Preparing Soluble Salts

Preparing soluble salts is one of the most tested practical procedures in the Cambridge 0620 syllabus. It appears on Paper 2, Paper 4, and Paper 6. There are two main methods, and choosing the right one depends on whether the base or metal used is soluble or insoluble.

Method 1: Acid + excess insoluble substance

This method is used when the base, carbonate, or metal is insoluble in water. The excess ensures all the acid reacts.

Suitable reagent combinations

Desired salt	Acid	Insoluble reagent	Reaction type
Copper(II) sulfate	Dilute H₂SO₄	Copper(II) oxide (CuO)	Acid + base
Zinc chloride	Dilute HCl	Zinc (Zn) or zinc oxide (ZnO)	Acid + metal or acid + base
Magnesium sulfate	Dilute H₂SO₄	Magnesium (Mg) or MgO or MgCO₃	Acid + metal/base/carbonate
Iron(II) sulfate	Dilute H₂SO₄	Iron (Fe)	Acid + metal

Note: you cannot make copper sulfate using copper + acid, because copper is below hydrogen in the reactivity series and does not react with dilute acid. Use copper(II) oxide instead.

Step-by-step procedure

1. Warm the dilute acid gently in a beaker
2. Add the insoluble base/carbonate/metal in small portions, stirring after each addition
3. Continue adding until the solid is in excess — some solid remains undissolved at the bottom, even after stirring. This confirms all the acid has reacted
4. Filter the mixture to remove the excess unreacted solid. The filtrate (liquid passing through) contains the dissolved salt
5. Evaporate the filtrate gently by heating until the volume is reduced to about half (or until crystals start to form at the edge of a glass rod dipped in the solution — the “point of crystallisation”)
6. Leave to cool and allow crystals to form slowly (slow cooling gives larger, purer crystals)
7. Filter off the crystals, wash with a small amount of cold distilled water, and pat dry between filter papers

Observations during preparation of CuSO₄

• CuO is a black powder; the acid is colourless
• As CuO is added, it dissolves and the solution turns blue (Cu²⁺ ions)
• When in excess, black CuO powder remains at the bottom and does not dissolve
• After filtering: a blue solution (copper sulfate) passes through
• After crystallisation: blue crystals of CuSO₄.5H₂O

Why use excess base?

Using excess ensures all the acid is consumed. If acid remained in the solution, the crystals would be impure (contaminated with acid). The excess insoluble solid is easily removed by filtration.

Method 2: Titration (acid + soluble alkali)

This method is used when both reactants are soluble — typically a soluble alkali (NaOH or KOH) with an acid. You cannot see excess soluble alkali (it dissolves and is invisible), so the excess method does not work.

Suitable reagent combinations

Desired salt	Acid	Soluble alkali
Sodium chloride	HCl	NaOH
Potassium sulfate	H₂SO₄	KOH
Sodium nitrate	HNO₃	NaOH

Step-by-step procedure

1. Use a titration to find the exact volume of acid needed to neutralise a known volume of alkali (using an indicator)
2. Repeat the reaction using the exact volumes found, but without indicator (indicator would contaminate the salt)
3. The resulting solution contains only the salt and water
4. Evaporate gently to the point of crystallisation
5. Cool slowly to form crystals
6. Filter, wash, and dry the crystals

Why no indicator in the repeat?

Indicator molecules would remain in the salt crystals, making them impure and discoloured. The first titration determines the volumes; the second run produces the pure salt.

Choosing the method — decision tree

1. Is the base/reactant insoluble? → Use Method 1 (excess + filter)
2. Is the base/reactant soluble? → Use Method 2 (titration)
3. If making an insoluble salt → Use precipitation (different method entirely)

Worked exam question

Describe how you would prepare pure, dry crystals of copper(II) sulfate starting from copper(II) oxide and dilute sulfuric acid. [5]

Warm the dilute sulfuric acid [1]. Add copper(II) oxide in small portions until in excess (solid remains undissolved) [1]. Filter to remove excess copper(II) oxide [1]. Heat the blue filtrate to evaporate some water / heat until the point of crystallisation [1]. Allow to cool and crystallise; filter, wash, and dry the crystals [1].

Common exam mistakes

1. Not using excess base — without excess, some acid remains and contaminates the product.
2. Boiling the solution to dryness instead of gentle evaporation — rapid boiling gives tiny, impure crystals or an amorphous solid. Heat to the point of crystallisation, then cool slowly.
3. Using the titration method with an insoluble base — there is no need for titration when the excess can be simply filtered off.
4. Forgetting to repeat the titration without indicator when preparing the salt.
5. Trying to make copper sulfate by reacting copper metal with sulfuric acid — copper does not react with dilute acid because it is below hydrogen in the reactivity series. Use CuO or CuCO₃ instead.