Preparing pure dry samples of soluble salts (Required Practical)
This required practical tests your ability to make a soluble salt by reacting an acid with an excess insoluble base (an oxide, hydroxide or carbonate). The "excess" trick guarantees no acid is left, and the unreacted excess can be filtered off.
Salt-making strategy summary
For a soluble salt of a metal whose hydroxide is insoluble (most metals), use this method.
For Group 1 metals (whose hydroxides are soluble), use titration instead (C4.7).
Method (worked for copper sulfate)
Aim: Prepare pure dry crystals of copper(II) sulfate (CuSO₄·5H₂O).
Apparatus: Beaker, Bunsen burner, tripod & gauze, glass rod, filter funnel + paper, evaporating basin, water bath, pipette.
Reagents: dilute H₂SO₄ (acid), CuO (excess insoluble base — black powder).
Procedure:
- Measure 25 cm³ dilute H₂SO₄ into a beaker.
- Warm gently (don't boil — speeds the reaction).
- Add excess CuO a spatula at a time, stirring, until no more dissolves (black powder remains undissolved). This guarantees all the acid is used up.
- Filter to remove the excess CuO. The filtrate is blue copper sulfate solution.
- Pour filtrate into an evaporating basin.
- Heat over a water bath until the solution is half its original volume (saturated).
- Leave on a windowsill — water evaporates slowly and blue crystals form.
- Filter, pat dry between filter paper.
Equation: CuO + H₂SO₄ → CuSO₄ + H₂O.
Why each step matters
- Warm the acid to speed reaction (collision theory).
- Add base in excess to make sure no acid is left in the final solution (acid would crystallise as well, contaminating the salt).
- Filter to remove unreacted excess base.
- Slow evaporation/cooling gives larger, purer crystals (rather than fast evaporation which gives a powder + impurities).
Alternative reagents for CuSO₄
You could also use copper(II) carbonate (CuCO₃ + H₂SO₄ → CuSO₄ + H₂O + CO₂; bubbles seen). Hydroxide also works.
Yield
It's rarely 100%: lost in transfer between vessels, some salt left in the filter paper, some still in solution at the cooling stage.
⚠Common mistakes
- Boiling dry the solution. Driving off all the water gives a powder, not crystals. Use a water bath.
- Insufficient excess base — leaves acid in solution, contaminating the salt.
- Forgetting to filter before evaporation.
- Confusing CuSO₄ (anhydrous, white) with CuSO₄·5H₂O (blue) — crystals are hydrated.
Links
Builds on C4.4 (acid reactions). Sets up C4.6 (preparing soluble salts more generally) and C4.7 (acid + alkali for Group 1 salts).
AI-generated · claude-opus-4-7 · v3-deep-chemistry