Acids, Bases and Salt Preparations
Inorganic Chemistry
Solubility Rules for Ionic Compounds
Why solubility matters
- Knowing which salts dissolve in water and which do not decides which preparation method to use
- A soluble salt is built up in solution first and then crystallised out by evaporation
- An insoluble salt is dropped out of solution by precipitation — mixing two soluble salts whose ions exchange to form the wanted insoluble product
Solubility patterns to memorise
| Type of salt | Soluble in water | Insoluble exceptions |
|---|---|---|
| Sodium, potassium and ammonium salts | All | — |
| Nitrates | All | — |
| Chlorides | Most | Silver chloride, lead(II) chloride |
| Sulfates | Most | Barium sulfate, calcium sulfate, lead(II) sulfate |
| Carbonates | Sodium, potassium and ammonium carbonates only | The rest are insoluble |
| Hydroxides | Sodium, potassium and ammonium hydroxides; calcium hydroxide is sparingly soluble (limewater) | Most other metal hydroxides |
- Calcium hydroxide solution is what you know as limewater, used as the test for carbon dioxide
Common exam question
Predicting which pair of solutions gives a precipitate
Question: Which pair of solutions gives a precipitate (an insoluble salt) when they are mixed? (1 mark, multiple choice)
Set in 2 of the 23 papers. Swap the partners of the two salts and test each new pairing against the solubility rules. Every sodium, potassium and ammonium salt and every nitrate stays dissolved, so a pair such as sodium sulfate with potassium nitrate can never precipitate. A precipitate needs one of the insoluble exceptions to form: calcium sulfate from copper(II) sulfate and calcium chloride, copper(II) carbonate from sodium carbonate and copper(II) sulfate, or calcium carbonate from potassium carbonate and calcium nitrate. Check carbonates and hydroxides first, because only their sodium, potassium and ammonium compounds dissolve (calcium hydroxide slightly).