When an acid reacts with a base, a salt is formed. Some salts are soluble while some salts are insolube. Knowing which salts are insoluble can help us design an appropriate experiment to prepare the salt we desire.
HaX(aq) + M(OH)b(aq) → MaXb(aq/s) + H2O(l)
The below will summarise the soluble and insoluble salts.
- All K+, Na+, NH4+
- All NO3–
- All HCO3–
- All X– (Cl–, Br–, I–) except Ag+, Pb2+
- All SO42- except Pb2+, Ca2+, Ba2+
- Ag+, Pb2+ halides (Cl–, Br–, I–)
- Pb2+, Ca2+, Ba2+ sulphates (SO42-)
- All CO32- except K+, Na+, NH4+
All hydroxides (except K+, Na+) are insoluble in water. Calcium hydroxide is slightly soluble in water to form limewater.
However, when some metal ions are in excess hydroxide or ammonia solution, they can form stable complexes, which are soluble in water.
In excess NaOH(aq):
Al(OH)3(s) + OH–(aq) → [Al(OH)4]–(aq)
Zn(OH)2(s) + 2OH–(aq) → [Zn(OH)4]2-(aq)
Pb(OH)2(s) + 2OH–(aq) → [Pb(OH)4]2-(aq)
In excess NH3(aq):
Zn(OH)2(s) + 4NH3(aq) → [Zn(NH3)4]2+(aq) + 2OH–(aq)
Cu(OH)2(s) + 4NH3(aq) → [Cu(NH3)4]2+(aq) + 2OH–(aq)
Ag2O(s) + H2O(l) + 4NH3(aq) → 2[Ag(NH3)2]+(aq) + 2OH–(aq)
Try the question below and see if you can get the answer!