Solubility Rules and Precipitation Reactions
Knowing which salts dissolve lets you predict whether mixing two solutions produces a precipitate — and that underpins salt preparation, ion tests and ionic equations alike.
1. The solubility rules
ALWAYS SOLUBLE:
- All sodium, potassium and ammonium salts
- All nitrates
MOSTLY SOLUBLE, with exceptions:
- Chlorides — except silver and lead
- Sulfates — except barium, calcium and lead
MOSTLY INSOLUBLE, with exceptions:
- Carbonates — except sodium, potassium and ammonium
- Hydroxides — except sodium, potassium and ammonium (calcium hydroxide is slightly soluble)
A summary table:
| Salt type | Soluble? | Exceptions |
|---|---|---|
| Sodium, potassium, ammonium | all soluble | none |
| Nitrates | all soluble | none |
| Chlorides | soluble | Ag, Pb |
| Sulfates | soluble | Ba, Ca, Pb |
| Carbonates | insoluble | Na, K, NH₄ |
| Hydroxides | insoluble | Na, K, NH₄ (Ca slightly) |
Not remembering the solubility chart was a recorded error — and it makes every question in this area impossible. Learn the two “always soluble” lines first; they cover most cases.
2. Predicting a precipitate
When two solutions are mixed, SWAP the partners. If either new combination is INSOLUBLE, it forms a PRECIPITATE.
Example: silver nitrate + sodium chloride
- Swap: silver chloride and sodium nitrate
- Silver chloride is INSOLUBLE → a white precipitate forms
- Sodium nitrate is soluble → stays in solution
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
Example: barium chloride + sodium sulfate
- Swap: barium sulfate (insoluble) and sodium chloride (soluble)
- A white precipitate of barium sulfate forms
If BOTH new combinations are soluble, NO precipitate forms and no reaction occurs.
3. Ionic equations
Write the full equation, then REMOVE the SPECTATOR IONS — the ones that appear unchanged on both sides.
Full: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
All ions: Ag⁺ + NO₃⁻ + Na⁺ + Cl⁻ → AgCl(s) + Na⁺ + NO₃⁻
Spectators: Na⁺ and NO₃⁻ — unchanged on both sides
Ionic equation: Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
Include STATE SYMBOLS — especially (s) for the precipitate, which is the whole point of the equation.
Only the ions that actually REACT appear. Leaving spectator ions in was recorded as an error in displacement work, and the same rule applies here.
Other common ionic equations:
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s) — a bright yellow precipitate Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s) — light blue
4. Making an insoluble salt
1. Choose TWO SOLUBLE solutions containing the required ions 2. MIX them 3. FILTER to collect the precipitate as the residue 4. WASH with distilled water 5. DRY
Choose the starting materials using the solubility rules — a nitrate and a sodium/potassium salt are always safe, since both are guaranteed soluble.
Example — making lead(II) iodide: mix lead(II) nitrate and potassium iodide, both soluble.
Wash the precipitate to remove the soluble by-product left on it.
5. Precipitate colours worth knowing
| Precipitate | Colour |
|---|---|
| Silver chloride | white |
| Silver bromide | cream |
| Silver iodide | yellow |
| Barium sulfate | white |
| Lead(II) iodide | bright yellow |
| Copper(II) hydroxide | light blue |
| Iron(II) hydroxide | green |
| Iron(III) hydroxide | red-brown |
| Calcium / aluminium / zinc hydroxide | white |
These colours are what the ion tests rely on — precipitation and qualitative analysis are the same chemistry.
6. Mistakes that cost marks
Not knowing the solubility rules.
Choosing an insoluble starting material for a precipitation.
Forgetting to swap the partners when predicting.
Leaving spectator ions in an ionic equation.
Omitting state symbols, especially (s).
Not washing the precipitate.
Confusing the precipitate colours.
Frequently asked questions
Which salts are always soluble? All sodium, potassium and ammonium salts, and all nitrates.
Which chlorides are insoluble? Silver and lead.
Which sulfates are insoluble? Barium, calcium and lead.
Are carbonates soluble? Mostly no — only sodium, potassium and ammonium.
How do I predict a precipitate? Swap the partners and check both new salts against the solubility rules.
What if both are soluble? No precipitate — no reaction occurs.
How do I write an ionic equation? Remove the spectator ions and include state symbols.
What is the ionic equation for silver chloride? Ag⁺(aq) + Cl⁻(aq) → AgCl(s).
How do I make an insoluble salt? Mix two soluble solutions, then filter, wash and dry.
Why wash the precipitate? To remove the soluble by-product.
Quick revision checklist
- I know the two always soluble categories
- I know the exceptions for chlorides and sulfates
- I know carbonates and hydroxides are mostly insoluble
- I can swap partners to predict a precipitate
- I know no precipitate means no reaction
- I can write ionic equations with spectators removed
- I include state symbols
- I can plan a precipitation using soluble reagents
- I know to wash and dry the product
- I know the common precipitate colours
These notes cover solubility rules and precipitation reactions in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and the qualitative analysis notes for your own exam series.
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