Solutions include different combinations in which a solid, liquid, or gas acts as either solvent
or solute. Usually the solvent is a liquid. For instance, sea water is an aqueous solution of many
salts and some gases such as carbon dioxide and oxygen. Carbonated water is a saturated solu-
tion of carbon dioxide in water. Solutions are common in nature and are extremely important
in all life processes, in all scientific areas, and in many industrial processes. The body fluids of
all forms of life are solutions. Variations in concentrations of our bodily fluids, especially those
of blood and urine, give physicians valuable clues about a person’s health. Solutions in which
the solvent is not a liquid are also common. Air is a solution of gases with variablecomposi-
tion. Dental fillings are solid amalgams, or solutions of liquid mercury dissolved in solid
metals. Alloys are solid solutions of solids dissolved in a metal.
It is usually obvious which of the components of a solution is the solvent and which is (are)
the solute(s): The solvent is usually the most abundant species present. In a cup of instant
coffee, the coffee and any added sugar are considered solutes, and the hot water is the solvent.
If we mix 10 grams of alcohol with 90 grams of water, alcohol is the solute. If we mix 10 grams
of water with 90 grams of alcohol, water is the solute. But which is the solute and which is the
solvent in a solution of 50 grams of water and 50 grams of alcohol? In such cases, the termi-
nology is arbitrary and, in fact, unimportant.
THE DISSOLUTION PROCESS
SPONTANEITY OF THE DISSOLUTION PROCESS
In Section 4-2, part 5, we listed the solubility guidelines for aqueous solutions. Now we
investigate the major factors that influence solubility.
A substance may dissolve with or without reaction with the solvent. For example, when
metallic sodium reacts with water, there is the evolution of bubbles of hydrogen and a
great deal of heat. A chemical change occurs in which H 2 and soluble ionic sodium
hydroxide, NaOH, are produced.
2Na(s)2H 2 O88n2Na(aq)2OH(aq)H 2 (g)If the resulting solution is evaporated to dryness, solid sodium hydroxide, NaOH, is
obtained rather than metallic sodium. This, along with the production of bubbles of
hydrogen, is evidence of a reaction with the solvent. Reactions that involve oxidation state
changes are usually considered as chemical reactions and not as dissolution.
Solid sodium chloride, NaCl, on the other hand, dissolves in water with no evidence
of chemical reaction.
H 2 O
NaCl(s)8888nNa(aq)Cl(aq)Evaporation of the water from the sodium chloride solution yields the original NaCl. In
this chapter we focus on dissolution processes of this type, in which no irreversible reac-
tion occurs between components.
The ease of dissolution of a solute depends on two factors: (1) the change in energy
and (2) the change in disorder (called entropy change) that accompanies the process. In
the next chapter we will study both of these factors in detail for many kinds of physical
and chemical changes. For now, we point out that a process is favoredby (1) a decrease in
the energyof the system, which corresponds to an exothermic process,and (2) an increase in
the disorder,or randomness, of the system.
14-1
Many naturally occurring fluids
contain particulate matter suspended
in a solution. For example, blood
contains a solution (plasma) with
suspended blood cells. Sea water
contains dissolved substances as well
as suspended solids.14-1 Spontaneity of the Dissolution Process 543Ionic solutes that do not react with the
solvent undergo solvation. This is a
kind of reaction in which molecules
of solvent are attached in oriented
clusters to the solute particles.