1. Properties of Solutions

2. Solution—a homogeneous mixture of two or more substances in a single phase.
Does not have to involve liquids. Air is a solution of nitrogen, oxygen, carbon dioxide
etc.;
solute—component in lesser concentration; dissolvee
solvent—component in greater concentration; dissolver
solubility—maximum amount of material that will dissolve in a given amount of
solvent at a given temperature to produce a stable solution. In other words, the solution
is saturated.
Saturated solution— a solution containing the maximum amount of solute that will
dissolve under a given set of pressure and temperature conditions. Saturated solutions
are at dynamic equilibrium with any excess undissolved solute present. Solute particles
dissolve and recrystallize at equal rates.
Unsaturated solution— a solution containing less than the maximum amount of
solute that will dissolve under a given set of conditions. (more solute can dissolve)

3. Supersaturated solution—oxymoron—a solution that has been prepared at an
elevated temperature and then slowly cooled. It contains more than the usual
maximum amount of solution dissolved. A supersaturated solution is very
unstable and agitation (stirring, pouring, etc.) or the addition of a “seed crystal’ will
cause all excess solute to crystallize out of solution leaving the remaining
solvent saturated. (rock candy is made this way as are those liquid hand warmer
packets of sodium acetate solution, pictured at left, folks use hunting, at football
miscible—When two or more liquids mix (ex. Water and food coloring)
immiscible—When two or more liquids DON’T mix.—they usually layer if allowed to
set for a while. (ex. Water and oil)

4. Solutions
• Solutions are homogeneous mixtures of two or
more pure substances.
• In a solution, the solute is dispersed uniformly
throughout the solvent.

5. Solutions
How does a solid dissolve
into a liquid?
What ‘drives’ the dissolution
process?
What are the energetics of
dissolution?

6. How Does a Solution Form?
1. Solvent molecules attracted to surface ions.
2. Each ion is surrounded by solvent molecules.
3. Enthalpy (∆H) changes with each interaction broken or
formed.
Ionic solid dissolving in water

7. How Does a Solution Form?
1. Solvent molecules attracted to surface ions.
2. Each ion is surrounded by solvent molecules.
3. Enthalpy (∆H) changes with each interaction broken or
formed.

8. How Does a Solution Form
The ions are solvated
(surrounded by
solvent).
If the solvent is water,
the ions are hydrated.
The intermolecular force
here is ion-dipole.

9. Energy Changes in Solution
To determine the enthalpy
change, we divide the
process into 3 steps.
1. Separation of solute
particles.
2. Separation of solvent
particles to make ‘holes’.
3. Formation of new
interactions between
solute and solvent.

11. Enthalpy Is Only Part of the Picture
Entropy is a measure of:
• Dispersal of energy in the
system.
• Number of microstates
(arrangements) in the
system.
b. has greater entropy,
∴ is the favored state

12. Entropy changes during dissolution
Each step also involves a
change in entropy.
1. Separation of solute
particles.
2. Separation of solvent
particles to make ‘holes’.
3. Formation of new
interactions between
solute and solvent.

13. Dissolution vs reaction
• Dissolution is a physical change—you can get back the original
solute by evaporating the solvent.
• If you can’t, the substance didn’t dissolve, it reacted.
Ni(s) + HCl(aq) NiCl2(aq) + H2(g) NiCl2(s)
dry

14. Degree of saturation
• Saturated solution
Solvent holds as much
solute as is possible at
that temperature.
Undissolved solid
remains in flask.
Dissolved solute is in
dynamic equilibrium
with solid solute
particles.

15. Degree of saturation
• Unsaturated Solution
Less than the maximum
amount of solute for that
temperature is dissolved
in the solvent.
No solid remains in flask.

16. Degree of saturation
• Supersaturated
Solvent holds more solute than is normally possible at
that temperature.
These solutions are unstable; crystallization can often
be stimulated by adding a “seed crystal” or scratching
the side of the flask.

17. Degree of saturation
Unsaturated, Saturated or Supersaturated?
⇒ How much solute can be dissolved in a solution?

18. Factors Affecting Solubility
• Chemists use the axiom
“like dissolves like”:
Polar substances tend to
dissolve in polar solvents.
Nonpolar substances tend to
dissolve in nonpolar
solvents.

19. Factors Affecting Solubility
Glucose (which has
hydrogen bonding) is
very soluble in water.
Cyclohexane (which only
has dispersion forces)
is not water-soluble.

20. Factors Affecting Solubility
• Vitamin A is soluble in nonpolar compounds (like
fats).
• Vitamin C is soluble in water.

21. Gases in Solution
• In general, the solubility
of gases in water
increases with
increasing mass.
Why?
• Larger molecules have
stronger dispersion
forces.

22. Gases in Solution
• The solubility of
liquids and solids
does not change
appreciably with
pressure.
• But, the solubility of a
gas in a liquid is
directly proportional
to its pressure.
Increasing
pressure above
solution forces
more gas to
dissolve.

23. Temperature
Generally, the
solubility of solid
solutes in liquid
solvents increases
with increasing
temperature.

24. Temperature
• The opposite is true of
gases. Higher
temperature drives gases
out of solution.
Carbonated soft drinks
are more “bubbly” if
stored in the refrigerator.
Warm lakes have less O2
dissolved in them than
cool lakes.