Figure 4-5 A precipitation reaction.
When K 2 CrO 4 solution is added to
aqueous Pb(NO 3 ) 2 solution, the yellow
compound PbCrO 4 precipitates. The
resulting solution contains Kand
NO 3 ions, the ions of KNO 3.and potassium chromate (Figure 4-5). The other product of the reaction is KNO 3 , a
soluble ionic salt.
The balanced formula unit, total ionic, and net ionic equations for this reaction follow.
Pb(NO 3 ) 2 (aq)K 2 CrO 4 (aq)88nPbCrO 4 (s)2KNO 3 (aq)[Pb^2 (aq)2 NO 3 (aq)][2K(aq)CrO 42 (aq)]88n
PbCrO 4 (s)2[K(aq)NO 3 (aq)]
Pb^2 (aq)CrO 42 (aq)88nPbCrO 4 (s)
Another important precipitation reaction involves the formation of insoluble carbon-
ates (solubility guideline 7). Limestone deposits are mostly calcium carbonate, CaCO 3 ,
although many also contain significant amounts of magnesium carbonate, MgCO 3.
Suppose we mix together aqueous solutions of sodium carbonate, Na 2 CO 3 , and calcium
chloride, CaCl 2. We recognize that bothNa 2 CO 3 and CaCl 2 (solubility guidelines 2, 4a,
and 7) are soluble ionic compounds. At the instant of mixing, the resulting solution contains
four ions:
Na(aq), CO 32 (aq), Ca^2 (aq), Cl(aq)One pair of ions, Naand Cl, cannotform an insoluble compound (solubility guidelines
2 and 4). We look for a pair of ions that could form an insoluble compound. Ca^2 ions
and CO 32 ions are such a combination; they form insoluble CaCO 3 (solubility guideline
7). The equations for the reaction follow.
CaCl 2 (aq)Na 2 CO 3 (aq)88nCaCO 3 (s)2 NaCl(aq)[Ca^2 (aq)2 Cl(aq)][2Na(aq)CO 32 (aq)]88n
CaCO 3 (s)2[Na(aq)Cl(aq)]
Ca^2 (aq)CO 32 (aq)88nCaCO 3 (s)
Seashells, which are formed in very
slow precipitation reactions, are
mostly calcium carbonate (CaCO 3 ),
a white compound. Traces of
transition metal ions give them
color.