Use these relationships to determine whether a double displacement reaction will result in the formation of a precipitate (ppt). 

Keep these relationships in mind as we are doing qualitative analysis.

Solubility Rules
1 All chlorides, bromides, and iodides are soluble except those containing Ag+, Pb2+, or Hg22+.
2 All compounds containing NO3-, ClO4-, ClO3-, and C2H3O2- anions are soluble.
3 All sulfates (SO4-2) are soluble except those containing Hg22+, Pb2+, Sr2+, Ca2+, or Ba2+.
4 All compounds containing alkali metal cations (Na+, K+, H+) and the ammonium ion (NH4+) are soluble
5 All compounds containing OH-,  PO43-, S2-, CO32-, O2-,CrO4-2 and SO32-ions are  insoluble, unless the cation is from Rule 4. (Rule 4 is stronger than Rule 5)

Solubility reactions. A double displacement reaction will "fail" if

  1. One or more of the reactants are insoluble. Example: PbS + AgNO3 --> Fails by rule 5 S2- compounds are insoluble. No reaction.
  2. If both products are soluble. Example: NaCl + HC2H3O2 --> NaC2H3O2 + HCl No reaction. No ppt. Rule 2 and Rule 4