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Acid-Base Chemistry

Topic 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
Pure only

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 example

Prepare copper(II) sulfate crystals

Choose reagents and explain the key 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 example

Make 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?

  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 understandingWhy repeat the measured acid and alkali volumes without indicator when preparing a pure salt?Think it through, then reveal the answer
The first titration finds the neutralising volumes. Repeating them without indicator avoids contaminating the salt with indicator; adding arbitrary excess alkali would also leave a soluble impurity.