. Explain each choice.
Give me a hint
Soluble target and insoluble target are in different fractions.
Compare my reasoning
- For CuSO₄, keep the filtrate containing dissolved product and discard the excess CuO residue.
- For BaSO₄, keep the precipitate residue and wash off the adhering solution.
- The target's solubility determines the retained fraction; there is no universal keep-the-residue rule.
CuSO₄: retain filtrate. BaSO₄: retain washed residue. Product solubility determines the choice.
Look for these in your work
- Both target identities are explicit.
- Separation decisions follow solubility.
Common mistakes
- Starting with insoluble feeds and assuming their ions are freely available in solution.
- Keeping the filtrate instead of the intended precipitate.
- Skipping washing because filtration already occurred.
- Keeping spectator ions in the net ionic precipitation equation.
What can you explain now?
Where is the intended AgCl after filtration in the stated precipitation method, and why wash it?
Compare with the explanation
In the residue; washing removes adhering soluble material.
AgCl is insoluble, while dissolved salts can remain in liquid clinging to the solid after filtration.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Design a precipitation from two soluble feeds, write the ionic equation and explain what filtration, washing and drying each accomplish.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.