8.8: Limits of Solubility
• When we dissolve a
solid in some liquid,
what is actually
happening?
– Take glucose added to
water
• H-bonding between water
molecules and glucose
molecules pulls the
individual molecules into
solution
• H-binding and Van der
Waals interactions keep
the glucose held together
as a solid
Solubility: Saturated Solutions
When we keep adding solid
to a liquid, the solid will
dissolve until…
The solid is in equilibrium
with the dissolved solute in
a saturated solution
Solubility
The molar solubility, s, of a substance is
it molar concentration in a saturated
solution
A saturated solution is one in which the
dissolved and undissolved solute are in
equilibrium
8.9: Like Dissolves Like
• What is a good solvent to remove butter
from your shirt? Soda from your
carpet?
• We want to use a solvent that has
similar chemical properties because of
the “Like Dissolves Like” rule
Like Dissolves Like
• It all comes down to intermolecular forces…
• When we dissolve something, we replace
some of the specific intermolecular forces in
the solution with the same kind of interactions
with the solute.
• Water (polar, hydrogen bonding solvent) will
work well to dissolve polar or ionic solutes
• Fatty stains like gravy or butter can be
removed with organic solvents like hexane or
dichloroethene
Soap
• Soap is made by treating fats with strong
hydroxide bases
– In the resulting chemical reactions, the fatty acids
are knocked off and have a polar head and an acyl
tail
• Soap molecules have hydrophobic parts (the
tails) and hydrophilic parts (the head)
Summary: Solutions
1. For most ionic substances, dissolution
increases the entropy of the system and
they dissolve more readily at higher
temperatures (since the free energy is
dependent on TS)
2. For gas molecules, the entropy decreases
when the temperature rises due to the
contribution of Ssurr (solvent)
3. Dissolution depends on the balance
between the change in entropy of the
solution and the change in entropy of the
surroundings.
Boiling Point Elevation and Freezing Point Depression
• The presence of a solute will increase the boiling
point of the solvent due to the decrease in Gibbs
Free Energy
This is one reason
why people put
salt into cooking
water: It makes
the temperature
higher
However: In
reality, this
elevation is
extremely low.
Freezing Point Depression
• The presence of the solute molecules in the solution
decreases the freezing point of the solvent relative to
its normal value
It does so because
of the increased
solution entropy
caused by the
presence of the
solute.
Boiling Point Elevation and Freezing Point
Depression
Freezing Point Depression = kf(molality)
Boiling Point Elevation = kb(molality)

8.17: Osmosis
• We’ve all heard of osmosis, but what is it really?
Osmosis is the flow of solvent through a semipermeable membrane from a region of low solute
concentration to one of high solute concentration
• In terms of entropy, this just makes sense, right?
Osmosis: Examples
Red Blood Cells in
a)
Normal Cells
b)
Dilute Solution
c)
Concentrated Solution
Osmosis in the Laboratory
Osmosis: The Science
• If we set up an apparatus with a semipermeable membrane on one end, filled
it with a sugar solution, and then placed
it into pure water, what would happen?
Osmosis: The Science
i = van’t Hoff i factor
 = iRTC
(determined experimentally and related to ion
charge)
R = Gas constant
T = Temperature
C = Molar concentration
• We can use this equation to determine
the molar mass of a solute from
osmotic pressure measurements
