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Learn0620Papers 2, 4 · ExtendedAcids, bases and saltsPreparation of salts
Acids, bases and salts · Subtopic 3 of 3

Preparation of Salts

A salt forms when the hydrogen of an acid is replaced by a metal (or ammonium). The acid gives the second part of the name: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate. Soluble salts are made by adding excess insoluble base, metal or carbonate to acid, filtering, then crystallising; or by titration when using an alkali. Insoluble salts are made by precipitation: mix two solutions, filter, wash and dry.

Cambridge 5070 Paper 1Cambridge 5070 Paper 2Cambridge 0620 Papers 2, 4Cambridge 0620 Papers 1, 3Edexcel 4CH1 Paper 1CEdexcel 4CH1 Paper 2C≈ 45 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Name salts from the acid and base used.
  • Use solubility rules.
  • Choose and describe a method to prepare a pure, dry salt.
Pause and think

Why add copper(II) oxide in excess when making copper(II) sulfate?

Section 2 of 14

Core idea

Start hereGrowing blue crystals

Black copper oxide powder, colourless acid, a little warmth and patience, and a few days later you have deep-blue crystals the size of your fingernail. Choosing the right method is everything.

The big idea

A salt is what's left when the hydrogen of an acid is replaced by a metal. The method used to make it depends on whether the salt, and the base you start from, are soluble.

1. Choosing the method

Which method?

Solubility decides.

Worked exampleZinc sulfate crystals

Describe how to make pure dry zinc sulfate crystals from zinc carbonate.

Goal
A complete method.
Strategy
Soluble salt from an insoluble carbonate: excess solid.
  1. ZnCO3+H2SO4→ZnSO4+H2O+CO2\mathrm{ZnCO_3+H_2SO_4\rightarrow ZnSO_4+H_2O+CO_2}
    The reaction.
  2. add ZnCO3 until fizzing stops\text{add }\mathrm{ZnCO_3}\text{ until fizzing stops}
    Excess makes sure all acid reacts.
  3. filter, warm to crystallisation point, cool\text{filter, warm to crystallisation point, cool}
    Remove excess solid, then crystals form on cooling.
  4. filter, dry between filter papers\text{filter, dry between filter papers}
    Collect pure dry crystals.
AnswerExcess-solid method: react, filter, crystallise, dry.
✓ CheckNo fizzing shows the acid is used up ✓.

2. Solubility rules

  • Always soluble: Na, K, NH4+\mathrm{NH_4^+} salts; all nitrates.
  • Chlorides: soluble except Ag, Pb.
  • Sulfates: soluble except Ba, Pb (Ca slightly).
  • Carbonates and hydroxides: insoluble except Na, K, NH4+\mathrm{NH_4^+}.
Exam tipWater of crystallisation Boiling a solution to dryness can turn blue CuSO4⋅5H2O\mathrm{CuSO_4\cdot5H_2O} into white anhydrous powder. Crystallise by cooling instead.
Pause and think

Why is barium sulfate washed with distilled water after filtering?

Summary

  • Name from the acid.
  • Three methods, chosen by solubility.
  • Crystallise by cooling.
Key words
Salt
Compound formed when the hydrogen of an acid is replaced by a metal or ammonium.
Precipitate
Insoluble solid formed when two solutions are mixed.
Filtrate
Liquid that passes through the filter paper.
HistoryGlauber's salt. In 1625 Johann Glauber made sodium sulfate and sold it as a "miracle salt" laxative. It was one of the first chemicals ever sold commercially.
Fun factBarium meals. Barium ions are poisonous, yet patients drink barium sulfate for X-rays. It's so insoluble that almost no barium ions get into the body.
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