http://www.ck12.org Chapter 16. Solutions
Predicting Precipitates
Some combinations of aqueous reactants result in the formation of a solid precipitate as a product. However, some
combinations will not produce such a product. If solutions of sodium nitrate and ammonium chloride are mixed, no
reaction occurs. One could write a molecular equation showing a double-replacement reaction, but both products,
sodium chloride and ammonium nitrate, are soluble and would remain in the solution as ions. Every ion is a spectator
ion, so there is no net ionic equation.
It is useful to be able to predict when a precipitate will form from a given mixture of ions. To do so, you can use a
set of guidelines called the solubility rules (Table16.4).
TABLE16.4: Solubility Rules for Ionic Compounds in Water
Solubility Ionic Compound
Soluble Compounds containing the alkali metal ions (Li+, Na+,
K+, Rb+, Cs+) or the ammonium ion (NH 4 +)
Soluble Compounds containing the nitrate ion (NO 3 −), acetate
ion (CH 3 COO−), chlorate ion (ClO 3 −), or bicarbonate
ion (HCO 3 −)
Mostly soluble Compounds containing the chloride ion (Cl−), bromide
ion (Br−), or iodide ion (I−)
Exceptions are those compounds that also contain sil-
ver (Ag+), mercury(I) (Hg 22 +), or lead(II) (Pb^2 +)
Mostly soluble Compounds containing the sulfate ion (SO 42 −)
Exceptions are the sulfate salts of silver (Ag+), calcium
(Ca^2 +), strontium (Sr^2 +), barium (Ba^2 +), mercury(I)
(Hg 22 +), or lead(II) (Pb^2 +) ions
Mostly insoluble Compounds containing the carbonate ion (CO 32 −),
phosphate ion (PO 43 −), chromate ion (CrO 42 −), sulfide
ion (S^2 −), or silicate ion (SiO 32 −)
Exceptions are those compounds that also contain the
alkali metals or ammonium
Mostly insoluble Compounds containing the hydroxide ion (OH−)
Exceptions are hydroxide salts of the alkali metals and
the barium ion (Ba^2 +)As an example of how to use the solubility rules, predict if a precipitate will form when solutions of cesium bromide
and lead(II) nitrate are mixed.
Cs+(aq)+Br−(aq)+Pb^2 +(aq)+2NO− 3 (aq)→?The potential precipitates from a double-replacement reaction are cesium nitrate and lead(II) bromide. According
to the solubility rules table, cesium nitrate is soluble because all compounds containing the nitrate ion, as well as all
compounds containing the alkali metal ions, are soluble. Most compounds containing the bromide ion are soluble,
but lead(II) is an exception. Therefore, the cesium and nitrate ions are spectator ions and the lead(II) bromide is a
precipitate. The balanced net ionic reaction is:
Pb^2 +(aq)+2Br−(aq)→PbBr 2 (s)