When preparing copper(II) sulfate from dilute sulfuric acid and insoluble copper(II) oxide, add a small excess of the oxide so the acid can be used up. Filter off the unreacted solid, then concentrate the filtrate and allow crystals to form.
The 2027 SEC G3 Chemistry K324 syllabus covers preparing salts using acids and insoluble bases. This guide shows where the desired salt is after each step and why the solid left on the filter paper is removed.
- Choose sulfuric acid as the source of sulfate ions.
- Use excess insoluble copper(II) oxide to consume the acid.
- Keep the filtrate after removing the remaining solid.
- Concentrate the filtrate and cool it to obtain crystals.
CHAPTER 1 OF 4 · Back to contents
Choose the reactants for the salt
Copper(II) oxide is a base that is insoluble in water. It reacts with sulfuric acid to form copper(II) sulfate and water. The balanced equation is CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l).
An excess of oxide can remain as a solid once the acid has reacted. Because the excess is insoluble, it can be separated by filtration.
This is an exam explanation of a school-laboratory method. In a practical, follow the supplied instructions on warming, quantities and equipment.
CHAPTER 2 OF 4 · Back to contents
Worked example | Follow the product through the equipment
A student adds copper(II) oxide in portions to dilute sulfuric acid as directed. After stirring and allowing the reaction to proceed, a little black solid persists in the mixture.
| Stage | What you keep | Reason |
|---|---|---|
| After reaction | The mixture for filtration | Dissolved salt is in the liquid |
| After filtration | The blue filtrate | Unreacted copper(II) oxide remains as residue |
| After crystallisation | The crystals | The dissolved salt has crystallised from the concentrated solution |
Use the words filtrate and residue carefully. The filtrate passes through the filter. In this method it contains the dissolved product. The black residue is excess starting material.
Contents · Previous chapter · Next chapter
CHAPTER 3 OF 4 · Back to contents
Practise diagnosing a changed procedure
Suppose a pupil adds too little copper(II) oxide and filters immediately. Unreacted acid can pass through with the dissolved salt, so filtration cannot remove that excess acid.
A suitable improvement is to allow the reaction to proceed with a small excess of oxide before filtering. The remaining solid can then be removed physically.
Concentrate the filtrate and allow it to cool as directed. The purpose is to obtain crystals from the solution, so the step does not call for boiling the sample to dryness.
Contents · Previous chapter · Next chapter
CHAPTER 4 OF 4 · Back to contents
Frequently asked questions
Why does filtration separate the excess oxide?
The oxide remains solid, while the dissolved salt passes through the filter with the solution.
Why use sulfuric acid?
It supplies sulfate ions for the copper(II) sulfate product.
Where are the crystals before concentration?
At that stage the copper(II) sulfate is dissolved in the filtrate. Crystals form later from the concentrated solution.
Contents · Previous chapter · Continue with the related guide