Preparation of a Soluble Salt

Aim

To prepare a pure, dry sample of a soluble salt (copper sulfate) by reacting an insoluble base (copper oxide) with dilute sulfuric acid.

Apparatus

• Beaker (250 cm3)
• Bunsen burner, tripod, gauze, and heatproof mat
• Glass stirring rod
• Filter funnel and filter paper
• Conical flask or beaker (to collect filtrate)
• Evaporating basin
• Measuring cylinder (50 cm3)
• Copper(II) oxide powder (CuO)
• Dilute sulfuric acid (H2SO4, approximately 1 mol/dm3)
• Spatula

Method

1. Pour approximately 50 cm3 of dilute sulfuric acid into a beaker.
2. Warm the acid gently using a Bunsen burner (do not boil). Warming speeds up the reaction.
3. Add one spatula measure of copper oxide powder to the warm acid. Stir with a glass rod.
4. Continue adding copper oxide, one spatula at a time, stirring after each addition, until the copper oxide no longer dissolves and excess black powder remains at the bottom of the beaker. This confirms all the acid has been neutralised.
5. Filter the mixture through filter paper in a funnel to remove the excess copper oxide. The blue filtrate contains dissolved copper sulfate.
6. Pour the filtrate into an evaporating basin. Heat gently to evaporate some of the water.
7. Test for saturation by dipping a clean glass rod into the solution and allowing it to cool — if crystals form on the rod, the solution is saturated.
8. Remove from heat and leave to cool slowly. Blue crystals of hydrated copper sulfate form.
9. Filter the crystals, wash with a small volume of cold distilled water, and pat dry between sheets of filter paper.

Expected results

The reaction is: CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)

The black copper oxide dissolves in the acid, and the solution turns blue as copper sulfate forms. When excess copper oxide is added, the solution remains blue and undissolved black powder is visible. After filtration, the blue solution is clear. On crystallisation, blue crystals of copper sulfate pentahydrate (CuSO4.5H2O) form.

Analysis

This method works because copper oxide is insoluble in water but reacts with the acid. The excess ensures complete neutralisation. Filtration removes only the unreacted solid — the dissolved salt passes through the filter paper.

The same method works for other combinations:

• Zinc oxide + hydrochloric acid → zinc chloride
• Magnesium carbonate + sulfuric acid → magnesium sulfate (with effervescence from CO2)
• Iron oxide + hydrochloric acid → iron chloride

If a carbonate is used instead of an oxide, the reaction also produces carbon dioxide gas (fizzing is observed), which stops when neutralisation is complete.

Sources of error

• Not adding enough excess base: If the acid is not fully neutralised, the crystals will be acidic and impure. Continue adding base until you can clearly see unreacted solid that does not dissolve even with stirring.
• Heating too strongly during crystallisation: Copper sulfate is hydrated (CuSO4.5H2O). Heating too strongly removes the water of crystallisation, producing a white anhydrous powder instead of blue crystals.
• Cooling too quickly: Rapid cooling produces many small, impure crystals. Slow cooling allows larger, purer crystals to form.
• Not washing the crystals: Residual acid or excess solution on the surface of the crystals makes them impure. Wash with a small amount of cold distilled water — cold water minimises the amount of salt that redissolves.

Exam tips

This is one of the most commonly examined practicals. Examiners expect you to state each step in order: warm acid, add excess base, stir, filter, evaporate, crystallise, filter, wash, dry. The word “excess” is essential — without it, you may lose 2 marks.

If asked why this method cannot be used to make sodium sulfate, the answer is: sodium oxide and sodium hydroxide are both soluble, so you cannot add excess and filter. For soluble bases (alkalis like NaOH), use a titration method instead: find the exact volume of alkali needed to neutralise the acid using an indicator, then repeat without indicator and crystallise.