Dr. Saidane
Chem. 152
Solutions
Skills you should have mastered
Conceptual
1. Distinguish between a solute and a solvent in a solution.
2. Explain the basis for the like-dissolves-like rule.
3. Identify three factors that affect the solubility of various substances.
4. Explain the differences among saturated, unsaturated, and supersaturated
solutions in terms of their composition.
5. List three different ways of expressing the concentration of solutions.
Problem-solving
1. Calculate the concentration of a solution as a percent by mass.
2. Calculate the concentration of a solution in terms of molarity.
3. Convert among different expressions for the concentration.
4. Determine the volume of solution needed to prepare a dilute solution of a given
molarity.
Descriptive
1. Describe the most common combinations of different states of matter to form a
solution.
A solution is a homogeneous mixture of two or more substances in a single phase.
In a solution, atoms, molecules, or ions are thoroughly mixed, resulting in a mixture that
has the same composition and properties throughout.
Components of Solutions
The dissolving medium in a solution is called a solvent and the substance dissolved in a
solution is called the solute.
Types of Solutions
Solutions can be solids, liquids, or gases.

Examples of solid solutions are alloys.

Any mixture of gas is a gaseous solution.

Liquid solutions can be aqueous or organic. An example of aqueous solution is saltwater; an example of organic solution is gasoline, which is a mixture of
hydrocarbons.
In this chapter, we will focus largely on aqueous solutions, but most of this material
apply to non-aqueous solutions.
Molecular Nature of Dissolving

Solids dissolve in water when individual molecules or ions are attracted to water
molecules and break away from the solid.

When dissolved, the molecules are surrounded by water and held in suspension by
interactions between the solute and the solvent.
Solubility

A given quantity of solvent can dissolve only so much solute. A solution is saturated
when the solvent has dissolved all the solute it can and some undissolved solute
remains.

The concentration of solid solute in a saturated solution has reached its greatest value
and no more can dissolve. In other words, a saturated solution represents the limit of
a solute’s ability to dissolve in a given quantity of solvent.

A solution that contains less solute than a saturated solution under the existing
conditions is an unsaturated solution.

A supersaturated solution is a solution that contains more dissolved solute than a
saturated solution under the same conditions. A supersaturated solution may remain
unchanged for a long time if it is not disturbed, but once it is disturbed, crystals start
to form until the solution becomes saturated.
FACTORS AFFECTING SOLUBILITY
The solubility of a substance in a given solvent depends on a number of factors. The
solubilities of all substances vary with temperature.
Solute-Solvent Interactions

The solubility of a substance depends on the choice of solvent as well as on the
substance itself. The rule is “like-dissolves-like”. This means that a polar solvent
dissolves an polar solute and a non-polar solvent dissolves a non-polar solute. This is
explained as follows: The interactions between solute molecules and the interactions
between solvent molecules must be replaced by solute-solvent interactions when a
solution forms. If the new interactions are similar to those replaced, very little energy
is required for the solution to form and the dissolution is favorable.

Soaps and detergents are a practical application of the “like-dissolves-like” rule.
Soaps are the sodium salts with anions of a carboxylic acid. Soaps have a polar head
at one end and a non-polar hydrocarbon chain on the other end. Because of its polar
character, the head group is hydrophilic, or water attracting, whereas the non-polar
hydrocarbon tail is hydrophobic, or water repelling. The hydrophobic tails dissolve
in grease, leaving the hydrophilic head dissolved in water. As a result, the whole
soap with the grease trapped in it dissolves in water and is washed away when rinsed.
Solubility of Ionic Compounds

We can predict whether a given ionic solid is likely to dissolve in water by using the
solubility rules.

In order to dissolve an ionic compound, the ion-dipole interaction must be greater
than the ion-ion interaction. This happens when ionic compounds are formed from
large anions with low charges. Ionic compounds with large anions with low charges
are often soluble.
Temperature and Solubility

The variation of solubility with temperature is difficult to predict, except in the case
of gases, where raising the temperature nearly always lowers the solubility.

Most ionic and molecular compounds are more soluble in hot water than in cold.
However, a few solids, such as lithium sulfate, are less soluble at high temperatures
than at low. A smaller number of compounds show a mixed behavior.
EXPRESSING CONCENTRATION
The amount of solute in a solvent, also called concentration, can be expressed in different
ways as shown in the following table:
Concentration
Definition
Units
Molarity
Moles of solutes per liter of solution
mol/L or
M
Mass
Mass of solute expressed as a
percentage
percentage of a total mass
Solubility
Mass of solute in 100 g of water
%
g/mL
(100 mL)
Molarity
The molarity, M, of a solution is the number of moles of solute molecules or
formula units divided by the volume of the solution (in liters).
Molarity = number of moles of solute / volume of solution (liters)
Molarity can be used to calculate the number of moles, n, of a solute in a solution or the
volume, V, of a solution.
n=MxV
or
V=n/M
Dilution
Diluting a solution is the reduction of the molarity from an initial value by adding
more solvent. When a solution is diluted, the same number of solute molecules occupies
a larger volume.
The amount of solute in the final solution is the same as the amount of solute in the initial
volume of solution.
ninitial = nfinal  Minitial x Vinitial = Mfinal x V final