Fig. 8.1 A student prepared crystals of copper(II) sulfate by reacting copper(II) oxide with dilute sulfuric acid, filtering, and then crystallising the fil...
Question 1 Report
Fig. 8.1
A student prepared crystals of copper(II) sulfate by reacting copper(II) oxide with dilute sulfuric acid, filtering, and then crystallising the filtrate. Fig. 8.1 shows the dry crystals the student obtained at the end of the experiment.
| Feature of the preparation | Description |
|---|---|
| appearance of the copper(II) sulfate crystals | |
| solid reactant used with the acid | |
| name of the acid used |
(a) Complete the table to describe the appearance of the crystals and to name the reactants used. [3]
(b) Describe how the student obtained these dry crystals from the aqueous copper(II) sulfate. [4]
(c) Suggest why the crystals should not be dried by heating them strongly. [2]
Answer Details
This tests the soluble-salt preparation route: an insoluble base (a metal oxide) is reacted with an acid, the excess solid removed by filtration, and the salt recovered by crystallisation.
(a) Appearance and reactants [3].
| Feature | Answer |
|---|---|
| appearance of the crystals | blue (regular-shaped) crystals [1] |
| solid reactant | copper(II) oxide (copper(II) carbonate also accepted) [1] |
| acid | (dilute) sulfuric acid [1] |
The neutralisation is:
\[ \text{CuO}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{CuSO}_4(aq) + \text{H}_2\text{O}(l) \](b) Obtaining dry crystals from the solution [4].
- Heat the filtrate in an evaporating basin to drive off some water, up to the point of crystallisation (until a hot spot of solution forms crystals on a cold glass rod) [1].
- Leave it to cool slowly so that well-formed crystals grow [1].
- Filter (or decant) off the remaining solution to separate the crystals [1].
- Dry the crystals between sheets of filter paper [1].
(c) Why not dry by strong heating [2]. Blue copper(II) sulfate is hydrated: its blue colour comes from water of crystallisation held in the crystal (\(\text{CuSO}_4\cdot5\text{H}_2\text{O}\)). Strong heating would drive off this water of crystallisation [1], so the blue crystals would turn to a white anhydrous powder and the sample would be spoilt [1].
Examination tip: "point of crystallisation" (not "evaporate to dryness") is the key phrase - evaporating to dryness also decomposes the hydrate.
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