Dissolving
Objectives: define dissolving, define and apply the solubility rule, review Lewis Dot Diagrams
Dissolving- formation of a homogeneous mixture
Solubility Rule- polar solutes will dissolve in polar solvents; non-polar solutes will dissolve in non-
polar solvents
POLARITY REVIEW for Bond Type
1. Ionic Bonds are polar because- the molecule is made from a positively charged cation
and a negatively charged anion
2. Polar Covalent Bonds are polar because- there is an unequal sharing of electrons and the
higher EN element has a partial negative charge since the electron is there more of
the time whereas the lower EN element has a partial positive charge so as to make
the molecule neutral
3. Non-Polar Covalent Bonds are not polar because- equal sharing of electrons means the
molecule will be symmetrical
POLARITY REVIEW for Lewis Diagrams
*If the molecule has an unshared pair of electrons around the central atom that atom will have a
partial negative charge. The opposite end of the molecule will have a partial positive charge.
The molecule will be polar.
Example: Draw the Lewis Dot Diagram for NH3
Be reminded of the steps:
1. Find the central atom and draw its electron dot diagram
from the group number
2. The subscript on the other atom is the number of them
that need to be attached to the central
atom adjacent to a single electron
3. Any remaining electrons around the
central atom should exist as pairs and will
compress the shared pairs together
decreasing the bond angle
T. O’Toole
Example: Draw the Lewis Dot Diagram for C2H5OH
Be reminded of the steps:
1. Determine the number of electrons available by
adding the valence electrons for every atom in
the formula
2. Determine the number of electrons
required for all atoms in the
formula to obey the octet rule—
remember Hydrogen follows the “octet” rule
with only 2 electrons
3. Subtract and divide by 2—this is the number of
shared pairs
4. Available minus Used (#1-#3) equals the unshared electrons
which are placed around the atoms that still need electrons
to obey the octet rule.
Polar Solute and Polar Solvent: NaCl in H2O
Why is NaCl polar? This ionic bond contains a positively charged sodium cation and a
negatively charged chlorine anion
Why is H2O polar? This molecule with polar covalent bonding also has 4 bonding sites which
are not identical—2 are bonds and 2 are unshared pairs of electrons
draw a sodium ion surrounded by water
draw a chlorine ion surrounded by water
Non-Polar Solute and Non-Polar Solvent: grease and soap
Even though we don’t know the formula for a grease stain—we do know that it won’t be
cleaned off with just water.
Soap molecules are super long molecules with a polar end AND a non-polar end. The
non-polar end dissolves the grease stain and the polar end allows the soap and stain to
be carried away with water!!
T. O’Toole
Minute Paper: Use the solubility rule to explain how this giant sink hole at Lake Peigneur,
Louisiana was created.
The salt dome is made of a polar substance sodium chloride which will be dissolved by a
polar solvent water when the two come in contact. This contact occurred during the
drilling.
How do I get from where I am to where I’m going?
10 Minute Makeover Customize your learning by choosing from the following options each night
 Complete the “Solubility Rule” section on the Portfolio Page
 Review the PPT file on the website
 Read pages 502-503 in the book
 Check out the tutorial sources in the LiveBinder
 Complete the SYS Assignment
 Review the skills from today’s Train Your Brain
T. O’Toole