SOLUBILITY RULES


Chemical reactions happen around us, and in us, everyday. To maintain our body temperature at 98.6°F (37°C) when the ambient temperature is cooler, requires heat-generating biochemical reactions that are sensitively regulated. In addition to heat formation, there are four other signs that signal the occurance of a chemical reaction: color change, gas (odor) formation, precipitation of a solid, and light. To help chemists (and chemistry students) manage the vast number of chemical reactions, four categories have been created into which most chemical reactions fall:

  1. Precipitation
  2. Acid-Base
  3. Reduction-Oxidation (RedOx)
  4. Decomposition





you're part of the precipitate.

Precipitation reactions occur when the solubility of a product is low and it "falls out of solution" as a precipitate. Observe the precipitation of silver iodide by clicking the Play button in the animation to the left. Since solid reaction products are denser than aqueous solutions, the solid settles to the bottom of the test tube. The saturated solution above the solid is called the supernatant (from Latin - above + swim . . . . float). In the animation, the AgI forms a yellow precipitate while the NaNO3 dissolves in water and does not form a precipitate. Click Play again to repeat the animation.

Is there a way to predict this behavior without "looking up" each products' solubility data?

Yes . . . . the solubility of hundreds of compounds can be predicted with the six solubility rules below.


Start at Rule 1 and proceed until a condition is met for the compound's cation or anion . . . . then, Stop sign . . . . a rule with a lower number takes precedence over a rule with a higher number.


The Solubility Rules

  1. if the anion is NO3-, HCO3-, C2H3O2-, ClO3- or ClO4-, the compound is  Soluble(aq) 
  2. if the cation is an alkali metal cation (Li+, Na+, K+, Rb+, Cs+) or NH4+, the compound is  Soluble(aq) 
  3. if the anion is Cl-, Br- or I-, the compound is  Soluble(aq)  except for  AgCl, Hg2Cl2 and PbCl2 .
  4. if the anion is SO42-, the compound is  Soluble(aq) except for  SrSO4, BaSO4 and PbSO4 .
  5. if the cation is Ca2+, Sr2+ or Ba2+, then Ca(OH)2, Sr(OH)2, Ba(OH)2, CaS, SrS, and BaS are  Soluble(aq) .
  6. all remaining salts are  Insoluble(s)↓ .


If you know the  Soluble(aq)  salts and the exceptions for chloride, bromide, iodide and sulfate, you will know the solubility rules because the rest of the salts are  Insoluble(s)↓ . Here's another way to view the Solubility Rules (Stop sign when a condition is met) . . . .

1. Soluble Anions (X is any cation):    XNO3       XHCO3       XC2H3O2       XClO3       XClO4

XCl, XBr, XI    except Ag+, Hg22+, Pb2+  XSO4    except Sr2+, Ba2+ and Pb2+ 

2. Soluble Cations (Y is any anion):    LiY       NaY       KY       RbY       CsY       NH4Y

3. Other Soluble Salts:   Ca(OH)2         Sr(OH)2         Ba(OH)2         CaS         SrS         BaS



Use the solubility rules to predict whether the compounds below are soluble or insoluble. Answer 10 questions correctly to display the Tutorial Complete message.










Predict the products of the following reactions and use the Solubility Rules to assign the state of matter to the reactants and products. After you balance the reaction, click the Show Answer link to check your work.

Barium acetate + Lithium phosphate →

3 Ba(C2H3O2)2  (aq)  + 2 Li3PO4  (aq)  → Ba3(PO4)2  (s)  + 6 LiC2H3O2  (aq) 

Solubility Rule #            1                           2                             6                             1 

Lead(II) nitrate + Ammonium iodide →

Pb(NO3)2  (aq)  + 2 NH4I  (aq)  → PbI2  (s)  + 2 NH4NO3  (aq) 

Solubility Rule #                1                          2                       3                     1 

Silver nitrate + Potassium chloride →

AgNO3  (aq)  + KCl  (aq)  → AgCl  (s)  + KNO3  (aq) 

Solubility Rule #                       1                    2                   3                 1 

Strontium acetate + Sodium sulfate →

Sr(C2H3O2)2  (aq)  + Na2SO4  (aq)  → SrSO4  (s)  + 2NaC2H3O2 (aq) 

Solubility Rule #              1                          2                        4                          1