5 Solubility Guidelines for Compounds in Aqueous Solution
Solubility is a complex phenomenon, and it is not possible to give a complete summary
of all of our observations. The following brief summary for solutes in aqueous solutions
will be very useful. These generalizations are often called the solubility guidelines.
Compounds whose solubility in water is less than about 0.02 mol/L are usually classified
as insoluble compounds, whereas those that are more soluble are classified as soluble
compounds. No gaseous or solid substances are infinitely soluble in water. You may wish
to review Tables 2-3 (page 55), 4-5, and 4-6. They list some common ions. Table 4-15 on
page 164 contains a more comprehensive list.1.The common inorganic acids are soluble in water. Low-molecular-weight organic
acids are also soluble.
2.All common compounds of the Group IA metal ions (Li, Na, K, Rb, Cs)
and the ammonium ion, NH 4 , are soluble in water.
3.The common nitrates, NO 3 ; acetates, CH 3 COO; chlorates, ClO 3 ; and perchlo-
rates, ClO 4 , are soluble in water.
4.(a) The common chlorides, Cl, are soluble in water except AgCl, Hg 2 Cl 2 , and
PbCl 2.
(b) The common bromides, Br, and iodides, I, show approximately the same
solubility behavior as chlorides, but there are some exceptions. As these halide
ions (Cl, Br, I) increase in size, the solubilities of their slightly soluble
compounds decrease.
(c) The common fluorides, F, are soluble in water except MgF 2 , CaF 2 , SrF 2 , BaF 2 ,
and PbF 2.
5.The common sulfates, SO 42 , are soluble in water except PbSO 4 , BaSO 4 , and
HgSO 4 ; CaSO 4 , SrSO 4 , and Ag 2 SO 4 are moderately soluble.
6.The common metal hydroxides, OH, are insolublein water except those of the
Group IA metals and the heavier members of the Group IIA metals, beginning with
Ca(OH) 2.
7.The common carbonates, CO 32 , phosphates, PO 43 , and arsenates, AsO 43 , are
insolublein water except those of the Group IA metals and NH 4 . MgCO 3 is moder-
ately soluble.
8.The common sulfides, S^2 , are insolublein water except those of the Group IA and
Group IIA metals and the ammonium ion.Table 4-8 summarizes much of the information about the solubility guidelines.
We have distinguished between strong and weak electrolytes and between soluble and
insoluble compounds. Let us now see how we can describe chemical reactions in aqueous
solutions.REACTIONS IN AQUEOUS SOLUTIONS
Many important chemical reactions occur in aqueous solutions. In this chapter you should
learn to describe such aqueous reactions and to predict the products of many reactions.
Let us first look at how we write chemical equations that describe reactions in aqueous
solutions. We use three kinds of chemical equations. Table 4-9 shows the kinds of infor-
mation about each substance that we use in writing equations for reactions in aqueous4-3
There is no sharp dividing line
between “soluble” and “insoluble”
compounds. Compounds whose
solubilities fall near the arbitrary
dividing line are called “moderately
soluble” compounds.
134 CHAPTER 4: Some Types of Chemical Reactions
The White Cliffs of Dover,
England, are composed mainly
of calcium carbonate (CaCO 3 ).