Preparing a sample of pure, dry hydrated copper

To make a pure, dry soluble salt you need a clever trick: how do you stop adding reactant at exactly the right point, and how do you make sure no leftover acid contaminates your salt?

The answer is to react a soluble acid with an insoluble base:

• Copper(II) sulfate (CuSO₄) is soluble — so it can dissolve, then crystallise out of solution.
• Copper(II) oxide (CuO) is an insoluble black base — so any leftover (excess) can simply be filtered off.

This is the key idea: because the base is insoluble, you can safely add it in excess to be certain every bit of acid has reacted, then filter out the unreacted solid.

The neutralisation reaction:

CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

(base + acid → salt + water)

You can swap the reactants and follow the same method to make other soluble salts, e.g. zinc oxide + sulfuric acid → zinc sulfate, or a metal carbonate such as copper(II) carbonate + sulfuric acid → copper sulfate + water + carbon dioxide.

This is a required-practical method. Examiners award marks for the correct steps in the correct order, with the right reasons.

Step 1 — Warm the dilute sulfuric acid. Measure dilute H₂SO₄ into a beaker and warm gently (e.g. on a tripod, gauze and Bunsen, or a water bath). Warming speeds up the reaction. Do NOT boil.

Step 2 — Add copper(II) oxide a little at a time, stirring, until in EXCESS. Add black CuO powder a spatula at a time, stirring after each addition. It dissolves to give a blue solution of copper sulfate. Keep adding until no more dissolves (some black solid stays at the bottom). This means the CuO is in excess — proof that all the acid has reacted, so no acid is left to contaminate the salt.

Step 3 — FILTER to remove the excess copper oxide. Filter the mixture through filter paper in a funnel. The unreacted black CuO is the residue left on the paper; the blue copper sulfate solution passes through as the filtrate. This removes the excess solid so the salt will be pure.

Step 4 — Heat the filtrate to the point of crystallisation. Gently heat the blue filtrate in an evaporating basin to evaporate some of the water. Stop when the solution is concentrated enough that crystals just begin to form at the edge (a "saturated" solution — test by dipping a cold glass rod). Do NOT evaporate to dryness.

Step 5 — Leave to cool so crystals form (crystallisation). Leave the hot, concentrated solution to cool slowly (ideally undisturbed). As it cools, blue hydrated copper(II) sulfate crystals, CuSO₄·5H₂O, grow out of the solution. Slow cooling gives larger, well-formed crystals.

Step 6 — Filter off the crystals and dry them. Filter to separate the crystals from the remaining liquid. Dry them by patting between sheets of filter paper (or in a warm oven). The product is a pure, dry sample of blue CuSO₄·5H₂O crystals.

Examiners love asking you to explain WHY a particular step is done. Three of these come up again and again.

Why add the copper oxide in EXCESS? So that you can be sure ALL of the acid has reacted (none is left over). If any acid remained, it would crystallise with your salt and the sample would be impure / contaminated with acid. You know it is in excess when no more solid dissolves.

Why FILTER after the reaction? To remove the excess (unreacted) copper oxide, which is an insoluble black solid. This separates it from the blue copper sulfate solution, so the final salt is pure.

Why evaporate to the point of crystallisation and NOT to dryness? You evaporate only some of the water to make a concentrated (saturated) solution, then let it cool so neat crystals form. If you evaporated to dryness you would:

• get a powder, not nice crystals, and
• risk driving off the water of crystallisation (so you'd lose the blue CuSO₄·5H₂O and get white anhydrous CuSO₄ instead).

Why dry between filter paper (or in a warm oven)? To remove surface water without heating the crystals strongly. Strong heating would, again, drive off the water of crystallisation and turn the blue crystals white.

Water of crystallisation is water that is chemically part of the crystal structure of a salt. It is built into the lattice — not just damp surface water. A salt that contains it is called hydrated; a salt with none is anhydrous.

Formula notation. A dot then the number of water molecules: CuSO₄·5H₂O means every CuSO₄ unit has 5 water molecules locked into the crystal.

Heating drives the water off — hydrated → anhydrous.

CuSO₄·5H₂O(s) → CuSO₄(s) + 5H₂O(g)

• Colour change: blue → white.
• The water leaves as steam (you may see it condense on cooler parts of the test tube).

Adding water reverses it — anhydrous → hydrated.

Add water to white anhydrous CuSO₄ and the blue colour returns (and it gets warm — exothermic). This is the basis of the chemical test for water: white anhydrous copper(II) sulfate turns blue when water is added.

So the salt you made in the practical, blue CuSO₄·5H₂O, is the hydrated form — and you protected its water of crystallisation by never evaporating to dryness or over-heating it.