Solubility Rules

Solubility Rules

1. Ionic solids that are insoluble in water form precipitates

2. All nitrates and salts of Na+, K+ and NH4+ are soluble, so will never form precipitates

3. Addition of ammonia, a basic solution containing OH- ions, will commonly results in the formation of a precipitate of the metal hydroxide

4. All chlorides are soluble except those of AgCl, PbCl2 (both white)

   • This means addition of a solution of AgNO3 can test for the presence of Cl- ions and in addition of HCl(aq) will produce a precipitate if Ag+ ions are present

5. All bromides and iodides are soluble except those of Ag+ and Pb 2+

   • PbI2 is bright yellow, AgI is a pale yellow, while AgBr is cream coloured

6. All sulfates are soluble except BaSO4 and PbSO4 (both white)

   • Addition of a solution of Ba(NO3)2 is often used as a test for the presence of sulfate ions, while addition of sulfuric acid or a solution of sulfate (e.g. Na2SO4) can test for Ba2+
   • CaSO4 is sparingly soluble only

7. All carbonates, sulfides, oxides and hydroxides are insoluble (except for the compounds of the cations identified under 'nitrates')

   • Precipitates of Zn2+, Al3+ and Group 2 ions are white
   • Fe(OH)2 is green, Fe(OH)3 is orange-brown, Cu(OH)2 is blue, Cr(OH)3 is green, Mn(OH)2 is off-white
   • Addition of NaOH to a solution containing Ag+ gives a brown ppt of Ag2O
   • CuO, CuS and FeS are all black
   • Both Fe(OH)2 and Mn(OH)2 are easily oxidised by O2 in air, and these precipitates will darken