Edexcel IGCSE Chemistry 2.42 Soluble salt practical
Practise the soluble-salt method for making pure, dry hydrated copper(II) sulfate crystals from copper(II) oxide and sulfuric acid.
- Syllabus
- First assessment 2019
- Course
- Chemistry 4CH1
Exam points
- state why warm acid increases the reaction rate before adding copper(II) oxide
- recognise excess copper(II) oxide from black solid remaining or settling in the beaker
- describe evaporating, cooling, filtering and drying to obtain hydrated copper(II) sulfate crystals
2.42 Soluble salt practical question 1
[Maximum number: 9]
A salt can be made by reacting an acid with an insoluble base.
A student has a sample of copper(II) oxide.
Stage 1 pour 50 cm3 of dilute sulfuric acid into a beaker
Stage 2 warm the acid using a Bunsen burner
Stage 3 add a small amount of copper(II) oxide to the warm acid and stir the mixture
Stage 4 add further amounts of copper(II) oxide until copper(II) oxide is in excess
Stage 5 filter the mixture
Stage 6 obtain crystals from the filtrate
Question (a)
(a)
State why the acid is warmed in stage 2.
[ 1 ]
to increase the rate of reaction
Accept:
- to make the reaction faster / speed up the reaction
Reject:
- any reference to increasing the solubility of copper(II) oxide
Question (b)
(b)
State how the student would know that the copper(II) oxide is in excess in stage 4.
[ 1 ]
Any one from:
- copper(II) oxide stops disappearing
- mixture turns cloudy black
- black solid settles at the bottom of the beaker
Accept:
- stops dissolving
- copper(II) oxide settles at the bottom of the beaker
Reject:
- any other colour
Ignore:
- precipitate
Question (c)
(c)
State why the mixture is filtered in stage 5.
[ 1 ]
to remove excess/unreacted copper(II) oxide/solid/base (from the mixture)
ACCEPT to separate the
copper(II) sulfate solution (from the
copper(II) oxide/unreacted solid/excess solid)
Question (d)
(d)
State the colour of the filtrate obtained in stage 5.
[ 1 ]
blue
Question (e)
(e)
Describe how the student could obtain a pure, dry sample of hydrated copper(II) sulfate crystals from the filtrate in stage 6.
[ 5 ]
- heat / boil the filtrate
- until crystals form in a cooled sample / on a glass rod
- leave the solution to cool / crystallise
- filter to remove the crystals
- dry the crystals on filter paper / paper towel / in a warm oven / in a desiccator / leave to dry
Accept:
- heat to crystallisation point / form a saturated solution / until crystals start to form / remove some of the water
- decant the excess solution
Reject:
- hot oven or direct heating, e.g. Bunsen burner, for drying
Marking guidance:
- If the solution is heated to remove all the water, only the heating mark can be awarded.
- If the solution is left to evaporate all the water without heating, only 1 mark can be awarded.
- Crystallisation-point mark depends on heating.
- If the solution is left to completely evaporate after heating, award max 3.
- No drying mark if crystals are washed after drying.
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