Solubility Rules
* Generally soluble:
* Group 1 cations (Li+, Na+, K+, Rb+, Cs+): All salts of these cations are soluble.
* Ammonium (NH4+): All ammonium salts are soluble.
* Nitrate (NO3-): All nitrates are soluble.
* Halides (Cl-, Br-, I-): Most halides are soluble, except for those with Ag+, Pb2+, and Hg22+.
* Sulfates (SO42-): Most sulfates are soluble, except for those with Ba2+, Sr2+, Ca2+, and Pb2+.
* Generally insoluble:
* Carbonates (CO32-): Most carbonates are insoluble, except for those with Group 1 cations and NH4+.
* Phosphates (PO43-): Most phosphates are insoluble, except for those with Group 1 cations and NH4+.
* Hydroxides (OH-): Most hydroxides are insoluble, except for those with Group 1 cations, Ba2+, Sr2+, and Ca2+.
* Sulfides (S2-): Most sulfides are insoluble, except for those with Group 1 cations, Group 2 cations, and NH4+.
Example
Let's say we have the following ion combinations:
1. Na+ and Cl- (Sodium chloride - NaCl)
2. Ag+ and Cl- (Silver chloride - AgCl)
3. Ba2+ and SO42- (Barium sulfate - BaSO4)
4. K+ and CO32- (Potassium carbonate - K2CO3)
Looking at the solubility rules:
* NaCl: Both Na+ and Cl- are generally soluble, so NaCl would be soluble.
* AgCl: While Cl- is generally soluble, Ag+ is an exception and forms an insoluble chloride. AgCl would precipitate out.
* BaSO4: While SO42- is generally soluble, Ba2+ is an exception and forms an insoluble sulfate. BaSO4 would precipitate out.
* K2CO3: While CO32- is generally insoluble, K+ is a Group 1 cation, making K2CO3 soluble.
Therefore, the combination least likely to produce a precipitate is Na+ and Cl- (NaCl).
Key Point: To determine if a combination will form a precipitate, look for any exceptions to the solubility rules. If both ions are generally soluble, it's less likely to form a precipitate.
