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wise otter.Chemistry
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K324 / 2027
Data and Experimental Skills

Lesson 8 of 8

Separation and recovery

Choose what to retain and how to recover it.

O-Level 6092 (2026) / SEC G3 K324 (2027)

  • I can use separation techniques.Syllabus

    Syllabus K324, T3. Use separation techniques

  • I can prepare salts.Syllabus

    Syllabus K324, T4. Prepare salts

Recover the substance you need

Choose a process from solubility, volatility and thermal stability.

The question determines what to keep
TaskMethodWhat is recovered
Remove an insoluble solid from a liquidFiltrationSolid is the residue; liquid passing through is the filtrate
Obtain a dissolved solid that crystallises on coolingConcentrate, cool, filter crystals, wash sparingly and dryCrystals; some solute remains dissolved in the mother liquor
Recover a solventSimple distillationCondensed vapour from the distillate; monitor the apparatus correctly
Compare soluble mixture componentsPaper chromatographySeparated spots, compared under the same conditions

For paper chromatography, use a pencil baseline above the solvent, small concentrated spots and a covered vessel where appropriate. Remove the paper before the solvent reaches the top and mark the solvent front immediately.

Pure only

Rf is the distance moved by the spot centre divided by the solvent-front distance, both measured from the baseline; it has no unit. Compare values only under the same conditions.

Pure only

Worked exampleInterpret chromatography conservatively

A spot moves 3.6 cm while the solvent front moves 6.0 cm. A reference compound in the same run has a spot at the same position.

Try it first. Then show the worked stepsHide the worked steps
  1. Rf = 3.6 / 6.0 = 0.60.
  2. Matching position under the same conditions is consistent with the reference identity.
  3. It does not prove identity by itself: different substances can have the same Rf, and an unresolved mixture can appear as one spot.
Answer

Rf = 0.60; use the comparison as evidence with the stated limitation.

Pure only

For a soluble salt from an acid and an insoluble base or carbonate, add the solid until it is in excess, filter off unused solid, then crystallise the filtrate. For two soluble reactants giving an insoluble salt, mix, filter the precipitate, wash and dry. For an acid and soluble alkali, use titration to establish the reacting volumes, then repeat without indicator before crystallisation. Avoid evaporating a heat-sensitive salt to complete dryness.

Check your understanding

Why is a filtered insoluble precipitate washed with a little distilled water?

  1. To rinse away soluble impurities left from the solution it was made in.
  2. To cool the precipitate, so that it can be weighed straight away.
  3. To dissolve the precipitate partly, so that its crystals become larger.
Show the answer

To rinse away soluble impurities left from the solution it was made in.

The solid is coated with the solution it formed in, which contains soluble ions. A small amount of cold distilled water removes them without dissolving much of the product.