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Making a soluble salt from an insoluble base
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Acids react with metals, bases and carbonates to make salts. Here is how to make pure, dry copper(II) sulfate crystals.
Reactions of acids
Hydrochloric acid makes chlorides, sulfuric acid makes sulfates, nitric acid makes nitrates.
acid + metal → salt + hydrogen
acid + base (metal oxide or hydroxide) → salt + water
acid + metal carbonate → salt + water + carbon dioxide
acid + metal → salt + hydrogen
acid + base (metal oxide or hydroxide) → salt + water
acid + metal carbonate → salt + water + carbon dioxide
The method
1. Warm some dilute sulfuric acid.
2. Add copper(II) oxide a little at a time, stirring, until some black solid stays undissolved. The oxide is in excess, so all the acid has reacted.
3. Filter to remove the excess copper(II) oxide. The blue filtrate is copper(II) sulfate solution.
4. Heat the filtrate to evaporate some water, until crystals start to form (the crystallisation point).
5. Leave it to cool so crystals form.
6. Filter off the crystals and dry them between filter papers.
2. Add copper(II) oxide a little at a time, stirring, until some black solid stays undissolved. The oxide is in excess, so all the acid has reacted.
3. Filter to remove the excess copper(II) oxide. The blue filtrate is copper(II) sulfate solution.
4. Heat the filtrate to evaporate some water, until crystals start to form (the crystallisation point).
5. Leave it to cool so crystals form.
6. Filter off the crystals and dry them between filter papers.
The equation
CuO + H2SO4 → CuSO4 + H2O
The crystals are hydrated copper(II) sulfate, CuSO4·5H2O, which is blue.
This method works when one reactant is insoluble, because the excess can be filtered off.
The crystals are hydrated copper(II) sulfate, CuSO4·5H2O, which is blue.
This method works when one reactant is insoluble, because the excess can be filtered off.
Why is copper(II) oxide added in excess?
- Excess makes sure all of the sulfuric acid reacts.
- The leftover oxide is insoluble, so it is easily filtered off.
- The solution then contains only copper(II) sulfate and water.
Answer: To use up all the acid; the unreacted solid is then removed by filtration.
Hydrochloric acid gives chlorides, sulfuric acid sulfates, nitric acid nitrates. To make copper(II) sulfate: warm acid, add excess copper(II) oxide, filter, evaporate to the crystallisation point, cool, filter and dry the crystals.
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