Chapter 13 – Solutions
13.1 – The Solution Process
• Know these terms: solution, solvent, solute
• The ability of substances to form solutions depends on
 natural tendency toward mixing.
 intermolecular forces.
• Natural tendency toward mixing: The formation of solutions
is favored by the increase in entropy that accompanies mixing.
• Intermolecular forces: Any of the intermolecular forces of
attraction can be the attraction between solute and solvent
molecules.
• In order for a solution to form, the following must occur
regarding the intermolecular forces:
• 1. Solute–solute interactions must be overcome to
disperse these particles when making a solution.
• 2. Solvent–solvent interactions must be overcome to
make room for the solute.
• 3. Solvent–solute interactions occur as the particles mix.
Energetics of Solution Formation
Enthalpy of Solution (Hsoln) = the sum of 3
enthalpies:
1) the enthalpy to separate solvent molecules
(always positive (endothermic))
2) the enthalpy to separate solute molecules
(always positive (endothermic))
3) the enthalpy of bringing together the solute and
solvent (always negative (exothermic))
Solution vs. Chemical Reaction?
Just because a substance disappears when it comes in contact
with a solvent, it does not mean the substance dissolved. It may
have reacted, like nickel with hydrochloric acid. Since nickel
changed from neutral to an ion, a chemical reaction occurred.
Dissolving does NOT involve a chemical reaction – it is a physical
process.
13.2 – Saturated Solutions and Solubility
• The solution-making process and crystallization are
opposing processes.
• When the rate of the opposing processes is equal (i.e. at
equilibrium), the maximum solute has been dissolved,
and additional solute will not dissolve unless some
crystallizes from solution. This is a saturated solution.
• If we have not yet reached the amount that will result in
crystallization, we have an unsaturated solution.
• Solubility is the maximum amount of solute that can
dissolve in a given amount of solvent at a given
temperature.
• Saturated solutions have that amount of solute
dissolved.
• Unsaturated solutions have any amount of solute
less than the maximum amount dissolved in solution.
Supersaturated Solutions
• In supersaturated solutions, the solvent holds more
solute than is normally possible at that temperature.
They are formed at a high temperature, and then slowly
cooled.
• These solutions are unstable; crystallization can usually
be stimulated by adding a “seed crystal” or scratching the
side of the flask.
13.3 – Factors Affecting Solubility
• The extent to which one substance dissolves
in another depends on:
• The nature of both substances (Solute–solvent
Interactions)
• Pressure (for gaseous solutes)
• Temperature
Solute – Solvent Interactions
• In general, the stronger the attractions between solute and
solvent molecules, the greater the solubility of the solute in
that solvent.
• General rule: “like dissolves like” (substances with similar
intermolecular attractive forces tend to be soluble in one
another)
• Liquids that mix in all proportions (i.e. ethanol in water) are
called miscible.
• Liquids that don’t dissolve in one another (i.e. gasoline and
water) are called immiscible.
• solubility of alcohols
Solubility and Biological Importance
• Fat-soluble vitamins (like vitamin A) are nonpolar; they are readily
stored in fatty tissue in the body.
• Water-soluble vitamins (like vitamin C) need to be included in the
daily diet, as they are not stored in the body.
Pressure Effects
• The solubility of solids
and liquids are not
appreciably affected by
pressure.
• Gas solubility is affected
by pressure.
Henry’s Law
• The solubility of a gas is
proportional to the partial
pressure of the gas above the
solution.
or
S1/P1 = S2/P2
Temperature Effects
• For most solids, as temperature
increases, solubility increases.
• For all gases, as temperature
increases, solubility decreases. Cold
rivers have higher oxygen content
than warm rivers.
• Cold soda is more carbonated than
warm soda.
13.4 – Expressing Solution Concentration
• Qualitative ways of expressing solution
concentration:
 Dilute – relatively small concentration of solute
 Concentrated – large concentration of solute
• Quantitative ways of expressing solution
concentration:
 Molarity (M) = moles of solute / liters of solution
 Mole fraction(i) = moles of component / total moles of all components
 Mole fractions have no units. The sum of all the mole fractions of a
solution equals 1.