Scientific Principles & Theory
Scientific Background: A soluble salt can be prepared by reacting an acid with an insoluble base (metal oxide or metal carbonate). The acid reacts with the base to form a salt and water:
To ensure that all the acid has reacted, the insoluble base is added in excess. The unreacted solid is then removed by filtration, leaving a pure solution of the salt. Crystals of hydrated copper(II) sulfate (CuSO₄·5H₂O) are obtained by gentle heating and slow cooling.
Experimental Variables
Independent Variable
The mass of copper(II) oxide added (to ensure excess).
Dependent Variable
The yield and purity of the hydrated copper(II) sulfate crystals.
Control Variables
Concentration of sulfuric acid (0.5 mol/dm³), volume of acid (25 cm³), heating temperature.
⚠️ Lab Risk Assessment
| Hazard | Associated Risk | Control Measure |
|---|---|---|
| Dilute sulfuric acid | Corrosive / skin irritant | Wear safety goggles; wash skin immediately if contact occurs. |
| Hot glassware / acid | Scalding / cuts | Heat the acid gently, do not boil it; use beaker tongs to handle hot glass. |
Apparatus & Procedure
Required Apparatus
- Beaker (100 cm³)
- Conical flask (100 cm³)
- Filter funnel and filter paper
- Evaporating basin
- Bunsen burner, tripod, gauze, and heatproof mat
- Measuring cylinder (25 cm³)
- Glass stirring rod
- Spatula
- Copper(II) oxide powder
- 0.5 mol/dm³ sulfuric acid
- Distilled water wash bottle
Step-by-Step Procedure
- Measure 25 cm³ of 0.5 mol/dm³ sulfuric acid using a measuring cylinder and pour it into a beaker.
- Heat the acid gently over a Bunsen burner until it is warm, but do not boil it. Warming the acid increases the rate of reaction.
- Use a spatula to add a small amount of copper(II) oxide powder to the warm acid and stir with a glass rod. The black powder will react to form a blue solution.
- Continue adding copper(II) oxide in small portions, stirring after each addition, until no more dissolves and a black powder remains at the bottom of the beaker. This indicates the oxide is in excess and all the acid has reacted.
- Filter the warm mixture through filter paper in a funnel into a conical flask to remove the excess copper(II) oxide.
- Pour the clear blue filtrate (copper(II) sulfate solution) into an evaporating basin.
- Heat the solution in the basin gently over a Bunsen burner to evaporate about half of the water to concentrate the solution. Stop heating when crystals start to form on a glass rod dipped into the solution. This is the crystallisation point.
- Leave the basin in a warm place to cool slowly. Slow cooling allows large, well-defined blue crystals of copper(II) sulfate to form.
- Decant the remaining liquid and filter the crystals, then dry them by patting gently with filter paper. Do not heat them in an oven, or they will lose water of crystallisation and turn into white anhydrous powder.
Fig 1. Laboratory experimental setup for Core Practical 2.42.
Sample Data & Calculations
This representative dataset illustrates the values typically obtained when carrying out this experiment in the laboratory:
| Parameter | Value |
|---|---|
| Volume of 0.50 mol/dm³ H₂SO₄ used | 25.0 cm³ |
| Mass of hydrated CuSO₄·5H₂O crystals obtained | 2.45 g |
Data Processing & Analysis
- Moles of H₂SO₄ = Concentration * Volume (dm³) = 0.50 * 0.025 = 0.0125 mol
- Reacting Ratio: 1 mol H₂SO₄ yields 1 mol CuSO₄·5H₂O
- Theoretical yield = Moles * Mr of CuSO₄·5H₂O = 0.0125 mol * 249.5 g/mol = 3.12 g
- Percentage yield = (Actual yield / Theoretical yield) * 100 = (2.45 g / 3.12 g) * 100 = 78.5%
Conclusion & Evaluation
Chemical Explanation: Saturated solutions are heavily dependent on temperature. Heating shifts solubility limits, allowing more solute to form coordinate bonds or ion-dipole interactions with solvent molecules. When cooled, the reverse process happens and solute precipitates out.
Experimental Error Analysis
| Error Type & Source | Effect on Final Result | Mitigation Strategy |
|---|---|---|
| Systematic Error Hydrated crystals heated too strongly during drying |
The crystals lose their water of crystallisation, turning into a dull white anhydrous powder, reducing yield and quality. | Never dry the crystals in an oven or over a flame. Pat them dry gently with filter paper at room temperature. |
| Random Error Incomplete filtration of excess copper oxide |
Black CuO powder contaminate the blue crystals, making them impure. | Ensure the filter paper has no tears and fits tightly to the funnel. Refilter if any black solid is in the filtrate. |
Exam Practice
Describe how to prepare a pure, dry sample of hydrated 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
Model Answer (6/6 Marks):
- Acid Heating: Warm the sulfuric acid in a beaker to increase the rate of reaction, but do not boil it to prevent splattering.
- Reactant Excess: Add copper(II) oxide powder in small portions and stir. Continue until a black solid remains at the bottom, ensuring all acid is neutralised.
- Filtration: Filter the mixture through filter paper and a funnel to remove the unreacted copper(II) oxide, leaving a pure solution of copper(II) sulfate.
- Evaporation: Heat the filtrate in an evaporating basin until the crystallisation point is reached (when crystals form on a glass rod).
- Crystallisation: Leave the concentrated solution to cool slowly at room temperature, allowing large hydrated crystals to form.
- Drying: Filter off the crystals and pat them dry gently between sheets of filter paper to remove surface water without dehydrating them.
Examiner Tip:
Clearly link each step to its scientific rationale (e.g. why we add excess copper oxide, why we filter, why we cool slowly, and how we dry).