8.3 THE DISSOLVING PROCESS
GENERAL STEPS TO DISSOLVING:
1) Intermolecular forces between particles in solid solute must be broken
2) Some of intermolecular forces between particles in solvent must be broken
3) Attraction between particles of solute and particles of solvent forms
DISSOLVING IONIC COMPOUNDS
 When dissolving ionic compounds in water, the
ionic bond, must
be broken.
 There is evidence of the compound breaking into its ions, as the solution conducts
electricity, making it an
.




Water is very
. As water molecules approach the crystal, the slightly
negative
is attracted to the
ion and the
slightly positive
is attracted to the
ion.
There is a competition of the
force between water and sodium
chloride and the
bonds that hold the sodium chloride together
Dissociation Hydration -
Writing Dissociation Equations
Example 1: Write a dissociation equation
for lithium phosphate
DISSOLVING MOLECULAR COMPOUNDS
Miscible –
Immiscible -
Example 2: Write a dissociation equation
for calcium nitride



Some compounds don’t have an overall dipole but have areas on them that are polar (i.e.
sugar). They have polar areas and non-polar areas. These polar bonds (O-H) often
(it’s an intermolecular force) with water.
These compounds will dissolve in polar solvents, because the polar parts of the solute are
able to attract the polar parts of the solvent molecules.
A sugar molecule dissolved in water becomes hydrated, but still has no charge (neutral).
They do not
.
Dissolving methanol, CH3OH in water






There are some covalent compounds that have no ions or polar bonds – non-polar. These
compounds are
in water. Non-polar compounds (solutes) dissolve in
non-polar solvents like benzene.
Hydrocarbons have many C-H bonds, which have a very low
,
and their bonds are almost non-polar
Most hydrocarbons are not only non-polar but are also
in shape
The only intermolecular forces between non-polar molecules are weak
forces. These forces hold non-polar molecules strongly, because
hydrocarbons tend to be very large.
Water is polar, so there are not many intermolecular forces of attraction between
hydrocarbons and water.
Will diatomic iodine dissolve in a hydrocarbon?
SUMMARY
RULE OF THUMB: Like dissolves like, unlikes don’t dissolve! So…
APPLICATIONS OF DISSOLVING:
QUESTION: HOW DO I GET STAINS OUT OF MY CLOTHES?
ANSWER: UNDERSTAND DISSOLVING




Soaps and detergents are
. Soap molecules usually have a
sodium ion following by a
tail.
The ionic portion of the soap molecule attracts the polar water molecule. We call this side
of the molecule
(water-loving)
The non-polar hydrocarbon attracts the non-polar grease particle. We call this side of the
molecule
(water-fearing)
The hydrophobic end of the molecule helps pull the grease off fabric and each grease globule
then becomes surrounded by soap molecule
Homework: Read Page 382-387 Page 389 #1-15