Factors That Affect Solubility
and Rate of Dissolving
Factors That
Affect Rate of Dissolving
• Temperature, more specifically . . .
as T ↑, rate of dissolving ↑.
• Agitation
Shaking, stirring
• Surface area of solids.
How would you use this information to
prepare orange juice from the frozen
concentrate as expeditiously as possible?
• defrost (liquefy) the concentrate;
• shake or stir when water added.
Review of Intermolecular Forces
What Does Dissolving Mean
at the Molecular Level?
• Dissolving is related to solute-solvent
attraction.
• When attractive forces between different
particles in mixture are greater than the
attractive forces between like particles …
a solution forms.
• Strength of attraction governs
the amount of solute that dissolves.
The Process of Dissolving
1. Intermolecular forces between solute
particles need to be broken—true for
both ionic and molecular solutes. This is
endothermic.
2. Some intermolecular forces in solvent
(liquid) also need to be broken. This is
endothermic.
3. Attraction between solute and solvent
particles. This is
exothermic.
NaCl dissolving in water
What simple rule governs solubilty?
• Like dissolves like
• Polar dissolves polar (and
ionic).
eg. CH3OH (methanol) dissolves in
H2O.
NaCl dissolves in
water.
Name something polar that doesn’t dissolve in
water.
A polar bear.
Take Note
• Not all ionic compounds are water soluble.
Some ionic compounds—with small, highcharge ions—have very strong lattice forces.
This explains poor solubility of some ionic
compounds.
NaCl(aq) + AgNO3(aq)  AgCl(s) + NaNO3(aq)
NB. Binary ionic compounds with a small
electronegativity difference tend not to be
water soluble.
eg. HgS, mercury (II) sulfide
↑ ↑
1.9 2.5  electronegativity values
∆eneg = 0.6  very small; HgS likely not
water soluble.
eg. NaCl, ∆eneg = 2.1  large, NaCl soluble
How to tell if an ionic compound is water
soluble?
Refer to a solubility chart.
• Non-polar solutes dissolve in non-polar
solvents.
eg. I2 dissolves in hexane, CH3(CH2)4CH3.
Which of the following
is/are polar molecules?
Solubility of Alcohols in H2O
CH3OH, miscible in all proportions
CH3CH2OH, miscible in all proportions
CH3CH2CH2OH, miscible in all proportions
CH3(CH2)3OH, reduced solubility
CH3(CH2)4OH, very reduced solubility
CH3(CH2)5OH, very,very reduced solubility
CH3(CH2)7OH, very, very,very reduced sol.
. . . You get the idea . . .
With water, the –OH group in alcohols can
form
hydrogen bonds.
The hydrocarbon chain in an alcohol is
non-polar.
As the length of the non-polar hydrocarbon
chain increases, water solubility of the
alcohol
decreases.
Effect of Temperature on Solubility
• For most solids in water: As T ↑,
solubility ↑
• Solubility of most liquids is not greatly
affected by temperature. Why?
The liquid-liquid intermolecular forces are
not as strong as the intermolecular forces
between solid solute
particles with the solvent.
Solubility of Gases in Liquid
Solubility of gas in liquid
α 1/T
- or –
As T of liquid ↑, solubility of gas ↓
Heat Pollution of Water
Many industries return warm water to a river
or lake.
What effect would this have?
Dissolved O2 ↓;
marine life can suffer.
At which temperature will you get a fizzier
glass of pop—cold or warm?
Warm—why?
Solubility of CO2 ↓ as T ↑
Solubility of Gas and Pressure
As P of a gas above a liquid ↑, solubility
of gas ↑.
A potential problem for SCUBA divers is
narcosis, aka
“the bends.”
N2 dissolves in blood at high P under
surface of water.
If diver ascends too fast, N2 bubbles out of
the blood, much like CO2 in pop.
Can be painful—sometimes fatal.
Review
• Factors that Affect Rate of Dissolving:
temperature
agitation,
particle size.
Also:
• Like dissolves like
• Forces between solute and solvent
particles
• Solubility of gas in liquid (T and P)
HW
P 370 RQs #2, 3, 4, 5, 6, 8, 9, 10, 11, 12,
13, 14, 15, 16