Solubility Rules

[Rule] How must the course solubility rules be applied?

In numerical order; an earlier rule overrides a later conflicting rule.

[Rule 1] Which cations make ionic compounds soluble?

Group 1 cations and NH4+ make compounds soluble.

[Rule 2] Which anions are always soluble in the course list?

NO3- and CH3CO2-.

[Rule 3] Which cations make compounds insoluble unless an earlier rule applies?

Ag+, Hg2^2+, and Pb2+.

[Rule 4] Halide rule

Cl-, Br-, and I- compounds are soluble unless an earlier rule applies.

[Rule 5] Which anions are insoluble unless an earlier rule applies?

PO4^3-, CO3^2-, and OH-.

[Rule 6] Sulfate rule

SO4^2- is soluble except with Ca2+, Ba2+, or Sr2+.

[Rule 7] Sulfide rule

S2- is insoluble except with Group 2 cations; earlier rules still take priority.

[Concept] What does insoluble mean in an aqueous reaction?

The compound forms a solid precipitate rather than remaining as separated aqueous ions.

[Practice] Is Ba(CH3CO2)2 soluble?

Yes. Acetates are soluble by Rule 2.

[Practice] Is PbBr2 soluble?

No. Pb2+ is insoluble by Rule 3, which comes before the halide rule.

[Practice] Is CaCO3 soluble?

No. Carbonates are insoluble by Rule 5.

[Practice] Is AgNO3 soluble?

Yes. Nitrates are soluble by Rule 2, which comes before the Ag+ rule.

[Practice] Is Na3PO4 soluble?

Yes. The Group 1 rule comes first and overrides the phosphate rule.

[Practice] Is AgCl soluble?

No. Ag+ makes it insoluble by Rule 3.

[Practice] Is CaCl2 soluble?

Yes. Chlorides are soluble and Ca2+ is not an earlier exception.

[Practice] Is SrSO4 soluble?

No. Sr2+ is an exception to the sulfate rule.

[Practice] Is Na2SO4 soluble?

Yes. Group 1 compounds are soluble.

[Practice] Is FeS soluble?

No. Sulfides are insoluble unless paired with Group 2 or overridden earlier.

[Practice] Is MgS soluble under the course rules?

Yes. Mg2+ is Group 2, an exception in Rule 7.

[Practice] Is (NH4)2CO3 soluble?

Yes. Ammonium compounds are soluble by Rule 1.

[Practice] Mix AgNO3(aq) and NaCl(aq). What precipitate forms?

AgCl(s); NaNO3 remains aqueous.

[Practice] Write the exchange reaction for AgNO3 and NaCl.

AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq).

[Practice] Mix CaCl2(aq) and Na2CO3(aq). What precipitate forms?

CaCO3(s); NaCl remains aqueous.

[Trap] What is the most common solubility-rule error?

Applying a familiar later rule without first checking whether an earlier rule overrides it.