Saturated solutions and solubility
What happens when a solid solute starts to
dissolve in a solvent?
Two processes occur:
Dissolution, e.g,., NaCl(s)  Na+(aq) + Cl-(aq)
Crystallization, e.g., Na+(aq) + Cl-(aq)  NaCl(s)
When these opposing processes occur at equal
rates, the system is at equilibrium
A solution which is in equilibrium with
undissolved solute is said to be saturated
The amount of solute required to form a saturated
solution for a given quantity of solvent is known
as the solubility of the solute
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E.g., the solubility of NaCl in water at 0oC is 35.7
g/100 mL of water
What is the solubility of NaCl in 200 mL water
at 0 C?
o
We can also have unsaturated & supersaturated
solutions
What factors determine the extent to which one
substance dissolves in another?
Factors affecting solubility
Solvent-solute interactions
Pressure
Temperature
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1. Solute - solvent interactions
polar liquids tend to dissolve readily in
polar solvents
E.g., acetone is miscible in H2O; what types of
interactions are possible?
Remember: Like dissolves like
E.g., which member of each of the following pairs
is the more soluble in H2O? Why?
CH3CH3 or CH3OH
CCl4 or CaCl2
CH3CH2OH or
CH3CH2CH2OH
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2. Pressure effects
Solubility of a gas in any solvent increases as the
pressure of the gas over the solvent increases
(why?)
Relationship between gas pressure and solubility
is expressed by Henry's Law,
Cg = kPg
Where
Cg = solubility of gas in solution phase
Pg = partial pressure of gas
k = Henry's Law constant
Notice that solubility is directly proportional to
pressure!
What are the implications of this??
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3. Temperature effects
Solubility of gases decreases with increasing
temperature (why?)
Solubility of ionic compounds (e.g., CaCl2)
increases with increasing temperature (Fig 13.7)
Now: in order to discuss quantitative aspects of
solutions (e.g., vapor pressure, osmotic pressure,
etc.), we need to be able to specify the
composition of a solution in terms of its solute
concentration.......
A detour is in order......
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