Making soluble salts (except sodium, potassium and ammonium salts)
4 marks · Normal| Step | Observation |
|---|---|
| Add nickel(II) oxide to warm dilute sulfuric acid | Black solid gradually disappears |
| More nickel(II) oxide added | Some solid remains at the bottom |
| Filter the mixture | Green solution collected |
| Heat solution until some water evaporates | Residue remains green |
| Cool the solution | Green crystals form |
Explain why nickel(II) oxide is added in excess (step 2).
Describe the purpose of step 3 (filtration).
Explain why the solution is cooled after heating (step 5).
Suggest a reason why it would be incorrect to heat the green solution to dryness in step 4.
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(a) To ensure all acid is reacted
(b) To remove any unreacted solid
(c) So hydrated crystals can form / slower cooling forms bigger crystals
(d) Heating to dryness would give the anhydrous salt, not hydrated crystals
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Marking points
B1 (a) · 1 mark · States nickel(II) oxide is added in excess so all acid reacts
B2 (b) · 1 mark · States filtration removes unreacted solid
B3 (c) · 1 mark · States cooling forms crystals (or allows hydrated crystals to form, or bigger crystals)
B4 (d) · 1 mark · Correctly explains heating to dryness forms anhydrous salt, not hydrated crystals
Worked solution
a) Nickel(II) oxide is added in excess to ensure all sulfuric acid reacts, so none remains in the solution.
b) Filtration removes any unreacted nickel(II) oxide, so the solution only contains nickel(II) sulfate and water.
c) Cooling allows hydrated crystals to form as the solubility of the salt decreases.
d) Heating to dryness would give anhydrous salt, not hydrated crystals.