Solubility Rules

Question 1

Rule 1

Answer 1

All compounds of Group IA elements (the alkali metals) are soluble.

Question 2

Rule 2

Answer 2

All ammonium salts (salts of NH4+) are soluble.

For example, NH4OH is a soluble compound. Molecules of NH4OH completely dissociate to give ions of NH4+ and OH- in aqueous solution.

Question 3

Rule 3

Answer 3

All nitrate (NO3-), chlorate (ClO3-), perchlorate (ClO4-), and acetate (CH3COO- or C2H3O2-, sometimes abbreviated as Oac-) salts are soluble.

Question 4

Rule 4

Answer 4

All chloride (Cl-), bromide (Br-), and iodide (I-) salts are soluble except for those of Ag+, Pb2+, and Hg22+.

For example, AgCl is a classic insoluble chloride salt:
AgCl(s) Ag+(aq.) + Cl-(aq.) (Ksp = 1.8 x 10-10).

Question 5

Rule 5

Answer 5

All sulfate ( SO4=) compounds are soluble except those of Ba2+, Sr2+, Ca2+, Pb2+, Hg22+, and Hg2+, Ca2+ and Ag+ sulfates are only moderately soluble.

For example, BaSO4 is insoluble (only soluble to a very small extent):
BaSO4(s) Ba2+(aq.) + SO42-(aq.) (Ksp = 1.1 x 10-10).
Na2SO4 is completely soluble:
Na2SO4(s) => 2 Na+(aq.) + SO42-(aq.).

Question 6

Rule 6

Answer 6

All hydroxide (OH-) compounds are insoluble except those of Group I-A (alkali metals) and Ba2+, Ca2+, and Sr2+

For example, Mg(OH)2 is insoluble (Ksp = 7.1 x 10-12).
NaOH and Ba(OH)2 are soluble, completely dissociating in aqueous solution:
NaOH(s) => Na+(aq.) + OH-(aq.), a strong base
Ba(OH)2(s) => Ba2+(aq.) + 2OH-(aq.) (Ksp = 3 x 10-4)

Question 7

Rule 7

Answer 7

All sulfide (S2-) compounds are insoluble except those of Groups I-A and II-A (alkali metals and alkali earths).

For example, Na2S(s) 2Na+(aq.) + S2-(aq.)
MnS is insoluble (Ksp = 3 x 10-11).

Question 8

Rule 8

Answer 8

All sulfites (SO3=), carbonates (CO3=), chromates (CrO4=), and phosphates (PO43-) are insoluble except for those of NH4+ and Group I-A (alkali metals)(see rules 1 and 2).