How to Memorize Solubility Rules
The easiest way to learn how to memorize solubility rules is to split them into two short lists: ions that are almost always soluble, and ions that are almost always insoluble, then memorize only the exceptions to each. Two mnemonics, "NAG SAG" and "PMS Castro Bear," cover most of what a first-year chemistry class asks, and a week of short card reviews locks them in.
If precipitation questions keep costing you points because you blank on which compounds dissolve, FlashDeck turns your solubility notes into cards and resurfaces each rule right before you'd forget it: Build my study plan.
What are the rules of solubility? The rules in a table
Solubility rules tell you whether an ionic compound will dissolve in water (soluble, aq) or form a solid (insoluble, s). You use them to predict precipitates in double replacement reactions and to write net ionic equations.
Here's a standard solubility rules table for intro chemistry. Your textbook may phrase a few lines differently, so compare it with your class notes.
| Ion in the compound | Usually… | Main exceptions |
|---|---|---|
| Group 1 ions (Li⁺, Na⁺, K⁺…) | Soluble | none you'll usually need |
| Ammonium (NH₄⁺) | Soluble | none you'll usually need |
| Nitrate (NO₃⁻) | Soluble | none |
| Acetate (C₂H₃O₂⁻) | Soluble | a few, like silver acetate, are only slightly soluble |
| Chloride, bromide, iodide | Soluble | Ag⁺, Pb²⁺, Hg₂²⁺ |
| Sulfate (SO₄²⁻) | Soluble | Ba²⁺, Sr²⁺, Pb²⁺, Ca²⁺, Hg₂²⁺ (Ag⁺ slightly) |
| Carbonate (CO₃²⁻) | Insoluble | Group 1 and NH₄⁺ |
| Phosphate (PO₄³⁻) | Insoluble | Group 1 and NH₄⁺ |
| Sulfide (S²⁻) | Insoluble | Group 1, NH₄⁺ (Group 2 often listed too) |
| Hydroxide (OH⁻) | Insoluble | Group 1, NH₄⁺; Ca²⁺, Sr²⁺, Ba²⁺ somewhat soluble |
Notice the shape: the top half is "soluble unless," the bottom half is "insoluble unless." That split is the whole trick.
Mnemonic: the easy way to memorize solubility rules
NAG SAG: the always-soluble list
NAG SAG gives you the ions that make a compound soluble:
- Nitrates
- Acetates
- Group 1 (alkali metal) ions
- Sulfates
- Ammonium
- Group 17 ions (chloride, bromide, iodide)
If a compound contains any of these, your first guess is "soluble." Then you check the exceptions.
PMS Castro Bear: the exceptions
For the halides and sulfates, the troublemakers are:
- Pb²⁺ (lead)
- Mercury (Hg₂²⁺)
- Silver (Ag⁺)
Those three ruin both halides and sulfates. Sulfates get extra exceptions:
- Castro → Ca²⁺ and Sr²⁺
- Bear → Ba²⁺
So: halides are soluble except with PMS. Sulfates are soluble except with PMS and Castro Bear.
The insoluble list
Everything else in the table (carbonates, phosphates, sulfides and hydroxides) is insoluble unless paired with a Group 1 ion or ammonium. One easy way to remember it: Group 1 and ammonium dissolve everything. That single sentence handles the exceptions to the whole bottom half.
Exceptions: how to memorize solubility rules for insoluble compounds
Most wrong answers come from exceptions, not the main rules. Learn these as their own mini-list.
- Silver chloride (AgCl) is insoluble. It's the classic white precipitate in lab.
- Lead(II) iodide (PbI₂) is insoluble. The bright yellow precipitate.
- Barium sulfate (BaSO₄) is insoluble. Castro Bear in action.
- Calcium carbonate (CaCO₃) is insoluble. Chalk and limestone don't dissolve in water.
- Sodium carbonate (Na₂CO₃) is soluble. Group 1 beats carbonate.
- Ammonium phosphate is soluble. Ammonium beats phosphate.
When two rules fight
What about lead(II) nitrate, Pb(NO₃)₂? Lead is on the exception list, but that list is for halides and sulfates. Nitrates have no exceptions, so it's soluble. The rule to follow: check the "always soluble" ions first. If Group 1, ammonium or nitrate is present, you're done.
Worked example: predicting a precipitate
Say you mix barium chloride and sodium sulfate solutions. Here's the thinking, step by step:
- Swap partners. The possible new products are barium sulfate (BaSO₄) and sodium chloride (NaCl).
- Check NaCl. Sodium is Group 1, so it's soluble. It stays dissolved as ions.
- Check BaSO₄. Sulfates are soluble, but barium is on the Castro Bear exception list. Insoluble.
- Write the answer. BaSO₄ is the precipitate. The net ionic equation is Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
Run every double replacement question through those four steps and the rules stop feeling like trivia. They become a tool you use in under 30 seconds.
Practice cards: the best way to memorize solubility rules
Here's the honest problem. You can recite NAG SAG today and still freeze on "Is SrSO₄ soluble?" three weeks from now in the middle of a net ionic equation. Rules fade unless you practice pulling them out.
Build two kinds of cards.
Rule cards
- Front: "Halides are soluble except with which three ions?" → Back: "Ag⁺, Pb²⁺, Hg₂²⁺"
- Front: "Which ions make every compound soluble?" → Back: "Group 1 and NH₄⁺ (plus nitrate and acetate)"
- Front: "Extra sulfate exceptions beyond PMS?" → Back: "Ca²⁺, Sr²⁺, Ba²⁺"
Compound cards
These are the ones that actually train you for tests.
- Front: "AgNO₃: soluble or not?" → Back: "Soluble (nitrate)"
- Front: "BaSO₄?" → Back: "Insoluble (sulfate + Ba²⁺)"
- Front: "K₃PO₄?" → Back: "Soluble (Group 1 beats phosphate)"
- Front: "Mg(OH)₂?" → Back: "Insoluble (hydroxide, not Group 1)"
- Front: "PbCl₂?" → Back: "Insoluble (halide + Pb²⁺)"
Aim for 15 to 20 compound cards that mix every rule. Then review them in short daily sessions.
The hard part is knowing which cards to redo and when. FlashDeck tracks that for you: paste your solubility notes and it writes one card per rule and exception. Each day you see only what's due. Missed cards come back tomorrow, and cards you knew come back after a day, then three days, then a week. Build my study plan
A quick practice routine
- Days 1–2: Learn NAG SAG and PMS Castro Bear. Write them from memory twice.
- Days 3–5: Five minutes of compound cards daily.
- Day 6: Predict the precipitate in five double replacement reactions from your textbook.
- Day 7 onward: Review only the due cards.
Solubility rules are built on polyatomic ions, so if "PO₄³⁻" still makes you pause, run through how to memorize polyatomic ions first. And since you'll use these rules mostly to predict products, pair this with how to memorize chemistry reactions.
Turn your solubility chart into cards in a minute
That's how to memorize solubility rules for good: two mnemonics, a short exceptions list, and a few minutes of daily recall. Paste the chart from your notes or upload your slides as a PDF, and FlashDeck writes one card per rule from exactly what you gave it, then serves up only the ones due each day.
FAQ
What is the easiest way to memorize solubility rules?
Use NAG SAG for the ions that make compounds soluble, and PMS Castro Bear for the exceptions. Then remember that carbonates, phosphates, sulfides and hydroxides are insoluble unless paired with Group 1 or ammonium.
How do you memorize solubility rules for AP Chemistry?
Learn the core rules with the mnemonics, then drill real compounds in net ionic equation problems. AP Chem leans hard on the always-soluble ions (sodium, potassium, ammonium and nitrate), so make sure those are automatic. Check your teacher's list for anything extra.
How can I quickly memorize solubility rules for the DAT?
Spend your time on the exceptions, since that's where questions get tricky. Make one card per exception and review them daily for a week or two before test day.
Which compounds are always soluble?
Compounds with Group 1 ions, ammonium or nitrate are soluble in intro chemistry. Most acetates are too, with a few slightly soluble exceptions.
Do I need to memorize the whole solubility table?
Not word for word. If you know the "always soluble" list, the insoluble list and the exceptions, you can rebuild the table on scratch paper in under a minute.