Content in a Nutshell

    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.

Place in the Curriculum

This module should follow a discussion of the states of matter, changes of state,
moles, and stoichiometry. It provides a good vehicle for either introducing
or augmenting a discussion of solution stoichiometry.

Central Concepts