How to memorize the solubility rules
The prettysmart teamUpdated 4 min readthe short answer
Memorize the exceptions, not the rules: 6 ion families are soluble (CASH N' GIA plus ammonium) and almost everything else isn't, and just 2 short metal lists, HAP and CBS, break the soluble side. For AP Chemistry you only need one rule: every sodium, potassium, ammonium and nitrate salt dissolves.
Key takeaways
- Ag⁺, Hg₂²⁺ and Pb²⁺ (HAP) make chlorides, bromides, iodides and sulfates insoluble; Ca²⁺, Sr²⁺ and Ba²⁺ (CBS) only sulfates.
- Carbonates, chromates, phosphates, sulfides and hydroxides are insoluble unless the cation is group 1 or NH₄⁺ (or Ba²⁺, for hydroxide).
- To find a precipitate, swap the partners of the two compounds and check each new one against the rules.
In this guide
The solubility rules chart
| Ions | Rule | Except with |
|---|---|---|
| Group 1 (Li⁺, Na⁺, K⁺…) and NH₄⁺ | Soluble | none |
| Nitrate NO₃⁻, acetate CH₃COO⁻, bicarbonate HCO₃⁻, chlorate ClO₃⁻ | Soluble | none |
| Chloride Cl⁻, bromide Br⁻, iodide I⁻ | Soluble | Ag⁺, Hg₂²⁺, Pb²⁺ |
| Sulfate SO₄²⁻ | Soluble | Ag⁺, Hg₂²⁺, Pb²⁺, Ca²⁺, Sr²⁺, Ba²⁺ |
| Fluoride F⁻ | Soluble | group 2, Pb²⁺, Fe³⁺ |
| Carbonate CO₃²⁻, chromate CrO₄²⁻, phosphate PO₄³⁻, sulfide S²⁻ | Insoluble | group 1, NH₄⁺ |
| Hydroxide OH⁻ | Insoluble | group 1, Ba²⁺ |
“Insoluble” means a low solubility: the compound readily precipitates. Your course’s table may differ in small ways; when it does, learn your course’s.
The trick: learn the exceptions, not the rules
Most of the chart is one sentence: if an ion isn’t on the soluble side, its compounds are insoluble unless the other ion is group 1 or ammonium. What’s left to learn is short lists of metals:
- CASH N’ GIA, the soluble families: Chlorates, Acetates, Sulfates, Halides, Nitrates, Group IA. Add ammonium yourself; the acronym leaves it out.
- HAP, the three that ruin chlorides, bromides, iodides and sulfates: Hg₂²⁺, Ag⁺, Pb²⁺.
- CBS, the three that ruin only sulfates: Ca²⁺, Ba²⁺, Sr²⁺.
- Fluoride is the odd halide: its exceptions are the group 2 ions, Pb²⁺ and Fe³⁺, not HAP.
Some CASH N’ GIA handouts, East Central College’s for one, write mercury as Hg²⁺ and give fluoride the same exceptions as chloride. Both are wrong against the textbook table: the ion is mercury(I), Hg₂²⁺, and AgF is soluble.
The mix-ups that cost points
| Mixed up | How to tell them apart |
|---|---|
| AgCl and AgF | AgCl is insoluble (HAP); AgF is soluble, because Ag⁺ isn’t a fluoride exception |
| Sulfate and sulfide | Sulfate, SO₄²⁻, is mostly soluble; sulfide, S²⁻, mostly isn’t |
| BaSO₄ and BaCl₂ | Ba²⁺ breaks only the sulfate rule: BaSO₄ precipitates, BaCl₂ dissolves |
| MgSO₄ and CaSO₄ | Mg²⁺ isn’t in CBS, so MgSO₄ dissolves; CaSO₄ doesn’t |
| Ba(OH)₂ and Mg(OH)₂ | Ba²⁺ is the one hydroxide exception outside group 1 |
| Hg²⁺ and Hg₂²⁺ | The exception is mercury(I), two atoms sharing a 2+ charge |
Using them: find the precipitate
Swap the partners, then check each product. Mix KI and Pb(NO₃)₂: the new pairs are PbI₂ and KNO₃. KNO₃ is soluble (K⁺, nitrate); PbI₂ is not (Pb²⁺ with iodide), so it’s the solid: Pb²⁺ + 2I⁻ → PbI₂(s).
AP Chemistry needs one rule: “All sodium, potassium, ammonium, and nitrate salts are soluble in water” (CED 4.7.A.5), and the exam doesn’t test rote memorization of any other. Any other solubility fact comes in the question, so the skill is spotting which product must stay dissolved.
A 5-day plan
Spread it over days: the same practice split across days sticks better than one long session (Cepeda et al., 2006).
| Day | What to do |
|---|---|
| 1 | CASH N’ GIA plus ammonium. Write the soluble families from memory 3 times |
| 2 | HAP and CBS. Write the chart with its exceptions from a blank page |
| 3 | The insoluble families and their exceptions, then fluoride and hydroxide |
| 4 | The mix-ups table, then 20 compounds: soluble or insoluble |
| 5 | The whole chart from memory, then 5 precipitate reactions |
Test tomorrow? Learn HAP and CBS tonight, write the chart twice, sleep, and write it once more before class.
Test yourself instead of rereading: 8 of the deck’s 50 cards are below, and the whole deck goes into prettysmart with one code.
Learn the ions first: the chart is written in polyatomic ions and their charges, and group 1 and group 2 are rows of the periodic table. The same groups give the strong bases.
Flashcards: Solubility rules for common ionic compounds
8 of the 50 cards, each checked against this guide's sources. Tap a card to see the answer.
1Which ion is the mercury exception: Hg²⁺ or Hg₂²⁺?
2Which is the hydroxide exception outside group 1?
3Soluble or insoluble: AgF
4Soluble or insoluble: BaSO₄
5Soluble or insoluble: MgSO₄
6Soluble or insoluble: (NH₄)₂CO₃
7KI(aq) + Pb(NO₃)₂(aq): what precipitates?
8KOH(aq) + Fe(NO₃)₃(aq): what precipitates?
Questions people ask
Is Ca(OH)₂ soluble?
Under the rules on this page it counts as insoluble: hydroxides are insoluble except with group 1 cations and Ba²⁺. Some handouts call calcium hydroxide marginally soluble, so follow your course's own table if it differs.
Sources
- Chemistry 2e, 4.2 Classifying Chemical Reactions (Table 4.1), OpenStax
- AP Chemistry Course and Exam Description (effective fall 2024), topic 4.7, College Board
- Solubility Rules Made Easy: CASH N' GIA, East Central College
- Distributed Practice in Verbal Recall Tasks (Cepeda et al., 2006), Psychological Bulletin