1. Precipitation Reactions
Graphic: Wikimedia Commons User Tubifex

2. Double Replacement Reactions
The ions of two compounds exchange places in an aqueous solution to form two new compounds.
AX + BY  AY + BX
One of the compounds formed is usually a
precipitate (an insoluble solid), an insoluble gas that bubbles out of solution, or a molecular compound, usually water.

3. Double replacement forming a precipitate…
Lead(II) nitrate + potassium iodide  lead(II) iodide + potassium nitrate
Double replacement (ionic) equation
Pb(NO3)2(aq) + 2KI(aq)  PbI2(s) + 2KNO3(aq)
Complete ionic equation shows compounds as aqueous ions
Pb2+(aq) + 2 NO3-(aq) + 2 K+(aq) +2 I-(aq)  PbI2(s) + 2K+(aq) + 2 NO3-(aq)
Net ionic equation eliminates the spectator ions
Pb2+(aq) + 2 I-(aq)  PbI2(s)

4. Solubility Rules – AP Chemistry
All sodium, potassium, ammonium, and nitrate salts are soluble in water. Memorization of other “solubility rules” is beyond the scope of this course and the AP Exam.
Therefore, the following slides are only for your amusement, and will not be tested 

5. Solubility Rules – Mostly Soluble
Ion
Solubility
Exceptions
NO3-
Soluble
None
ClO4-
Na+ K+
NH4+
Cl-, I-
Pb2+, Ag+, Hg22+
SO42-
Ca2+, Ba2+, Sr2+, Pb2+, Ag+, Hg2+

6. Solubility Rules – Mostly Insoluble
Ion
Solubility
Exceptions
CO32-
Insoluble
Group IA and NH4+
PO43-
OH-
Group IA and Ca2+, Ba2+, Sr2+
S2-
Groups IA, IIA, and NH4+

7. Solubility Chart: Common salts at 25C
S = Soluble
I = Insoluble
P = Partially
Soluble
X = Other