Unit 4 Solubility and Solutions
4.1.1—4.1.3
 Assignment #1- finding definitions
4.1.2 General Guidelines Concerning
Solubility
 If you add oil to water, what happens? Not much -
the two don't mix, even if shaken. Why can you clean
some paints with water, but others require a much
different type of cleaning solvent? Why do some
substances readily form solutions but others don't?
 There is a general saying that "like dissolves like“.
This refers to the type of bonding between molecules,
if the bonding between molecules of the substance to
be dissolved is similar to the bonding between
solvent molecules, there is a good chance that the
substance will dissolve.
 Time for a quick review of chemical bonding.
Intramolecular Bonding
 Intramolecular bonding refers to the chemical bonding
that holds atoms together within a compound.
Covalent bonding involves the sharing of electrons
between two non-metal atoms. For example, hydrogen
and oxygen atoms form a water molecule, H2O, using
covalent bonding.
2. Ionic bonding involves the transfer of electrons
from a metal atom to a non-metal. One atom loses
electrons, forming a positive cation, while the other
atom gains electrons to form a negative anion. An
example is sodium chloride, NaCl
1.
Intermolecular Bonding
 Intermolecular bonding, is what holds two or more
separate molecules together in the solid and liquid
phases. What type of intermolecular bonding is
involved largely depends on two main factors:
 whether the bonds within a single molecular are
polar or not, and
 the overall shape of the molecule
 A polar molecule will have one end of the molecule
bearing a partial positive charge while another end
carries a partial negative charge. Polar molecules
must contain polar bonds.
Like Dissolves Like
 Often, nonpolar substances dissolve best in nonpolar
solvents, while polar substances dissolve best in
polar solvents.
4.1.3 Factors Affecting Solubility
 Imagine pouring salt into a glass of water. Eventually no
more salt will dissolve, and any more you add will simply
sit on the bottom. Whenever a solute is dissolved in a
liquid, a point is reached in which no more solute will
dissolve. This is the point of saturation.
 Saturation is an equilibrium system, which you'll
remember is a dynamic state in which both the forward
and reverse reactions continue but at equal rates. We can
write a general equation that illustrates this:
solute (s, l, g)
solute (aq)
 Because a saturated solution is at equilibrium, the
same factors that affect a system at equilibrium will
affect our saturated system. The two key factors to
consider are:
 the effect of temperature and
 the effect of pressure on the solubility of gases.
The Effect of Temperature on Solubility
 If you want to get more salt to dissolve in our glass of
water, what would you do? If you heated the water
you would be able to dissolve more salt.
 Generally, increasing the temperature will increase
solubility of solids and liquids. But increasing
temperature will lower the solubility of gases
 We can explain this using Le Châtelier's Principle. The
dissolving process in the solid and liquid form is an
endothermic process:
 solid or liquid + heat
aqueous
 Adding heat will favor the endothermic direction; in this case
favoring the product (or aqueous) side of the reaction.
 However, the dissolving process of gases is exothermic:
 Gas
aqueous + heat
 Adding heat to this system will again favor the endothermic
reaction, which in this case is the reverse reaction.
The Effect of Pressure on the Solubility of
Gases
 The solubility of gases increases when the pressure
above the gas is increased; i.e more gas will dissolve
when pressure is increased.
 This can be explained by Le Châtelier's Principle. As
our example we will examine a can of coke
 Carbonated beverages contain carbon dioxide which
is dissolved under pressure. Our equilibrium system
is:
CO2 (g)
CO2 (aq)
CO2 (g)
CO2 (aq)
 On the reactant side is one mole of gas; on the
product side are zero moles of gas. Increasing
pressure will favor the side with the fewest moles of
gas
 When you open a can of pop, you increase the
volume and decrease the pressure. According to Le
Châtelier's Principle, this will favor the side with the
most moles of gas. And that's what happens - when
you open a can of pop it begins to fizz - this is the
CO2 coming out of solution. The "fizz" are the gas
bubbles forming and popping
 Assignment 4.1.3