Preparation of Salts

The salt-preparation question is a 0620 fixture on Papers 3, 4 and 6 (typically “name the method, then describe it” for 4-6 marks), and the first decision controls everything after it. Pick the wrong method for the salt named and the marks are gone before the description starts. The decision needs two facts: is the target salt soluble, and are the starting materials soluble?

Solubility rules first (Core)

Soluble	Insoluble (exceptions)
All sodium, potassium and ammonium salts	none
All nitrates	none
Chlorides	except silver and lead(II) chloride
Sulfates	except barium, calcium and lead(II) sulfate
Sodium, potassium, ammonium carbonates/hydroxides	all other carbonates and hydroxides

These rules are the routing table. Copper(II) sulfate: soluble → method 1 or 2. Sodium chloride: soluble, but every sodium compound is also soluble → titration. Barium sulfate: insoluble → precipitation.

Method 1: excess insoluble base, metal or carbonate (Core)

For soluble salts where the other reactant is insoluble, copper(II) sulfate from copper(II) oxide is the set-piece:

1. Warm dilute sulfuric acid.
2. Add copper(II) oxide in excess: keep adding until no more reacts and unreacted solid remains.
3. Filter to remove the excess unreacted oxide.
4. Heat the filtrate to evaporate part of the water, to the point of crystallisation.
5. Leave to cool so crystals form; filter or decant, then dry the crystals between filter paper.

The excess guarantees no acid is left over; the filtration removes the excess. Those two reasons are themselves marking points. The same scheme works with an excess of a (moderately reactive) metal such as zinc, or an insoluble carbonate. The underlying reactions are the characteristic acid reactions from Acids, Bases and Indicators. Do not propose sodium or potassium with acid; reactive Group I metals are dangerous and never accepted.

Drying note: gentle evaporation then cooling, never boiling to dryness: hydrated salts like CuSO4·5H2O lose their water of crystallisation if overheated.

Method 2: titration (Core)

When both reactants are soluble (sodium, potassium or ammonium salts from an alkali plus an acid), excess cannot be filtered out, so the exact neutralisation volume is found by titration:

1. Pipette 25.0 cm³ of sodium hydroxide into a conical flask; add a few drops of indicator (thymolphthalein or methyl orange).
2. Add dilute hydrochloric acid from a burette until the indicator just changes: the end-point.
3. Record the volume, then repeat without the indicator, adding exactly that volume of acid.
4. Evaporate to the point of crystallisation, cool, and dry the crystals.

The repeat-without-indicator step is the most forgotten mark: it gives a salt uncontaminated by indicator. The apparatus precision and technique detail live on Titration and Solubility.

Method 3: precipitation for insoluble salts (Core)

Mix two solutions, each supplying one of the target ions. Barium sulfate: mix aqueous barium chloride with dilute sulfuric acid (or any soluble sulfate).

1. Mix the two solutions; a white precipitate forms immediately.
2. Filter: the precipitate is the residue.
3. Wash the residue with distilled water to remove other ions.
4. Dry in a warm oven or between filter paper.

Filter–wash–dry is the three-word skeleton of every precipitation answer. (S) The ionic equation collects a further mark: Ba2+ + SO4^2− → BaSO4, spectators omitted. Precipitation chemistry is also how sulfate and halide ions are tested in qualitative analysis: same reaction, different purpose.

Worked exam question

Describe how a pure dry sample of copper(II) sulfate crystals can be prepared from copper(II) oxide and dilute sulfuric acid. [5]

Model answer: Warm the dilute sulfuric acid (1). Add copper(II) oxide in excess, until solid stops reacting and a residue remains (1). Filter to remove the unreacted copper(II) oxide (1). Heat the filtrate to evaporate part of the water, to the point of crystallisation, then leave to cool so crystals form (1). Remove the crystals and dry them between filter paper (1).

Mark-by-mark: the five marks are the five steps in order, and three carry trigger words: excess, filter, crystallisation. “Evaporate all the water” replaces the fourth mark with an error, and skipping the drying step quietly surrenders the fifth. Method questions are marked like recipes: each named action, in a workable order.

The mistakes that cost marks

1. Adding excess acid instead of excess base. Leftover acid cannot be filtered off; leftover insoluble base can.
2. Boiling the solution to dryness. Crystallise by partial evaporation and cooling, or the hydrated crystals are destroyed.
3. Titration answers without the no-indicator repeat. The first run finds the volume; the clean run makes the salt.
4. Forgetting wash and dry in precipitation. Filter–wash–dry: each word is routinely its own mark.
5. Choosing a method before checking solubility. Sodium salts can never use the excess-and-filter method: everything sodium is soluble.

How examiners want it phrased

Student wording	Mark-scheme wording
”Add enough copper oxide"	"Add copper(II) oxide in excess until no more reacts"
"Get rid of the leftover solid"	"Filter to remove the excess unreacted copper(II) oxide"
"Boil it down to get crystals"	"Evaporate to the point of crystallisation, then cool to crystallise"
"Clean the solid"	"Wash the precipitate with distilled water and dry between filter paper”

Method choice, then trigger words in sequence: that is the entire mark scheme. The full Section 7 route map is on the Acids, Bases and Salts pillar, and rehearsing these three recipes until the trigger words come automatically is a single-session job in a free 1-hour trial lesson with a Chemistry specialist.

Test yourself

Route each salt through the solubility table first; the answers are hidden until you click.

Q1 (3 marks). Name the preparation method for each salt: (a) potassium nitrate; (b) silver chloride; (c) magnesium sulfate, starting from magnesium metal.

Show answer

• (a) titration: both the salt and all possible potassium starting materials are soluble [1] • (b) precipitation: silver chloride is insoluble [1] • (c) add excess magnesium to warm dilute sulfuric acid, filter, then crystallise [1]

Q2 (3 marks). Describe how to prepare a pure dry sample of silver chloride from aqueous silver nitrate and aqueous sodium chloride.

Show answer

• mix the two solutions; a white precipitate of silver chloride forms [1] • filter (the precipitate is the residue) and wash it with distilled water [1] • dry the solid between filter paper or in a warm oven [1]

Q3 (2 marks). Sodium chloride is made by titrating sodium hydroxide with dilute hydrochloric acid. Explain why the excess-and-filter method cannot be used, and why the titration is repeated without the indicator.

Show answer

• every sodium compound is soluble, so excess solid could not be removed by filtration [1] • repeating with the recorded volume but no indicator gives a salt solution uncontaminated by indicator [1]