Solubility Rules
Not all ionic compounds dissolve in water. The MCAT expects you to know which salts are soluble and which form precipitates. The good news: the rules are straightforward, and a few simple patterns cover nearly every compound you will encounter.
The Rules - Soluble Compounds
Always soluble (no exceptions):
| Ion | Rule |
|---|---|
| Na⁺, K⁺, Li⁺ (Group 1) | All Group 1 salts dissolve |
| NH₄⁺ (ammonium) | All ammonium salts dissolve |
| NO₃⁻ (nitrate) | All nitrates dissolve |
| CH₃COO⁻ (acetate) | All acetates dissolve |
| ClO₄⁻ (perchlorate) | All perchlorates dissolve |
Usually soluble (with exceptions):
| Ion | Soluble EXCEPT with… |
|---|---|
| Cl⁻, Br⁻, I⁻ (halides) | Ag⁺, Pb²⁺, Hg₂²⁺ |
| SO₄²⁻ (sulfate) | Ba²⁺, Pb²⁺, Ca²⁺, Sr²⁺ |
The Rules - Insoluble Compounds
Usually insoluble (with exceptions):
| Ion | Insoluble EXCEPT with… |
|---|---|
| OH⁻ (hydroxide) | Group 1, NH₄⁺, Ba²⁺, Ca²⁺ (slightly), Sr²⁺ (slightly) |
| CO₃²⁻ (carbonate) | Group 1, NH₄⁺ |
| PO₄³⁻ (phosphate) | Group 1, NH₄⁺ |
| S²⁻ (sulfide) | Group 1, NH₄⁺, Group 2 |
| CrO₄²⁻ (chromate) | Group 1, NH₄⁺ |
How to Apply the Rules
When asked “Is this compound soluble?”:
- Check the cation first. Is it Na⁺, K⁺, or NH₄⁺? If yes, it is soluble. Done.
- Check the anion. Is it NO₃⁻ or CH₃COO⁻? If yes, it is soluble. Done.
- Check for exceptions. Is it a halide with Ag⁺, Pb²⁺, or Hg₂²⁺? Then it is insoluble. Is it a sulfate with Ba²⁺ or Pb²⁺? Insoluble.
- Default for carbonates, phosphates, hydroxides, sulfides: insoluble (unless paired with Group 1 or NH₄⁺).
Predicting Precipitation Reactions
When two aqueous solutions are mixed, a precipitate forms if any combination of the ions produces an insoluble compound:
Example: Mix AgNO₃(aq) and NaCl(aq)
- Possible products: AgCl and NaNO₃
- AgCl: Ag⁺ + Cl⁻ = insoluble (halide exception with Ag⁺)
- NaNO₃: Na⁺ + NO₃⁻ = soluble (Na⁺ is always soluble)
- Result: AgCl precipitates as a white solid
The net ionic equation shows only the ions that actually participate:
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
Na⁺ and NO₃⁻ are spectator ions - they remain dissolved and do not participate in the reaction.
Is lead(II) sulfate (PbSO₄) soluble or insoluble in water?
Click to reveal answer
Insoluble. Sulfates are generally soluble, BUT there are exceptions: Ba²⁺, Pb²⁺, Ca²⁺, and Sr²⁺ sulfates are insoluble or slightly soluble. PbSO₄ is one of the classic sulfate exceptions. If the MCAT gives you this compound, expect it to precipitate out of solution.
Solutions of Ba(NO₃)₂ and Na₂SO₄ are mixed. Write the net ionic equation for any precipitation reaction.
Click to reveal answer
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). Check possible products: BaSO₄ (insoluble - Ba²⁺ is a sulfate exception) and NaNO₃ (soluble - Na⁺ is always soluble). The Na⁺ and NO₃⁻ are spectator ions. BaSO₄ precipitates as a white solid.