Precipitation Reactions
The process of making a solid come from a solution is called precipitation. The solid itself is called a precipitate. An insoluble salt (one that doesn’t dissolve) can be made by reacting the appropriate soluble salt with an acid or alkali or another salt.
| STATE SYMBOLS | |
| (s) | solid |
| (l) | liquid |
| (g) | gas |
| (aq) | aqueous solution (dissolved in water) |
You are normally asked to prepare a solid from two soluble solutions – so know your solubility rules:
- All nitrates are soluble.
- All sodium, potassium and ammonium compounds are soluble.
- Most carbonates and hydroxides are insoluble except for sodium, potassium and ammonium.
- All sulphates are soluble except barium and lead(II) sulphate.
- All chlorides are soluble except lead(II) and silver chloride.
Example: Prepare Silver Chloride
You’ll need:
- A soluble silver salt – what about silver nitrate?
- A soluble chloride – like magnesium chloride?
You can make it up really. So the equation:
AgNO3 (aq) + MgCl2(aq) -> AgCl(s) + Mg(NO3)2(aq)