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Aqueous Solutions 51
4-2 Solubility and Precipitation
Don’t Forget!
Be Careful!
Many of the reactions that you will study occur in aqueous solution. Water
readily dissolves many ionic compounds as well as some covalent compounds.
Ionic compounds that dissolve in water (dissociate) form electrolyte solutions—
solutions that conduct electrical current due to the presence of ions. We may
classify electrolytes as either strong or weak. Strong electrolytes dissociate
(break apart or ionize) completely in solution, while weak electrolytes only partially
dissociate. Even though many ionic compounds dissolve in water, many
do not. If the attraction of the oppositely charged ions in the solid is greater
than the attraction of the water molecules to the ions, then the salt will not dissolve
to an appreciable amount.
Compounds like alcohols are nonelectrolytes—substances than do not conduct
an electrical current when dissolved in water. However, certain covalent compounds,
like acids, will ionize in water, that is, form ions:
HCl(aq) l H (aq) Cl (aq)
Very few molecular compounds, other than acids, are electrolytes. Molecular
compounds, as introduced in Chapter 2, are generally compounds composed
entirely of nonmetals.
Precipitation reactions involve the formation of an insoluble compound, a precipitate,
from the mixing of two aqueous solutions containing soluble compounds. To
predict if precipitation will occur upon the mixing of two solutions, you must know
and be able to apply the following solubility rules. You should apply these rules to
all combination of cations with anions in each of the mixed solutions.
Be sure, however, that you do not try to break apart molecular species such as
organic compounds.