Solubility rules: check the family and the exception
Use the complete syllabus rule set to decide which substances remain dissolved and which can precipitate.
Core and Extended · Chemistry 0620, 2026–2028.
Before you begin
- Read salt names and ionic formulas.
- Distinguish aqueous solution from a suspended solid.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which ion group identifies a nitrate salt?
What you will learn
- Recall the six required salt and hydroxide solubility rules with their exceptions.
- Apply the rules to formulas without confusing soluble products with solids.
Start with the broadly soluble families
For the syllabus general rules, sodium, potassium and ammonium salts are soluble, and all nitrates are soluble. Chlorides are soluble except lead(II) and silver chlorides. Sulfates are soluble except barium, calcium and lead(II) sulfates in this rule set. Thus silver nitrate is soluble although silver chloride is not; a metal name alone cannot decide every salt containing it.
Check the exception after identifying the salt family. Calcium chloride is soluble, but calcium sulfate belongs to the stated sulfate exceptions. Barium nitrate is soluble, while barium sulfate is insoluble in the school rule set. General soluble/insoluble classifications are practical syllabus approximations rather than a claim that an insoluble substance has mathematically zero solubility under every condition.
Carbonates and hydroxides need their own rules
Carbonates are insoluble except sodium, potassium and ammonium carbonates. Hydroxides are insoluble except sodium, potassium, ammonium and calcium, with calcium hydroxide only partially soluble. The ammonium-hydroxide wording follows the syllabus aqueous-ammonia convention; it does not imply an isolated stable crystal of NH₄OH. Keep calcium's partial solubility rather than assigning it the same fully soluble behaviour as sodium hydroxide.
A soluble substance disperses as a solution and is represented by (aq) when dissolved in water. An insoluble product can appear as a precipitated solid, represented by (s), if suitable solutions react to form it. Solubility rules help choose a preparation method: dissolved salts can be recovered by suitable crystallisation, while an insoluble precipitate can be filtered and washed. The Extended precipitation lesson develops that method.
Pause and explain
Which pair correctly applies the chloride rule?
Worked example
Classify AgNO₃, AgCl, CaSO₄ and K₂CO₃ using the required rules.
Show the worked solution
- AgNO₃ is soluble because nitrates are soluble; AgCl is the silver-chloride exception and is insoluble.
- CaSO₄ is one of the stated sulfate exceptions and is treated as insoluble in this rule set.
- K₂CO₃ is soluble because potassium carbonate is an exception to the mostly insoluble carbonate family.
Answer AgNO₃ and K₂CO₃ are soluble; AgCl and CaSO₄ are insoluble in the stated rules.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Classify sodium sulfate, barium sulfate and lead(II) nitrate, giving a rule for each.
Give me a hint
Check the sulfate exceptions and the nitrate family separately.
Compare my reasoning
- Sodium sulfate is soluble under the sodium-salt rule.
- Barium sulfate is a specified sulfate exception and is insoluble.
- Lead(II) nitrate is soluble because nitrates are soluble.
Sodium sulfate soluble; barium sulfate insoluble; lead(II) nitrate soluble.
Look for these in your work
- Every answer cites its matching family or exception.
- Lead is not treated as making all its salts insoluble.
Classify magnesium carbonate, ammonium carbonate and calcium hydroxide.
Give me a hint
Apply carbonate exceptions, then retain the qualified hydroxide rule.
Compare my reasoning
- Magnesium carbonate is insoluble; magnesium is not a carbonate exception.
- Ammonium carbonate is soluble under the ammonium exception.
- Calcium hydroxide is partially soluble, rather than fully soluble or treated as completely insoluble.
MgCO₃ insoluble; (NH₄)₂CO₃ soluble; Ca(OH)₂ partially soluble.
Look for these in your work
- Carbonate exceptions are applied.
- Calcium's partial solubility is stated.
A plan expects calcium sulfate and calcium chloride to remain equally dissolved merely because both contain calcium. Evaluate the plan using the stated rules.
Give me a hint
The negative ion family changes the result.
Compare my reasoning
- Calcium chloride is soluble because it is not a lead or silver chloride exception.
- Calcium sulfate is a sulfate exception in this syllabus rule set.
- The two calcium salts therefore cannot be assigned the same outcome just from calcium identity.
CaCl₂ is soluble; CaSO₄ is a stated sulfate exception. The shared calcium does not determine both outcomes.
Look for these in your work
- Both salt families are checked.
- The flawed shared-metal inference is corrected.
Common mistakes
- Deciding solubility from the metal name alone.
- Forgetting calcium in the sulfate exceptions.
- Calling every carbonate insoluble.
- Calling calcium hydroxide fully soluble like sodium hydroxide.
What can you explain now?
How does calcium hydroxide differ from sodium hydroxide in the stated solubility rule?
Compare with the explanation
Calcium hydroxide is partially soluble; sodium hydroxide is soluble.
Calcium is a qualified hydroxide exception, so its partial solubility must be retained.
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Make it stick
Write the six rule families and every exception without notes. Classify a fresh salt by family first, exception second.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.