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How to Tell if a Salt Dissolves in Water
Manesh Jayawardhana
CIO & Co-founder
Mix two clear solutions and sometimes you get a cloudy precipitate and sometimes you don’t. Predicting which is a standard exam question and a routine bench judgement, and it comes down to a short list of rules — plus their exceptions, which is where the actual answer usually lives.
The rules worth knowing
Reliably soluble:
- All nitrates (NO₃⁻)
- All Group 1 salts (Li⁺, Na⁺, K⁺…)
- Almost all ammonium (NH₄⁺) salts
These three have essentially no exceptions worth remembering, which makes them the useful starting point: if either ion in your product is a nitrate, a Group 1 metal or ammonium, it stays in solution.
Generally soluble, with exceptions:
- Chlorides, bromides, iodides — except silver, lead(II) and mercury(I)
- Sulfates — except barium, lead(II), calcium and strontium
Generally insoluble, with exceptions:
- Carbonates, phosphates, hydroxides, sulfides — except with Group 1 and ammonium
Notice the structure: the exceptions are the informative part. Sulfates being soluble is unremarkable. Barium sulfate being insoluble is the basis of the standard test for sulfate ions, and of barium meals in medical imaging.
| Anion | Rule | Key Exceptions |
|---|---|---|
| NO₃⁻ | Soluble | None worth noting |
| Cl⁻, Br⁻, I⁻ | Soluble | Ag⁺, Pb²⁺, Hg₂²⁺ |
| SO₄²⁻ | Soluble | Ba²⁺, Pb²⁺, Ca²⁺, Sr²⁺ |
| CO₃²⁻, PO₄³⁻ | Insoluble | Group 1, NH₄⁺ |
| OH⁻ | Insoluble | Group 1, and partly Ca²⁺, Ba²⁺ |
How to use them for a precipitation reaction
Swap the partners and check both products.
Mix barium chloride and sodium sulfate. The possible products are barium sulfate and sodium chloride. Sodium chloride is a Group 1 salt — soluble. Barium sulfate is a sulfate with barium — one of the exceptions, so insoluble. That’s your precipitate, and the reaction goes.
Mix sodium chloride and potassium nitrate. Products are sodium nitrate and potassium chloride — both soluble. Nothing precipitates, and nothing happens.
What “insoluble” actually means
Nothing is completely insoluble. Barium sulfate has a solubility product of around 10⁻¹⁰, which means a very small amount does dissolve — just not enough to see.
The rules are a qualitative shortcut for a quantitative property. Where precision matters — calculating an equilibrium concentration, predicting whether a precipitate forms at a specific concentration — you need the solubility product Ksp and the actual ion concentrations, not a rule of thumb.
Why people get stuck here
- Learning rules without exceptions. The exceptions are what questions test.
- Insoluble read as zero. It means very low, not none.
- Hydroxide edge cases. Calcium and barium hydroxides are partially soluble, which makes them awkward.
- Concentration ignored. A precipitate forms only when the ion product exceeds Ksp, so dilute solutions of “insoluble” salts can stay clear.
Common mistakes to avoid
- Memorising the soluble list and skipping the exceptions.
- Forgetting to swap both ion partners when predicting products.
- Assuming a reaction occurs whenever two solutions are mixed.
- Treating the rules as exact when a Ksp calculation is what the question needs.
- Ignoring that some “insoluble” carbonates dissolve in acid, which is a separate and very useful behaviour.
How to do it with Solubility Rules Reference
The Solubility Rules Reference puts each rule next to its exceptions, which is the arrangement that’s actually useful.
- Enter the compound or select the anion you’re working with.
- Read the general rule and its exceptions together.
- For a precipitation reaction, swap the partners and check both products.
- Where precision matters, move to Ksp values rather than the rules.
Other chemistry references are in the tools directory.
Frequently asked questions
Which rules have no exceptions worth remembering?
Nitrates and Group 1 salts are reliably soluble, and ammonium salts nearly always are. Almost every other rule carries exceptions you need to know.
Why do the exceptions matter so much?
Because precipitation reactions are defined by them. The standard test for sulfate ions works precisely because barium sulfate is an exception to sulfates being soluble.
Is “insoluble” really zero solubility?
No. It means very low solubility, quantified properly by the solubility product. The rules are a rapid qualitative shortcut, not a substitute for Ksp.
Final thought
Learn three reliably soluble groups and the four exception lists. That’s the whole system, and the exceptions are where every interesting reaction happens.