Soluble salts from excess insoluble reactants
Explain reaction, filtration and crystallisation for a suitable metal, insoluble base or insoluble carbonate.
Section 7.3 · Preparation of salts
Exam scope and assessment notes
Core and Extended · Chemistry 0971, 2026–2028. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Predict acid-reaction products and salt solubility.
- Distinguish a filtrate from the solid residue on a filter.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which material can a filter remove from a solution?
What you will learn
- Describe soluble-salt preparation using a suitable excess metal, insoluble base or insoluble carbonate.
- Explain how excess-solid removal and crystallisation separate and purify the product.
Use a suitable solid that can be removed
For an appropriate soluble salt, an acid reacts with a suitable metal, insoluble base or insoluble carbonate. Illustrative choices are magnesium with dilute sulfuric acid for MgSO₄, CuO with sulfuric acid for CuSO₄, or an appropriate copper(II) carbonate sample with sulfuric acid for CuSO₄. The metal route can make hydrogen; the carbonate route also makes carbon dioxide and water; the oxide route makes water. Not every metal or acid is suitable, and an insoluble product coating can prevent an intended preparation from proceeding well.
In a teacher-supervised method, an excess of the suitable solid is used so the acid is consumed. Remaining solid is then filtered away, leaving the dissolved salt in the filtrate. A solid excess is useful because it can be removed physically; an excess of soluble alkali would remain dissolved and cannot simply be removed by filtration. Do not add an excess acid and expect filtration to remove it.
Recover crystals without confusing residue and filtrate
The filtrate contains the soluble salt solution and is the part retained for crystallisation. In the appropriate supervised procedure, gently evaporate some water to concentrate it to a suitable crystallisation point, then allow it to cool so crystals form. Separate the crystals from remaining solution, wash with a small amount of suitable cold solvent where appropriate and dry them by a suitable method. Washing helps remove adhering solution; excessive water can dissolve a soluble product.
Do not simply evaporate all water to dryness and call that the required crystallisation-and-purification method for hydrated crystals. Drying conditions must preserve the desired crystal form rather than decompose it or remove its water of crystallisation. Distinguish a method explanation from proof of practical competence: these online problems assess the sequence and reasons, while actual heating, handling and apparatus work require supervision.
Pause and explain
Why use an excess insoluble solid in the appropriate method?
Worked example
Explain why a soluble copper(II) sulfate preparation can use excess CuO with sulfuric acid, and identify what to keep after filtration.
Show the worked solution
- CuO+H₂SO₄→CuSO₄+H₂O forms the desired soluble salt.
- Excess CuO makes acid consumption possible, and its insoluble remainder is removable by filtration.
- Keep the CuSO₄-containing filtrate, concentrate appropriately and cool to obtain crystals.
Answer Use excess CuO, filter off the unreacted solid and crystallise the retained copper(II) sulfate solution.
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.
Give the reaction and purification sequence for soluble CuSO₄ from sulfuric acid and excess CuO.
Give me a hint
Separate the reaction, excess removal and crystal recovery stages.
Compare my reasoning
- React CuO with H₂SO₄ to form CuSO₄ and water under appropriate supervision.
- Filter away excess CuO, retaining the salt solution as filtrate.
- Concentrate appropriately, cool to crystallise, separate and suitably dry the crystals.
React with excess CuO → filter and keep filtrate → concentrate, cool, separate and dry crystals.
Look for these in your work
- The correct material is retained.
- Each purification stage has a purpose.
Compare the other products of dilute sulfuric acid with suitable magnesium metal and with an insoluble carbonate, then explain the shared excess-solid separation.
Give me a hint
A metal and carbonate do not give the same gas.
Compare my reasoning
- Magnesium gives a soluble sulfate and hydrogen in the stated suitable-metal pattern.
- An appropriate carbonate gives sulfate, water and carbon dioxide.
- For a suitable excess insoluble reactant, filter the remaining solid and retain the soluble product solution for crystallisation.
Metal route: hydrogen. Carbonate route: CO₂ and water. Both suitable methods remove excess solid by filtration.
Look for these in your work
- The reaction patterns differ correctly.
- The shared separation is correctly explained.
A student keeps the first filter residue and calls it pure soluble product, then evaporates the discarded filtrate to dryness. Explain the two errors and the corrected sequence.
Give me a hint
The unreacted insoluble reactant and desired dissolved salt are in different places.
Compare my reasoning
- The first residue is excess starting solid, not the intended soluble salt crystals.
- The filtrate should be kept and concentrated appropriately before cooling to crystallise.
- Recover and suitably dry the crystals; evaporating entirely to dryness is not the default controlled crystallisation method.
Keep the filtrate, not excess-solid residue; concentrate and cool before recovering and suitably drying crystals.
Look for these in your work
- Residue and filtrate identities are correct.
- Controlled crystallisation replaces the flawed recovery.
Common mistakes
- Keeping the excess-solid residue instead of the salt-containing filtrate.
- Expecting filtration to remove dissolved excess acid or alkali.
- Using copper metal with dilute sulfuric acid as if it were magnesium.
- Evaporating a hydrated salt completely to dryness as the default purification method.
What can you explain now?
After excess insoluble base reacts with acid, where is the intended soluble salt after filtration?
Compare with the explanation
In the filtrate.
The unreacted solid is the residue; the dissolved salt passes through with the water.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain all three suitable reactant routes and their other products. Draw the separation sequence and label which material is retained at each step.
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.