Crystallisation: concentrate, cool and recover the solid
Explain recovery of a soluble solid from a solution, using supplied temperature-dependent solubility data and recognising that real yield can differ from a model.
Section C2.3 · Methods of purification
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Define saturation at a specified temperature.
- Scale supplied solubility data to a stated solvent mass.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does the mother liquor contain in this example?
What you will learn
- Explain concentration, cooling, crystal separation and drying in an appropriate supervised method.
- Calculate a model crystal yield and explain remaining solute or practical losses.
Reduce solvent without assuming total dry heating
Crystallisation can recover a soluble solid from its solution. In an appropriate supervised method, remove enough solvent to make a concentrated solution, then allow cooling so crystals form as the solubility permits. Separate the crystals from the remaining solution, wash appropriately with a small amount of cold suitable solvent when required, and dry them by a suitable method.
Heating every salt solution to complete dryness is not a universally interchangeable method: some substances can decompose or lose water of crystallisation under unsuitable heating. The target substance and supplied method matter. Crystallisation can leave some soluble impurities in the mother liquor, but it does not guarantee removal of every impurity; contamination and inclusions can still occur.
Use the same solvent mass for a cooling calculation
For a supplied hypothetical salt, solubility is 60 g per 100 g water at 80°C and 20 g per 100 g water at 20°C. With 50 g water, the hot saturated solution contains 30 g dissolved salt; at 20°C, 10 g can remain dissolved. If no water is lost during cooling and equilibrium is reached, the predicted crystals are 20 g.
Some solute remains dissolved in the mother liquor, so complete recovery is not predicted merely by cooling. A measured crystal mass can also be affected by transfer loss, incomplete drying or remaining impurities. A damp mass can overstate recovered dry target mass. Always state the model assumptions and avoid treating a theoretical recovery as an observed yield.
Sources for this lesson
Pause and explain
A recovered crystal sample is still damp. What can its mass establish?
Worked example
Use the supplied 60 g/100 g water at 80°C and 20 g/100 g water at 20°C data for 50 g water. Calculate predicted crystals after cooling with no solvent loss.
Show the worked solution
- Hot dissolved capacity is 60×50/100=30 g.
- Cool dissolved capacity is 20×50/100=10 g.
- The difference 30−10=20 g crystallises in the stated equilibrium model.
Answer 20 g crystals; 10 g salt remains dissolved at 20°C under the stated assumptions.
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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.
Describe an appropriate crystallisation sequence from a soluble solid's solution.
Give me a hint
Separate concentrating, cooling and solid recovery.
Compare my reasoning
- Concentrate by removing a suitable amount of solvent using the supervised method.
- Cool so the stated solubility change permits crystals to form.
- Separate crystals, wash appropriately if required and dry by a suitable method.
Concentrate, cool, separate crystals and dry appropriately; avoid assuming every salt should be heated to total dryness.
Look for these in your work
- The stages and their purpose are clear.
- The method is linked to the actual substance and supervised procedure.
Under the supplied 80°C/20°C solubilities, repeat the ideal calculation with 25 g water and no solvent loss.
Give me a hint
Scale both dissolved capacities to the same water mass.
Compare my reasoning
- Hot salt amount is 60×25/100=15 g.
- Cool dissolved amount is 20×25/100=5 g.
- Predicted crystal mass is 15−5=10 g.
10 g crystals and 5 g salt still dissolved at 20°C in the stated model.
Look for these in your work
- Both scaled capacities are correct.
- Dissolved remainder is not mistaken for lost matter.
The 50 g-water model predicts 20 g dry crystals. A sample measures 21 g while damp and 18 g after appropriate drying. Calculate recovery relative to the model and evaluate the damp reading.
Give me a hint
Use the dry mass for dry product recovery.
Compare my reasoning
- The damp 21 g contains retained solvent and cannot be treated as 21 g dry target.
- The dry mass gives 18/20×100=90% of the predicted crystal mass.
- Losses or model limitations can explain a lower recovery, but the figures alone do not identify one unique cause.
90% of the predicted dry crystal mass; the 21 g damp reading overstates dry recovery.
Look for these in your work
- The correct dry mass and denominator are used.
- Measured recovery and a specific causal explanation remain distinct.
Common mistakes
- Using the initial solution mass as though it were all solvent.
- Assuming cooling precipitates every gram of dissolved solute.
- Calling a damp crystal mass the dry product yield.
- Promising that one crystallisation guarantees perfect purity.
What can you explain now?
Why can some salt remain after crystals are collected?
Compare with the explanation
Some stays dissolved in the mother liquor at the final temperature.
Cooling changes the dissolved capacity; it does not necessarily reduce it to zero.
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Make it stick
Explain why crystallisation leaves mother liquor and how temperature data, solvent mass and drying affect a recovery calculation.
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.