Solubility Rules
Section: 7. Acids, Bases & Salts | Syllabus: Cambridge IGCSE Chemistry (0620)
The General Solubility Rules
These rules predict whether a salt will dissolve or form a precipitate - essential for choosing reactants for both soluble- and insoluble-salt preparations.
| Ion / salt type | Solubility | Exceptions |
|---|---|---|
| Sodium, potassium, ammonium salts | ALL soluble | none |
| Nitrates | ALL soluble | none |
| Chlorides | soluble | lead(II) and silver chlorides are insoluble |
| Sulfates | soluble | barium, calcium and lead(II) sulfates are insoluble |
| Carbonates | insoluble | sodium, potassium and ammonium carbonates are soluble |
| Hydroxides | insoluble | sodium, potassium and ammonium hydroxides are soluble; calcium hydroxide is partially soluble |
Worked Example: Predicting a Precipitate
- Question: is calcium sulfate soluble or insoluble in water?
- Answer: most sulfates are soluble, but calcium sulfate is one of the three named exceptions (Ba, Ca, Pb) - it is insoluble
Worked Example: Sulfate Precipitation
- Question: dilute sulfuric acid is added to barium chloride solution. State what is observed. [1]
- Answer: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s); a white precipitate forms (barium sulfate is one of the three insoluble sulfates)
Using the Rules to Test for Ions
| Ion tested for | Add | Positive result |
|---|---|---|
| Chloride (Cl⁻) | silver nitrate (+ dilute HNO₃) | white precipitate (AgCl) |
| Sulfate (SO₄²⁻) | barium chloride (+ dilute HCl) | white precipitate (BaSO₄) |
Dilute acid is added alongside the test reagent to remove any carbonate ions first, since carbonates would also give a false-positive white precipitate with either test.
Common Mistakes
- ❌ Assuming all chlorides/sulfates/carbonates follow one simple soluble-or-insoluble rule
- ✓ each rule has specific, memorable exceptions - Pb/Ag for chlorides, Ba/Ca/Pb for sulfates, Na/K/NH₄⁺ for carbonates and hydroxides
- ❌ Forgetting that calcium hydroxide is only partially soluble
- ✓ Ca(OH)₂ dissolves just enough to make limewater weakly alkaline - it is not fully soluble like NaOH or KOH
Interactive revision notes, videos and practice questions load below.