Solutions make up a large, important part of chemistry.
Usually solutions are
considered in terms of the ability of liquids to dissolve solids, other
liquids, or gases,
forming a homogeneous mixture. However, solutions can be solids in
solids, liquids
in solids, gases in solids, solids in liquids, liquids in liquids,
gases in liquids, and
gases in gases. The major component of a solution is termed the solvent
and the other
component(s) is(are) termed the solute(s). In some cases these terms
are arbitrary.
A somewhat arbitrary distinction is often made between homogeneous
solutions and
colloids, which are solid, liquid, or gaseous particles (made up of
particles large
enough to diffract or scatter light—the Tyndall effect) dispersed in
solid, liquid, or
gaseous media. The dispersed particles are called colloids. The thermal
motion
(Brownian movement) of such small particles is sufficient to keep them
from settling
out in the earth’s gravitational field.
“Like dissolves like” is an expression that qualitatively
expresses the experimental
observations that polar and ionic substances dissolve in polar solvents
and nonpolar
substances dissolve in nonpolar solvents—other things being equal.
A solvation
model that considers interactions between polar solvent molecules and
polar and
ionic solutes is helpful in considering the solvation properties of
water and other
polar solvents. However, some aqueous solutions, and most solutions
involving nonpolar
solvents, depend on the randomness (entropy increase) obtained in the
solution process.
Solution concentrations (e.g., mol/L) should be distinguished from
amounts (e.g., moles).
Several methods for expressing solution concentrations are important—reaction
stoichiometry in solution is normally expressed in terms of the molarity
(M) of the
reacting species, whereas colligative properties are compared using
either molality (m)
or mole fractions (X), and impurities are often quoted in parts per
million (ppm) or parts
per billion (ppb). Relative concentrations are often expressed as dilute
or concentrated,
or by the terms unsaturated, saturated or supersaturated.
Solution components can be detected and evaluated
by a variety of methods and can
often be separated by distillation, crystallization, or chromatography.
Colligative
properties are useful in determining the nature of solutes in solutions.
These
properties together with conductivity have allowed scientists to quantify
electrolytes
(ionic solutes) in solutions. Solutes in water are often categorized
as strong electrolytes,
weak electrolytes, and non-electrolytes, reflecting the conductivity
of the substance
when dissolved.
2. Various types of solutions are possible, based on whether the intermixed
species are solids, liquids, or gases (Demonstration 1).
.3. A solution is a homogeneous mixture of two or more substances where
the
solvent and solute particles are of typical molecular dimensions (i.e.,
< 1 nm).
A mixture made up of larger particles is considered a colloidal suspension,
which is observable via the Tyndall effect (Demonstration 2).
4. Like dissolves like, in general. Nonpolar solvents tend to dissolve
nonpolar
substances and polar solvents tend to dissolve polar and ionic substances.
5. A microscale model of the solution process within a liquid solvent
can be
constructed based on the principle of solvation.
6. Concentration (e.g., mol/L) should be distinguished from amount (i.e.,Êmoles).
7. Common quantitative methods for expressing solution concentration
include
percent by mass, percent by volume, molality (m), molarity (M), mole
fraction
(X), and parts per million (ppm).
8. Stoichiometry in solutions is conveniently based on molar concentrations
(M) of the reacting species (Laboratory Activity 1).
9. Relative concentrations of a solute/solvent system can often be expressed
by
the terms dilute and concentrated, or by the terms unsaturated, saturated,
and supersaturated.
10. A solution can be separated into its components through processes
such as
distillation, crystallization, and chromatography (Demonstration 3).
11. Colligative properties of solutions include vapor pressure lowering,
boiling
point elevation, freezing point depression, and osmotic pressure (Laboratory
Activity 2).
12. Solutes in water are often categorized as either electrolytes (strong,
if completely
ionized in water or weak, if only partially ionized) or non-electrolytes
(nonionized).
13. Generally, gases become less soluble in liquid solvents as the temperature
is
increased. On the other hand, most (but not all) solids and liquids
become
more soluble in liquid solvents as the temperature is increased.
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