Salt Preparation Methods
Summary: Three methods for preparing salts: (1) excess metal/insoluble base + acid, (2) titration for soluble Group 1/ammonium salts, (3) precipitation for insoluble salts. Choosing the right method depends on the salt’s solubility. Tags: igcse chemistry salts practical preparation Created: 2026-07-14 Last Updated: 2026-07-14
The Decision Tree
Which method should you use? This is the #1 question examiners ask. Follow this flow:
What salt do you want to make?
├─ Is the salt INSOLUBLE?
│ └─ YES → Method C: Precipitation
│ (e.g., BaSO₄, AgCl, PbI₂, CaCO₃)
│
└─ Is the salt SOLUBLE?
└─ Is it a Group 1 (Na, K, Li) or ammonium (NH₄⁺) salt?
├─ YES → Method B: Titration
│ (e.g., NaCl, KNO₃, Na₂SO₄, NH₄Cl)
│
└─ NO → Method A: Excess Metal / Insoluble Base + Acid
(e.g., CuSO₄, MgCl₂, Zn(NO₃)₂, FeSO₄)
Solubility Rules (What You MUST Know)
| Soluble | Insoluble (Except) |
|---|---|
| All Group 1 (Na⁺, K⁺, Li⁺) salts | None |
| All ammonium (NH₄⁺) salts | None |
| All nitrates (NO₃⁻) | None |
| Most chlorides (Cl⁻) | AgCl (silver chloride) — white ppt; PbCl₂ — white ppt (soluble in hot water) |
| Most sulfates (SO₄²⁻) | BaSO₄ (barium sulfate) — white ppt; PbSO₄ — white ppt; CaSO₄ — slightly soluble |
| Most carbonates (CO₃²⁻) | Group 1 and NH₄⁺ carbonates are soluble |
| Most hydroxides (OH⁻) | Group 1 and NH₄⁺ hydroxides are soluble; Ca(OH)₂ partially soluble |
Method A: Excess Metal / Insoluble Base + Acid
When: Making soluble salts of metals below sodium in the reactivity series (e.g., CuSO₄, Mg(NO₃)₂, ZnCl₂, FeSO₄)
Procedure
- Add excess solid (metal, metal oxide, or metal carbonate) to a fixed volume of dilute acid in a beaker
- Warm gently using a Bunsen burner / water bath (speeds up reaction)
- Stir until no more solid dissolves and the solid stops reacting (no more effervescence if using carbonate)
- Filter the mixture to remove excess (unreacted) solid — the filtrate is the salt solution
- Heat the filtrate to evaporate some water — heat until crystallisation point (when crystals first appear on cooling / a glass rod dipped in and withdrawn shows crystals forming)
- Allow to cool slowly at room temperature for crystals to form
- Filter, wash with a little cold distilled water, and dry the crystals between filter paper / in a warm oven
Why Excess Solid?
- Ensures all the acid is neutralised
- The solid that doesn’t react can be removed by filtration
Suitable Starting Materials
| Salt Wanted | Best Starting Solid | Equation |
|---|---|---|
| CuSO₄ | CuO (copper(II) oxide) | CuO + H₂SO₄ → CuSO₄ + H₂O |
| CuSO₄ | CuCO₃ (copper carbonate) | CuCO₃ + H₂SO₄ → CuSO₄ + H₂O + CO₂ |
| MgCl₂ | MgO (magnesium oxide) | MgO + 2HCl → MgCl₂ + H₂O |
| ZnSO₄ | Zn (zinc metal) | Zn + H₂SO₄ → ZnSO₄ + H₂ |
| FeCl₂ | Fe (iron metal) | Fe + 2HCl → FeCl₂ + H₂ |
Why NOT Use Group 1 Metals?
- Na, K, Li react too violently with acids — dangerous and uncontrollable
- For Group 1 salts, use titration (Method B)
Method B: Titration
When: Making soluble Group 1 (Na, K, Li) or ammonium salts where you cannot use excess metal (too reactive) or the base is soluble (can’t filter out excess)
Procedure
- Pipette 25.0 cm³ of alkali (NaOH, KOH, NH₃ solution) into a clean conical flask
- Add 2–3 drops of indicator (phenolphthalein or methyl orange)
- Fill burette with the acid and record the initial reading
- Titrate: Add acid from the burette, swirling the flask, until the indicator just changes colour (endpoint)
- Record the volume of acid used (titre) — this is the exact volume needed to neutralise 25.0 cm³ of alkali
- Repeat the titration without the indicator:
- Pipette 25.0 cm³ of alkali into a clean flask
- Add the exact same volume of acid from the burette (no indicator needed)
- Now you have a pure salt solution with no indicator contamination
- Evaporate some water by heating
- Crystallise: Allow to cool, filter, wash with cold distilled water, and dry
Why No Indicator in the Final Step?
- The indicator would contaminate the salt crystals — they would be coloured/impure
- By repeating with the known exact volume, you get pure salt
Why Can’t You Use Method A Here?
- NaOH/KOH are soluble bases — adding “excess” NaOH(aq) means you cannot filter out the excess (it’s dissolved)
- Na/K react violently with acids
Method C: Precipitation
When: Making insoluble salts (BaSO₄, AgCl, PbI₂, CaCO₃, PbSO₄)
Procedure
- Mix two solutions — one containing the cation, one containing the anion
- e.g., BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
- The insoluble salt forms immediately as a precipitate (solid)
- Filter the mixture — the insoluble salt stays on the filter paper as the residue
- Wash the residue with distilled water to remove soluble impurities (NaCl in the example)
- Dry the residue in a warm oven or between filter paper
Example Pairs
| Insoluble Salt Wanted | Solution 1 (cation) | Solution 2 (anion) | Precipitate Colour |
|---|---|---|---|
| BaSO₄ | BaCl₂(aq) or Ba(NO₃)₂(aq) | Na₂SO₄(aq) or H₂SO₄(aq) | White |
| AgCl | AgNO₃(aq) | NaCl(aq) or HCl(aq) | White |
| PbI₂ | Pb(NO₃)₂(aq) | KI(aq) | Yellow |
| CaCO₃ | CaCl₂(aq) | Na₂CO₃(aq) | White |
| PbSO₄ | Pb(NO₃)₂(aq) | Na₂SO₄(aq) | White |
Summary: Which Method?
| Salt Type | Method | Key Step |
|---|---|---|
| Soluble, NOT Group 1/NH₄⁺ | A: Excess solid + acid | Filter off excess, then crystallise |
| Soluble, Group 1 or NH₄⁺ | B: Titration | Find exact volume without indicator |
| Insoluble | C: Precipitation | Mix two solutions, filter, wash, dry |
Common Question Types
Type 1: “Describe how you would prepare pure, dry crystals of…”
- Frequency: ~50% of papers, 4-6 marks
- Must include: method choice, excess/filtration OR titration steps, evaporation to crystallisation point, cooling, filtration, washing, drying
- Common trap: Forgetting to say “add in EXCESS” or “wash with COLD distilled water”
Type 2: “Why is Method X not suitable for making Y?”
- e.g., “Why can’t you make potassium chloride by adding excess potassium to HCl?”
- Answer: Potassium reacts too violently with acid / Potassium is Group 1 → too dangerous
Type 3: Identify the Method
- “Which preparation method should be used to make silver chloride?”
- Answer: Precipitation (AgCl is insoluble)
Common Mistakes
- Forgetting to use EXCESS of the solid — acid won’t be fully neutralised
- Washing crystals with hot water — hot water dissolves more salt, losing product. Always use COLD distilled water
- Evaporating to dryness instead of crystallising — this decomposes some salts and gives a powder, not crystals
- Forgetting to repeat titration without indicator — indicator contamination
- Using Bunsen burner for flammable solvents — use a water bath instead
- Not drying crystals properly — mass measurement will be wrong
Key Facts to Memorize
- Method A: Excess insoluble solid + acid → filter → heat to crystallisation point → cool → filter → wash (cold) → dry
- Method B: Titration to find exact volume → repeat without indicator → crystallise
- Method C: Mix two soluble salt solutions → precipitation → filter → wash → dry
- Always wash with COLD distilled water (minimises solubility loss)
- Slow cooling produces larger, purer crystals; fast cooling produces smaller crystals
- Group 1 metals are never added to acid (too violent)
Related Notes
- Making Salts — General overview of salts
- Solubility Rules — Which salts are soluble/insoluble
- Acids and Bases — Acid-base reactions (neutralisation)
- Qualitative Analysis — Testing for cations and anions
- Neutralization — Neutralisation reactions and applications
- Experimental Techniques — Filtration, crystallisation, distillation
- IGCSE-Chem-Index
Past Paper Sources
- 0620/62 May/June 2019 Q4: Preparation of copper sulfate crystals (6 marks)
- 0620/52 Oct/Nov 2018 Q3: Describe how to prepare pure dry lead(II) sulfate (5 marks)
- 0971/62 May/June 2023 Q5: Preparation of zinc chloride by Method A (6 marks)
- 0620/63 Oct/Nov 2020 Q4(b): Explain why titration is used for potassium nitrate (3 marks)
- 0620/43 May/June 2017 Q7(c): Identify preparation method for silver chloride (2 marks)
IGCSE Chemistry (0620/0971) wiki. This topic is frequently assessed in Paper 5/6 (Alternative to Practical).