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172 CHEMISTRY FOR THE UTTERLY CONFUSED
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The major goal of this chapter is to help you master the concepts associated with
solutions—concentration units, solubility, and especially colligative properties.
We will also examine the properties of colloids. If you are still unsure about calculations
and the mole concept, review Chapters 1, 3, and 4. And again, the only
way to master these concepts is to Practice, Practice, Practice.
12-1 Concentration Units
A solution is a homogeneous mixture composed of solvent and one or more
solutes. The solvent is normally the substance present in the greatest amount. The
solute is the substance that is present in the smaller amount. If water is the solvent,
it is an aqueous solution. You may have more than one solute in a solution.
Some substances will dissolve in a particular solvent and others will not. There
is a general rule in chemistry that states “like dissolves like.” Polar substances
(such as alcohols) will dissolve in polar solvents like water. Nonpolar solutes
(such as iodine) will dissolve in nonpolar solvents such as carbon tetrachloride.
The mass of solute per 100 mL of solvent (g/100 mL) is a common alternative
to expressing the solubility as molarity. It is necessary to specify the temperature
because the solubility of a substance will vary with the temperature. The
solubility of a solid dissolving in a liquid normally increases with increasing
temperature. The reverse is true for a gas dissolving in a liquid.
A solution containing the maximum amount of solute per given amount of solvent
at a given temperature is a saturated solution. An unsaturated solution has
less than that maximum amount of solute dissolved. Sometimes, there may be
more that that maximum amount of solute, resulting in a supersaturated solution.
Supersaturated solutions are unstable and eventually expel the excess
solute, forming a saturated solution.
There are many ways of expressing the relative amounts of solute(s) and solvent
in a particular solution. The terms saturated, unsaturated, and supersaturated
discussed above give a qualitative measure of solubility, as do the terms
dilute and concentrated. Dilute refers to a solution that has a relatively small
amount of solute in comparison to the amount of solvent. Concentrated refers
to a solution that has a relatively large amount of solute in comparison to the
solvent. These terms are very subjective and chemists prefer to use quantitative