2.7.1Inorganic Chemistry
Solubility Rules — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 35 of 52
Part 1 of 5Solubility Rules for Common Salts
Most ionic compounds dissolve in water, but some common exceptions must be learned.
Sodium, potassium, ammonium
- All compounds of these ions are soluble in water, which makes them good sources of ions in reactions.
Nitrates
- All nitrates are soluble, so lead(II) nitrate and silver nitrate are common starting solutions.
Chlorides
- Most chlorides are soluble, but silver chloride and lead(II) chloride are insoluble.
Sulfates
- Most sulfates are soluble, but barium sulfate, lead(II) sulfate and, to a lesser extent, calcium sulfate are insoluble.
| Salt type | Usually soluble | Usually insoluble (exceptions) |
|---|---|---|
| Sodium, potassium, ammonium | All | None |
| Nitrates | All | None |
| Chlorides | Most | Silver and lead(II) |
| Sulfates | Most | Barium, calcium, lead(II) |
| Carbonates | Sodium, potassium, ammonium | Most others |
| Hydroxides | Sodium, potassium (calcium slightly) | Most others |
Part 2 of 5Carbonates and Hydroxides
Carbonates and hydroxides are mostly insoluble, with the group 1 and ammonium salts as the main exceptions.
Carbonates and Hydroxides
Soluble carbonates
- Sodium, potassium and ammonium carbonates dissolve in water, so they can be used as sources of carbonate ions.
Insoluble carbonates
- Most other carbonates, such as copper(II) carbonate, do not dissolve and can be prepared by precipitation.
Soluble hydroxides
- Sodium and potassium hydroxides are soluble, and their solutions are strongly alkaline.
Calcium hydroxide
- Calcium hydroxide is only slightly soluble, and its solution, limewater, is used to test for carbon dioxide.
Part 3 of 5Predicting a Precipitate
Solubility rules are used to predict whether a precipitate will form when two solutions are mixed.
1
Write the formulae of the possible products by swapping the ions of the two salts.
2
Check each product against the solubility rules to see whether it is soluble or insoluble.
3
If one product is insoluble, it forms a solid precipitate, which can be separated by filtration.
4
If both products are soluble, no precipitate forms and the solutions remain clear.
Part 4 of 5Worked Example
Question
- Predict whether a precipitate forms when silver nitrate solution is mixed with sodium chloride solution.
- Give the state symbols of the products if one forms.
- Swap the ions to get possible products: silver chloride and sodium nitrate.
- Silver chloride is insoluble (silver is an exception for chlorides);
- sodium nitrate is soluble because all nitrates and all sodium salts are soluble.
- So silver chloride forms as a precipitate.
Answer
- AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq).
- A white precipitate of silver chloride forms.
Part 5 of 5Exam Checklist & Tips
2.7.1 – Key Points for the Exam
- Soluble Able to dissolve in water to form a solution.
- Insoluble Does not dissolve appreciably in water.
- Precipitate An insoluble solid that forms when two solutions react together.
- Solubility rule A general statement about which ionic compounds dissolve in water.
Common Mistakes
- Assuming all chlorides are soluble; silver and lead(II) chlorides are the important exceptions.
- Forgetting that lead(II) sulfate and barium sulfate are insoluble despite most sulfates dissolving.
- Stating that calcium hydroxide is insoluble; it is only slightly soluble, which is why limewater can be made.
- Predicting a precipitate from the reactants instead of from the products formed by swapping the ions.
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