Making Salts
Summary: Salts are produced by neutralising acids with metals, bases, or carbonates. Three preparation methods: excess solid + acid (for insoluble reactants), titration (for soluble reactants), and precipitation (for insoluble salts). Salt names derive from the parent acid. Tags: igcse chemistry acids-bases Created: 2026-07-14 Last Updated: 2026-07-16
Learning Objectives
By the end of this topic, you should be able to:
- Name salts from the parent acid (hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate, ethanoic → ethanoate)
- Choose the correct preparation method for a given salt
- Describe Method A: excess solid + acid → filtration → evaporation → crystallisation
- Describe Method B: titration for soluble salts from soluble reactants
- Describe Method C: precipitation for insoluble salts
- Write balanced equations for salt preparation reactions
Content
1. Naming Salts
A salt’s name comes from:
- The metal (or ammonium, NH4+) — first part
- The acid it is made from — second part
| Parent Acid | Acid Formula | Salt Name Ending | Example Salt | Formula |
|---|---|---|---|---|
| Hydrochloric acid | HCl | -chloride | Sodium chloride | NaCl |
| Sulfuric acid | H2SO4 | -sulfate | Copper(II) sulfate | CuSO4 |
| Nitric acid | HNO3 | -nitrate | Potassium nitrate | KNO3 |
| Ethanoic acid | CH3COOH | -ethanoate | Sodium ethanoate | CH3COONa |
| Phosphoric acid | H3PO4 | -phosphate | Calcium phosphate | Ca3(PO4)2 |
| Carbonic acid | H2CO3 | -carbonate | Sodium carbonate | Na2CO3 |
General neutralisation pattern:
Acid + Base → Salt + Water
Acid + Metal → Salt + Hydrogen (for reactive metals above hydrogen)
Acid + Carbonate → Salt + Water + Carbon dioxide
2. Choosing the Right Preparation Method
The key question: Is the salt soluble or insoluble?
Solubility rules to know for IGCSE:
| Always Soluble | Exceptions / Insoluble |
|---|---|
| ALL sodium, potassium, and ammonium salts | — |
| ALL nitrates | — |
| Most chlorides | Silver chloride (AgCl), lead(II) chloride (PbCl2) |
| Most sulfates | Barium sulfate (BaSO4), lead(II) sulfate (PbSO4), calcium sulfate (CaSO4 — slightly soluble) |
| Most carbonates | ALL carbonates EXCEPT Na+, K+, NH4+ salts are insoluble |
| Most hydroxides | ALL hydroxides EXCEPT Na+, K+, NH4+, Ba2+ are insoluble |
3. Method A — Excess Solid + Acid (for Soluble Salts from Insoluble Reactants)
When to use: The metal, metal oxide, or metal carbonate is insoluble in water.
Principle: Add an excess of the insoluble solid to the acid → the acid is fully neutralised → filter off excess solid → evaporate to crystallise.
Step-by-Step (Copper(II) Sulfate Example)
- Add excess copper(II) oxide (CuO, black powder) to warm dilute sulfuric acid (H2SO4)
- Stir until no more CuO dissolves (the excess solid settles at the bottom — this proves all the acid has been neutralised)
- Filter to remove the excess/unreacted CuO (the filtrate is copper(II) sulfate solution)
- Heat the filtrate to evaporate some water (concentrate the solution)
- Leave to cool and crystallise — blue copper(II) sulfate crystals form
- Filter, wash with a small amount of cold distilled water, and dry between filter papers or in a warm oven
Equation: CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
Why Excess Solid?
- Ensures ALL the acid reacts (acid is the limiting reagent)
- The excess solid can be removed by filtration (unlike excess acid, which would contaminate the salt crystals)
Other Examples Using Method A
| Salt | Reactants | Equation |
|---|---|---|
| Magnesium sulfate | MgO(s) + H2SO4(aq) | MgO + H2SO4 → MgSO4 + H2O |
| Zinc chloride | ZnO(s) + HCl(aq) | ZnO + 2HCl → ZnCl2 + H2O |
| Calcium nitrate | CaCO3(s) + HNO3(aq) | CaCO3 + 2HNO3 → Ca(NO3)2 + H2O + CO2 |
| Iron(II) sulfate | Fe(s) + H2SO4(aq) | Fe + H2SO4 → FeSO4 + H2 |
4. Method B — Titration (for Soluble Salts from Soluble Reactants)
When to use: Both the acid and the base are soluble in water (e.g., NaOH(aq) + HCl(aq)). You cannot use an excess of solid because both are solutions.
Principle: Use titration to find the exact volumes that neutralise each other → mix those volumes without an indicator → evaporate to crystallise.
Step-by-Step (Sodium Chloride Example)
- Pipette a known volume of NaOH(aq) into a conical flask
- Add a few drops of indicator (phenolphthalein or methyl orange)
- Titrate with HCl(aq) from a burette until the indicator changes colour — record the volume
- Repeat the titration without indicator using the exact same volumes (indicator would contaminate the salt)
- Mix NaOH and HCl in exact neutralising proportions → NaCl(aq) + H2O(l)
- Evaporate the solution to obtain NaCl crystals
Equation: NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Why No Indicator in the Final Mix?
Indicators are themselves weak acids or bases — they would contaminate the pure salt. The first titration (with indicator) tells you the volumes needed; the second (without indicator) produces pure salt.
Other Examples Using Method B
| Salt | Reactants | Equation |
|---|---|---|
| Potassium nitrate | KOH(aq) + HNO3(aq) | KOH + HNO3 → KNO3 + H2O |
| Sodium sulfate | NaOH(aq) + H2SO4(aq) | 2NaOH + H2SO4 → Na2SO4 + 2H2O |
| Ammonium chloride | NH4OH(aq) + HCl(aq) | NH4OH + HCl → NH4Cl + H2O |
5. Method C — Precipitation (for Insoluble Salts)
When to use: The desired salt is insoluble in water.
Principle: Mix two solutions containing the required ions → the insoluble salt precipitates → filter, wash, dry.
Step-by-Step (Silver Chloride Example)
- Mix silver nitrate solution (AgNO3(aq)) with sodium chloride solution (NaCl(aq))
- A white precipitate of silver chloride (AgCl) forms immediately
- Filter the mixture — the AgCl precipitate stays on the filter paper
- Wash the precipitate with distilled water (removes soluble impurities like NaNO3)
- Dry the precipitate between filter papers or in a warm oven
Ionic Equation: Ag+(aq) + Cl-(aq) → AgCl(s)
Full Equation: AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
The Spectator Ion Concept
In the above reaction, Na+ and NO3- are spectator ions — they remain in solution and do not participate in the formation of the precipitate. The ionic equation shows only the ions that react.
Other Examples Using Method C
| Insoluble Salt | Reactants | Ionic Equation | Colour |
|---|---|---|---|
| Barium sulfate (BaSO4) | BaCl2(aq) + Na2SO4(aq) | Ba2+(aq) + SO4 2-(aq) → BaSO4(s) | White |
| Lead(II) iodide (PbI2) | Pb(NO3)2(aq) + KI(aq) | Pb2+(aq) + 2I-(aq) → PbI2(s) | Yellow |
| Calcium carbonate (CaCO3) | CaCl2(aq) + Na2CO3(aq) | Ca2+(aq) + CO3 2-(aq) → CaCO3(s) | White |
| Silver bromide (AgBr) | AgNO3(aq) + KBr(aq) | Ag+(aq) + Br-(aq) → AgBr(s) | Cream |
| Silver iodide (AgI) | AgNO3(aq) + KI(aq) | Ag+(aq) + I-(aq) → AgI(s) | Yellow |
6. Summary — Choosing the Right Method
| Is the salt soluble? | Are the reactants soluble? | Method | Key Steps |
|---|---|---|---|
| YES | NO (insoluble solid) | A: Excess solid + acid | Add excess solid → filter → evaporate → crystallise |
| YES | YES (both solutions) | B: Titration | Titrate with indicator → repeat without indicator → evaporate |
| NO | n/a (mix two solutions) | C: Precipitation | Mix solutions → filter → wash → dry |
Worked Examples (Exam-Style)
Example 1: Choosing the Method
Question: A student wants to prepare pure, dry crystals of copper(II) sulfate from copper(II) oxide and sulfuric acid. Describe the method. [5 marks]
Solution (model answer):
- Add excess copper(II) oxide to warm sulfuric acid and stir ✓
- Filter to remove excess CuO ✓
- Heat the filtrate to evaporate some water (to point of crystallisation) ✓
- Leave to cool — blue crystals form ✓
- Filter, wash with cold distilled water, and dry between filter papers ✓
Example 2: Ionic Equation for Precipitation
Question: Write the ionic equation for the preparation of silver chloride by precipitation. [2 marks]
Solution: Ag+(aq) + Cl-(aq) → AgCl(s) ✓✓ (1 mark for correct ions, 1 mark for state symbols and correct formula)
Example 3: Naming a Salt
Question: Name the salt formed when zinc oxide reacts with nitric acid. [1 mark]
Solution: Nitric acid produces nitrate salts → zinc nitrate ✓
Practice Questions
-
Name the salts formed when: (a) potassium hydroxide reacts with sulfuric acid; (b) magnesium reacts with hydrochloric acid; (c) calcium carbonate reacts with nitric acid. [3 marks]
-
Describe how you would prepare pure, dry crystals of magnesium sulfate starting from magnesium oxide and sulfuric acid. [5 marks]
-
Explain why an excess of the solid is used in Method A and why this is important. [2 marks]
-
Write the ionic equation for the precipitation of barium sulfate. Include state symbols. [2 marks]
-
State which preparation method (A, B, or C) you would use to prepare: (a) lead(II) sulfate; (b) potassium nitrate. [2 marks]
Key Facts to Memorise
- Salt naming: HCl → chloride, H2SO4 → sulfate, HNO3 → nitrate, CH3COOH → ethanoate
- Method A (excess solid + acid): metal/metal oxide/metal carbonate + acid → filter → evaporate → crystallise
- Method B (titration): for soluble salts from soluble reactants — titrate with indicator first, then without
- Method C (precipitation): mix two solutions → insoluble salt precipitates → filter → wash → dry
- Ionic precipitation equation: only the ions that form the precipitate are shown (spectator ions omitted)
- Three reactions producing salts: acid + metal, acid + base/oxide, acid + carbonate (the third produces CO2)
- Excess solid ensures all acid is neutralised — excess can be removed by filtration
Common Misconceptions
| Students often think… | But the correct understanding is… |
|---|---|
| ”Titration is always used to make salts” | Titration (Method B) is ONLY used when both reactants are soluble. If one reactant is an insoluble solid, use Method A (excess solid + acid) |
| “Evaporation means boiling to dryness” | The solution should be heated to the point of crystallisation (some water remains), then cooled to crystallise. Boiling to dryness gives an anhydrous powder, not crystals |
| ”You mix two solutions to make any salt” | Mixing two solutions (precipitation) only works for INSOLUBLE salts. For soluble salts, the salt stays dissolved |
| ”Sulfuric acid always makes sulfate salts” | Yes, but be careful — if you want a chloride, use hydrochloric acid; nitrate, use nitric acid. The acid determines the salt’s anion |
| ”Copper reacts with sulfuric acid to make copper sulfate” | Copper does NOT react with dilute sulfuric acid (copper is below hydrogen in the reactivity series). You must use copper(II) oxide (CuO) or copper(II) carbonate (CuCO3) |
Key Concepts from Past Papers
Definitions You MUST Know (Exact Mark Scheme Wording)
- Salt: a compound formed when the hydrogen ion(s) of an acid is/are replaced by a metal ion or ammonium ion
- Neutralisation: a reaction between an acid and a base to form a salt and water
- Precipitate: an insoluble solid formed when two solutions are mixed
- Crystallisation: the process of forming crystals from a saturated solution by cooling or evaporation
Recurring Mark Scheme Answers
- “Add excess solid to ensure all acid reacts / is neutralised”
- “Filter to remove excess unreacted solid”
- “Heat the filtrate to evaporate water / concentrate the solution”
- “Leave to cool — crystals form”
- “Filter, wash with distilled water, dry between filter papers”
- “Repeat titration without indicator to avoid contaminating the salt”
- substances required:
- steam / water
- conditions: one mark each for any two of:
- 6000 kPa / 60 atm (pressure)
- acid catalyst
- salt carbon dioxide water
- add (hydrochloric) acid
- test gas given off with limewater
Common Question Types
- Short recall (state/give/name): “Name the salt formed when magnesium reacts with sulfuric acid”
- Extended writing (describe/explain/suggest): “Describe how you would prepare crystals of copper(II) sulfate”
- Multiple choice / tick-box: “Which method is used to prepare an insoluble salt?”
- Equation writing: “Write the ionic equation for the precipitation of silver chloride”
- Method selection: “State which preparation method (A, B, or C) you would use for potassium nitrate”
Exam Tips
- This topic appears in 34 papers in the database
- Total Q+A entries: 86
- Average marks per question: 1.1
- The 5-mark “describe how to prepare crystals” question is one of the most frequent extended writing tasks in the IGCSE — learn the sequence: excess solid → filter → evaporate → crystallise → dry
- Always include “excess” when describing Method A — this is a key marking point
- For Method B, the phrase “repeat without indicator” is the mark that most students miss
- When writing ionic equations for precipitation, state symbols are essential — AgCl(s) with (s) for solid
- Know the colours of common precipitates: AgCl = white, AgBr = cream, AgI = yellow, PbI2 = yellow, BaSO4 = white
Keywords from Past Papers
acid, add, aqueous, precipitate, indicator, carbon, dioxide, white, sodium, hydroxide, colour, nitrogen, limewater, temperature, point
Related Notes
- Acids and Bases — Properties of acids and bases, strong vs weak acids
- Indicators and pH — pH scale, indicators
- Neutralization — H+ + OH- → H2O, applications
- Qualitative Analysis — Ion tests, precipitate colours
- Reactivity Series — Which metals react with acids
- IGCSE-Chem-Index — Full IGCSE Chemistry index
Sources
- OpenStax Chemistry 2e — [Chapter 14: Acid-Base Equilibria (Salts)], Rice University (free, CC BY 4.0)
- BBC Bitesize GCSE Chemistry — [Making Salts], BBC (free educational resource)
- Cambridge IGCSE Chemistry 0620 — Syllabus Section 7: Acids, Bases and Salts (Preparation of Salts), Cambridge Assessment International Education
- CK-12 Chemistry for High School — [Chapter 21: Salts and Their Preparation], CK-12 Foundation (free, CC BY-NC 3.0)
Past Paper Sources
- 0620/31 May/June 2015: Q11(c)(i) (0m), Q66(a)(i) (0m), Q66(a)(ii) (0m)
- 0620/32 Feb/March 2015: Q66(e)(ii) (2m), Q66(b)(ii) (1m), Q77(d)(i) (0m)
- 0620/32 Feb/March 2017: Q22(f)(ii) (1m), Q44(d)(i) (1m), Q55(f)(i) (1m) (+1 more)
- 0620/32 Feb/March 2018: Q88(b)(ii) (3m), Q22(a)(iv) (2m), Q77(b)(i) (1m) (+1 more)
- 0620/32 Feb/March 2019: Q44(b)(ii) (1m)
- 0620/32 Feb/March 2020: Q55(d)(iii) (2m), Q66(a)(ii) (1m)
- 0620/32 Feb/March 2021: Q66(a)(ii) (1m)
- 0620/32 Feb/March 2022: Q88(d)(ii) (2m), Q44(a)(ii) (0m)
- 0620/32 Feb/March 2023: Q33(b)(ii) (1m), Q88(c)(iii) (1m)
- 0620/32 May/June 2018: Q33(b)(i) (1m), Q33(b)(ii) (1m)
- 0620/32 May/June 2020: Q88(e)(i) (1m)
- 0620/33 May/June 2016: Q88(c)(ii) (1m), Q88(c)(iii) (0m)