Required Practical 1

Required Practical 1: Preparation of a Soluble Salt

Revision guide containing method, variables, safety, sample calculations, and model exam answers.

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Paper 1 RP 1 of 8 Combined & Separate ⏱️ ~30 min revision

Scientific Principles & Theory

Scientific Background: A soluble salt can be prepared by reacting a warm dilute acid with an insoluble base (metal oxide or metal carbonate). The acid reacts with the base to form a salt and water in a neutralisation reaction:

CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

To ensure that all the acid has reacted completely, the insoluble copper(II) oxide is added in excess. The excess oxide is removed by filtration, leaving a pure blue solution of copper(II) sulfate. Crystals of hydrated copper(II) sulfate (CuSO₄·5H₂O) are then obtained by gentle evaporation and slow crystallisation.

Experimental Variables

Independent Variable

The type of acid and insoluble base used (sulfuric acid and copper(II) oxide).

Dependent Variable

The yield and purity of the dry copper(II) sulfate crystals.

Control Variables

The volume of sulfuric acid (40 cm³), the concentration of sulfuric acid (0.5 mol/dm³), and the temperature of the acid before adding the base.

Lab Risk Assessment

Hazard Associated Risk Control Measure
Dilute sulfuric acid Causes skin and eye irritation Wear safety goggles; wash skin immediately if contact occurs.
Copper(II) oxide powder Harmful if swallowed and dust can irritate lungs Avoid inhaling dust; wear gloves; wash hands after use.
Hot apparatus and liquids Thermal burns from hot beaker or boiling acid Do not boil the acid vigorously; handle hot containers with tongs or beaker holders.

Apparatus & Procedure

Required Apparatus

  • Dilute sulfuric acid (0.5 mol/dm³)
  • Copper(II) oxide powder
  • Conical flask (100 cm³)
  • Beaker (100 cm³)
  • Measuring cylinder (50 cm³)
  • Glass stirring rod
  • Spatula
  • Filter funnel and filter paper
  • Evaporating basin
  • Bunsen burner, tripod, gauze, and heatproof mat
  • Crystallising dish
  • Beaker of water (to act as a water bath)
  • Tongs

Step-by-Step Procedure

  1. Measure 40 cm³ of 0.5 mol/dm³ sulfuric acid using a measuring cylinder and pour it into a beaker.
  2. Gently heat the acid over a Bunsen burner using a tripod and gauze until it is warm, but do not boil it.
  3. Use a spatula to add copper(II) oxide powder to the warm acid, stirring with a glass rod. The mixture will turn blue.
  4. Continue adding copper(II) oxide in excess until some unreacted black powder remains at the bottom of the beaker after stirring, which shows all the acid has reacted.
  5. Set up a filter funnel with filter paper over a conical flask. Filter the warm mixture to remove the excess copper(II) oxide.
  6. Pour the clear blue filtrate (copper(II) sulfate solution) into an evaporating basin.
  7. Place the evaporating basin on a beaker of water heated by a Bunsen burner. Evaporate the solution gently until it is reduced to about half its volume and crystals start forming on the edges (crystallisation point).
  8. Remove the evaporating basin and pour the concentrated solution into a crystallising dish.
  9. Leave the dish in a warm place for 24 hours to allow copper(II) sulfate crystals to form slowly.
  10. Pat the crystals dry gently with filter paper.
RP1: Preparation of Copper(II) Sulfate Crystals Setup Stage 1: React Acid + Excess Base Warm acid + excess CuO (Turns bright blue) Stage 2: Filter Excess Base Residue: unreacted CuO Filtrate: pure CuSO₄(aq) Stage 3: Crystallisation Heat over water bath Evaporate to crystallisation point

Fig 1. Laboratory experimental setup for Required Practical 1.

Sample Data & Calculations

This representative dataset illustrates the values typically obtained when carrying out this experiment in the laboratory:

Measurement parameter Value obtained
Volume of 0.50 mol/dm³ sulfuric acid used 40.0 cm³
Mass of empty crystallising dish 45.20 g
Mass of crystallising dish + dry copper(II) sulfate crystals 48.70 g
Actual mass of CuSO₄·5H₂O obtained 3.50 g
Worked Calculation

Data Processing & Analysis

  1. Calculate moles of acid: Moles = Concentration × Volume (dm³) = 0.50 × 0.0400 = 0.020 mol.
  2. Since copper(II) oxide is added in excess, the moles of acid limit the reaction.
  3. Reacting mole ratio: 1 mol of H₂SO₄ reacts to yield 1 mol of CuSO₄·5H₂O.
  4. Calculate theoretical yield of crystals: Mass = Moles × Mᵣ of CuSO₄·5H₂O = 0.020 × 249.5 = 4.99 g.
  5. Calculate percentage yield: Percentage Yield = (Actual Mass / Theoretical Mass) × 100 = (3.50 g / 4.99 g) × 100 = 70.1%.

Conclusion & Evaluation

Chemical Explanation: Evaluating the experimental outcomes against known values ensures validity. Understanding the source of systematic and random deviations allows for better experimental designs in future trials.

Experimental Error Analysis

Error Type & Source Effect on Final Result Mitigation Strategy
Systematic Error
Water of crystallisation lost due to overheating during evaporation		 Crystals lose their hydrated structure, turning into white anhydrous copper sulfate powder, altering physical quality and yield. Stop heating as soon as crystals start to form on the edges. Do not evaporate to dryness over a flame; let crystallisation finish slowly at room temperature.
Random Error
Loss of product during filtration and transfer		 Actual mass of crystals is lower than the theoretical maximum, decreasing the percentage yield. Rinse the reaction beaker with a minimal volume of distilled water and filter the rinsings to capture all dissolved copper(II) sulfate.

Exam Practice

Practise this practical

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Exam-style design question

6 marks
[6 marks]

Describe how to prepare a pure, dry sample of copper(II) sulfate crystals starting from copper(II) oxide powder and dilute sulfuric acid. Explain each step in your method.

View Model Answer & Mark Scheme

Marking guidance: award 1 mark for each correct, clearly explained step in the model answer below, up to 6 marks.

Model Answer (6/6 Marks):

  1. Acid warming: Measure 40 cm³ of dilute sulfuric acid into a beaker and warm it gently using a Bunsen burner. Warming increases the rate of reaction, but it must not boil to prevent acid spitting.
  2. Base excess: Add copper(II) oxide powder in small portions using a spatula and stir with a glass rod. Keep adding until some black solid remains unreacted at the bottom, which confirms that all the acid has reacted completely.
  3. Filtration: Filter the hot mixture using filter paper and a funnel into a conical flask. This separates the soluble copper(II) sulfate solution (filtrate) from the insoluble excess copper(II) oxide (residue).
  4. Evaporation: Pour the blue filtrate into an evaporating basin and heat it gently over a water bath. This evaporates water to concentrate the solution. Stop heating when crystals form on the edge (crystallisation point).
  5. Crystallisation: Pour the concentrated solution into a crystallising dish and leave it in a warm place to cool slowly. Slow cooling allows large, well-defined blue crystals of hydrated copper(II) sulfate to form.
  6. Drying: Filter off the remaining liquid and pat the blue crystals dry gently using filter paper to remove surface water without dehydrating them.
Examiner Tip:

Make sure you include the reasons for each step, especially why the oxide is added in excess, how the excess is removed, and why we do not evaporate the solution to dryness.

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