AP Chemistry Solubility Rules
The full table, and why you won't find it on the reference sheet College Board hands you on exam day.
Every AP Chemistry student eventually hits the same surprise: the official reference sheet gives you equations and constants, but it doesn't tell you which compounds actually dissolve in water. Solubility rules aren't on it, and neither are polyatomic ion charges. Both have to be memorized outside of exam day. Here's the full solubility table, organized the way it's actually easiest to learn: a short list of things that are always soluble, then the handful of exceptions worth memorizing individually.
Why This Matters Beyond Memorization
Solubility rules aren't just a trivia list. They're how you predict whether mixing two solutions produces a precipitate, which is the first step in writing a net ionic equation and a routine setup for equilibrium and Ksp problems. Get the rule wrong and everything downstream of it, the equation, the stoichiometry, the equilibrium expression, is wrong too.
The Short List: Always Soluble
A handful of ions form soluble compounds no matter what they're paired with. Learn these first, since they resolve a large share of solubility questions immediately without needing to check any exception list.
| Ion | Rule |
|---|---|
| Group 1 cations (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) | Always soluble, no exceptions |
| Ammonium (NH₄⁺) | Always soluble |
| Nitrate (NO₃⁻) | Always soluble, the most reliable rule on this list |
| Acetate (CH₃COO⁻) | Always soluble |
| Chlorate / Perchlorate (ClO₃⁻ / ClO₄⁻) | Always soluble |
Ammonium and the Group 1 metals both work the same way for the same underlying reason: a low-charge-density cation forms a weak enough lattice that water's pull on the ions wins out every time.
Halides: Soluble, With Three Exceptions
Chlorides, bromides, and iodides are soluble as a general rule, including everyday compounds like NaCl and CaCl₂. The exceptions are small in number but come up constantly on the exam:
- Silver (Ag⁺) — AgCl, AgBr, and AgI are all insoluble.
- Lead(II) (Pb₂⁺) — PbCl₂, PbBr₂, and PbI₂ are insoluble at room temperature, though PbCl₂ becomes noticeably more soluble in hot water.
- Mercury(I) (Hg₂₂⁺) — forms insoluble halides as well, though it's less frequently tested than the other two.
Sulfates: Soluble, With Four Exceptions
Most sulfate compounds are soluble. The exceptions are concentrated among Group 2 metals and lead, since the doubly-charged sulfate ion forms an unusually strong lattice with doubly-charged cations of similar size:
- Barium and strontium (Ba₂⁺, Sr₂⁺) — BaSO₄ and SrSO₄ are insoluble.
- Lead(II) (Pb₂⁺) — PbSO₄ is insoluble.
- Calcium (Ca₂⁺) — CaSO₄ is a genuine borderline case, commonly treated as an exception (not freely soluble) but less insoluble than the others on this list.
The Generally Insoluble Anions
Hydroxides, carbonates, phosphates, and sulfides run the opposite direction: insoluble by default, soluble only with the same "always soluble" cations from the top of this page.
| Anion | Soluble exceptions |
|---|---|
| Hydroxide (OH⁻) | Group 1 hydroxides and Ba(OH)₂ are soluble; Ca(OH)₂ and Sr(OH)₂ are only slightly soluble |
| Carbonate (CO₃₂⁻) | Group 1 and ammonium carbonates are soluble |
| Phosphate (PO₄₃⁻) | Group 1 and ammonium phosphates are soluble |
| Sulfide (S₂⁻) | Group 1, Group 2, and ammonium sulfides are soluble |
Notice the pattern repeating: whenever you're unsure about an anion's exception list, checking whether the cation is one of the "always soluble" ones from the top of this page resolves most of these instantly.
Common Mistakes
- Assuming every chloride, bromide, or iodide is soluble because most are, then missing AgCl, PbCl₂, or Hg₂Cl₂ on a mixing question.
- Treating "slightly soluble" compounds like CaSO₄ and Ca(OH)₂ as fully soluble, when the exam expects you to recognize them as exceptions.
- Forgetting that ammonium behaves like a Group 1 metal for solubility purposes, and marking an ammonium salt of an otherwise-insoluble anion as insoluble.
- Confusing solubility rules (a qualitative yes/no) with Ksp (a quantitative value), and trying to memorize numbers that were never meant to be memorized.
Related Resources
- AP Chemistry Reference Sheet
- Unit 3: Properties of Substances and Mixtures
- Unit 7: Equilibrium
- AP Chemistry Particulate Diagrams
- AP Chemistry Study Guide
Frequently Asked Questions
Are solubility rules on the AP Chemistry reference sheet?
No. College Board's reference sheet provides equations and constants, but not a solubility table, polyatomic ion names, or ion charges. All three have to be memorized separately, and solubility rules are the one most students don't realize is missing until they're stuck on an exam question.
Is calcium sulfate (CaSO4) soluble or insoluble?
It's genuinely borderline, and different textbooks classify it differently. For AP exam purposes, treat CaSO4 as an exception to the sulfates-are-soluble rule (i.e., not freely soluble), but know that it's the least insoluble of the sulfate exceptions and some sources list it as slightly soluble rather than fully insoluble.
Do I need to memorize exact Ksp values along with the solubility rules?
No. The solubility rules themselves are qualitative: soluble, insoluble, or an exception. Ksp values are quantitative and specific to each compound, and any you need for a calculation are either given in the question or derived from data in the problem, not memorized.
What's the single most commonly missed solubility exception on the AP exam?
Assuming all chlorides, bromides, and iodides are soluble. Most are, which is exactly why the three exceptions (Ag+, Pb2+, and Hg2 2+) are so commonly tested: they're the case that breaks the general pattern, which makes them a natural target for exam questions.
How do solubility rules connect to Ksp and equilibrium?
Solubility rules tell you qualitatively whether a precipitate forms at all. Once you know a compound is genuinely insoluble, Ksp and molar solubility calculations describe quantitatively how much of it actually stays dissolved, covered in the equilibrium unit.
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