Polar Molecules and Solubility
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Students will understand that physical
properties such as the polarity of molecules
are related to a compound’s solubility.
Students will be able to predict the solubility
of compounds in water
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Students will understand how intramolecular
and intermolecular forces affect
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Compounds with covalent
bonds usually form nonpolar molecules since neither
atom gains a positive or
negative charge.
There is a small
electronegativity difference
between the atoms.
In some molecules, there are
polar bonds, but the atom is
symmetrical leading to an
overall non-polar molecule.
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Water (H2O)
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Water (H2O) is the classic polar molecule.
Oxygen has a higher electronegativity
(3.5) than hydrogen (2.1). The
electronegativity difference is 1.4 and a
polar covalent bond forms. This means
that the oxygen atom gains a negative
charge and the hydrogen gains a positive
charge. Water is a polar molecule.
A similar effect is seen in Hydrofluoric
Acid, but carbon tetrachloride is
symmetrical and is therefore non-polar!
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HCl
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NH3
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BF3
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CCl4
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CH3Cl
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Solutions are homogenous mixtures of substances
composed of at least one solute and one solvent
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Homogenous mixture is a uniform mixture of only
one phase
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Solute is a substance that is dissolved in a solvent
(e.g. NaCl)
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Solvent is the medium in which a solute is dissolved;
often the liquid component of a solution (e.g. water)
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An aqueous solution is a solute dissolved in water
(e.g. NH3(aq))
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Electrolyte is a compound that, in aqueous solution,
conducts electricity
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Nonelectrolyte is a compound that, in aqueous solution,
does not conduct electricity
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Acid is a substance that, in aqueous solution, turns blue
litmus paper red
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Base is a substance that, in aqueous solution, turns red
litmus paper blue
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Neutral is a substance that, in aqueous solution, has no
effect on either red of blue litmus paper; neither acidic or
basic
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Polar molecules only dissolve into
polar liquids. For example, salt
dissolves into water.
Salt (NaCl) is ionic and water is a
polar molecule (polar covalent
bonds)
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Non-polar molecules only dissolve into nonpolar liquids
Grease dissolves in gasoline
Oil paint dissolves in paint thinner
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A solution is a homogeneous mixture of two
substances. It is composed of a solvent (the
substance that does the dissolving) and a
solute (the substance that gets dissolved).
For example Kool Aid is a solution with the
drink crystals being the solute and water being
the solvent.
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Water dissolves Sodium Chloride, the polar water
molecules surround the NaCl crystals and rip it apart!
This causes the Ionic Crystal (NaCl) to Dissociate or
Separate in Na+ ion and a Cl- ion.
In cases where water is the solvent, the solution is
considered to be an aqueous solution.
Polar solutions conduct electricity. We call these
solutions electrolytes.
Solute in
Example of solution
solvent
gas in gas
gas in liquid
gas in solid
liquid in gas
liquid in liquid
liquid in solid
solid in liquid
solid in solid
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Students will review concepts such as
electronegativity, properties of matter and
bonding.
Students will understand that intramolecular
forces such as covalent bonding, polar
covalent bonding and ionic bonding affect the
solubility of a substance
Student will learn the terminology related
solutions.
Predicting High and Low Solubilities (pp. 275-277)
Highest Solubility - Acetic Acid and Methanol
 Molecules are polar
 Formation of hydrogen bonds with water molecules
High Solubility - Dimethyl ether
 Molecule is polar
 Presence of oxygen atom means some hydrogen
bonding with O-H ends of water molecules
Slightly Soluble - Carbon dioxide and Oxygen
 Should be non-polar, but oxygen presence means that
there is a possibility of some hydrogen bonding with
water molecules.
Insoluble - Propane and Tetrachloromethane
 Nonpolar molecules
 Little or no solubility in water, cannot participate in
dipole-dipole forces and hydrogen bonding
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When a solute and
a solvent do not
mix, the solute is
said to be
insoluble. The
solute and solvent
are immiscible.
When the solvent
and solute do mix
they are soluble
and miscible.
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Why is it important to know about polar and nonpolar molecules?
Polar molecules dissolve into other polar molecules
Non-polar molecules dissolve into other non-polar
molecules but often not as easily!
BUT polar molecules do not dissolve non-polar
molecules – they form mixtures.
LIKE
DISSOLVES LIKE!
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Oil and water do
not mix. They are
immiscible. (Oil is
insoluble in water).
Oil is non-polar
and water is polar.
Salt dissolves in
water. They are
miscible. (Salt is
soluble in water).
Both salt (NaCl)
and water are
polar molecules.
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Whipped Cream
Colloids are
mixtures that
look a lot like
solutions. They
are very fine
particles that are
non-polar and
mix into other
non-polar
compounds and
sometimes water.
Milk
Jello
Mayonnaise
Solid stick deodourant