GCSE-readyCore practical: making copper sulfate crystals
Core practical 3.17 makes pure, dry crystals of hydrated copper sulfate from an insoluble base. You also need the titration method for soluble salts made from alkalis.
From an insoluble base
CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
1. Warm dilute sulfuric acid in a water bath.
2. Add copper(II) oxide one spatula at a time, stirring, until some black solid stays undissolved: the copper oxide is in excess, so all the acid has reacted.
3. Filter to remove the excess copper oxide.
4. Heat the blue filtrate to evaporate some of the water, until crystals start to form.
5. Leave to cool so crystals form. Pour off the liquid and pat the crystals dry with filter paper (or dry in a warm oven).
Why each step
Filtering: removes the unreacted solid.
Not evaporating to dryness: slow cooling gives bigger, better crystals, and overheating would remove the water of crystallisation (blue hydrated crystals would turn white).
Hazards: copper sulfate is harmful; wear eye protection; hot solutions can spit.
From an alkali: titration
1. Do a titration with an indicator to find the exact volume of acid needed to neutralise the alkali.
2. Repeat with the same volumes but no indicator.
3. Evaporate some water and leave to crystallise.
Explain how you know that enough copper(II) oxide has been added to the acid.
- Copper(II) oxide is a black solid that reacts and dissolves in the acid.
- When the acid is all used up, no more can react.
- Black solid stays in the beaker even after stirring.
Answer: Black solid remains undissolved, so the copper oxide is in excess and all the acid has reacted.
Add excess insoluble base to warm acid, filter, evaporate some water, cool to crystallise and pat dry. Excess base makes sure no acid is left. For an alkali, titrate to find exact volumes, then repeat without indicator.
- Hydrated
- Containing water of crystallisation, for example blue hydrated copper(II) sulfate, CuSO_{4}·5H_{2}O.
- Base
- A substance that neutralises an acid to form a salt and water (a proton acceptor), such as a metal oxide or hydroxide.
- Titration
- A method to find the exact volume of one solution that reacts with a measured volume of another, using a burette, pipette and indicator.
- Acid
- A substance that releases hydrogen ions, H^{+}, in water (a proton donor). Its solution has a pH below 7.
- Alkali
- A soluble base. It releases hydroxide ions, OH^{-}, in water, giving a pH above 7.
- Indicator
- A dye that changes colour depending on pH, such as litmus, methyl orange or phenolphthalein.
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Excess zinc oxide is added to hydrochloric acid to make zinc chloride. How do you know all the acid has reacted?
- Bubbles of gas appear
- Some zinc oxide stays undissolved
- The solution turns blue
- The solution gets hotter
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Answer: Some zinc oxide stays undissolved
Undissolved solid shows the base is in excess.
In making copper sulfate, what passes through the filter paper?
- Solid copper sulfate crystals
- Excess copper(II) oxide
- Copper sulfate solution
- Black solid
Show the answer
Answer: Copper sulfate solution
The filtrate is the salt solution.
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- Pearson Edexcel GCSE Chemistry (1CH0) specification, topic 3.15, 3.16, 3.17, 3.18
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification
Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.
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