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K324 / 2027
Acid-Base Chemistry

Lesson 6 of 7

Prepare and purify the salt you want

The method follows the solubility of both product and reactants.

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

Pure only
  • I can prepare, separate and purify salts.Syllabus

    Syllabus K324, 5.2(a). Prepare, separate and purify salts

  • I can select a salt-preparation route.Syllabus

    Syllabus K324, 5.2(c). Select a salt-preparation route

Prepare and purify the salt you want

The method follows the solubility of both product and reactants.

Choose the route
  1. Insoluble salt

    Mix two suitable soluble salts. Filter the precipitate, wash with distilled water, then dry.

  2. Soluble salt with an insoluble reactant

    React dilute acid with excess suitable metal, insoluble base or insoluble carbonate. Filter away excess solid, concentrate the filtrate, cool to crystallise, filter and dry.

  3. Soluble salt from acid and alkali

    Use titration to find exact reacting volumes. Repeat those volumes without indicator, then concentrate and crystallise.

Worked examplePrepare copper(II) sulfate crystals

Choose reagents and explain the key steps.

Try it first. Then show the worked stepsHide the worked steps
  1. Warm dilute sulfuric acid gently and add copper(II) oxide in small portions with stirring until some remains unreacted.
  2. Excess solid ensures the acid is used up. Filter away the unreacted oxide.
  3. Gently evaporate some water from the filtrate, then allow it to cool and crystallise.
  4. Filter crystals, wash with a little cold distilled water and dry between filter papers.
Answer

CuO(s) + H2SO4(aq) -> CuSO4(aq) + H2O(l). Copper metal is unsuitable with dilute sulfuric acid because it does not release hydrogen.

A suitable metal such as magnesium can prepare a soluble salt with dilute acid, but extremely reactive metals such as sodium are unsuitable. An insoluble carbonate such as zinc carbonate is another route; effervescence helps track acid consumption. For sodium chloride, both hydrochloric acid and sodium hydroxide remain dissolved, so excess cannot be removed by filtration: titration is needed.

Worked exampleMake a soluble salt when both reactants are solutions

Prepare pure potassium chloride crystals from dilute hydrochloric acid and potassium hydroxide solution. Why is titration needed?

Try it first. Then show the worked stepsHide the worked steps
  1. Use a pipette to put a known fixed volume of KOH(aq) in a conical flask. Add a few drops of a suitable indicator such as methyl orange.
  2. Add HCl(aq) from a burette while swirling, dropwise near the endpoint. Record initial and final readings to find the volume giving the indicator colour change. Repeat to obtain consistent volumes.
  3. Mix the same measured neutralising volumes in clean apparatus without indicator. KOH(aq) + HCl(aq) -> KCl(aq) + H2O(l).
  4. Gently evaporate some water, cool to crystallise, then filter the crystals, wash with a little cold distilled water and dry.
Answer

Titration avoids soluble excess acid or alkali, which filtration cannot remove. The indicator-free repeat prevents indicator contaminating the salt.

Check your understanding

When preparing a pure salt by titration, why repeat the mixing with the measured volumes but no indicator?

  1. So more salt forms, because the indicator uses up part of the acid in the flask.
  2. To check the end-point again, because the first titration result might be wrong.
  3. So the indicator does not stay in the salt as an impurity when it crystallises.
Show the answer

So the indicator does not stay in the salt as an impurity when it crystallises.

The first titration finds the exact volumes that neutralise. Repeating with the same volumes and no indicator gives a solution of the salt alone. Adding extra alkali would leave a soluble impurity.

Exam-style question

Describe how you would prepare pure, dry crystals of sodium sulfate from dilute sulfuric acid and aqueous sodium hydroxide. [4]

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Put these in order

Put the steps for making copper(II) sulfate crystals from copper(II) oxide in order.

Show the answer

1. Warm some dilute sulfuric acid in a beaker Warm acid reacts faster with the oxide that is added next.

2. Add copper(II) oxide a little at a time, stirring, until some stays undissolved Undissolved solid shows all the acid has been used, so the excess must now be removed.

3. Filter to remove the excess copper(II) oxide The filtrate is copper(II) sulfate solution, which must be concentrated.

4. Heat the filtrate until it is saturated A saturated solution forms crystals as it cools.

5. Leave to cool, then filter, rinse with a little cold water and dry the crystals