Making soluble salts (except sodium, potassium and ammonium salts)
3 marks · EasyThe table shows observations during a student's preparation of hydrated copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.
| Step | Observation |
|---|---|
| Add excess copper(II) oxide to warm acid | Black solid remains |
| Filter the mixture | Blue solution collected |
| Heat solution until nearly saturated | Blue crystals begin to form when cooled |
What does the black solid remaining in the beaker show about the reaction?
Why is the solution filtered at this stage?
Explain how the appearance of blue crystals shows the solution is nearly saturated.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) All the acid has been neutralised; copper(II) oxide is in excess.
(b) To remove unreacted copper(II) oxide from the solution.
(c) Blue crystals form when the solution is saturated with copper(II) sulfate, so any further cooling causes crystals to appear.
Common mistakes: Not recognising excess base indicates complete reaction.; Not correctly describing purpose of filtration.; Not explicitly stating that crystals form when the solution is saturated.
Show mark scheme and worked solution
Marking points
C1 (a) · 1 mark · States that the presence of solid shows excess base and complete neutralisation.
C2 (b) · 1 mark · States that filtration is to remove unreacted copper(II) oxide.
C3 (c) · 1 mark · Explains crystals form when the solution is saturated (cannot hold more dissolved salt).
Worked solution
(a) The black solid shows that excess copper(II) oxide has been added, and all the acid has reacted.
(b) Solution is filtered to remove the unreacted solid base.
(c) Blue crystals begin to appear when the solution is saturated, meaning it contains as much dissolved copper(II) sulfate as possible.