Properties of Water
Properties of Water
• Objectives:
1. Why are water molecules
polar?
2. What are acidic solutions?
3. What are basic solutions?
The Water Molecule
• Water:
– Covers ¾ of the
Earth’s surface
– Is the most abundant
compound in living
things
– Water is essential for
life
– Water has unique
properties
The Water Molecule
• Polarity
– A water molecule is
polar because there is
an uneven distribution
of electrons between
the oxygen and
hydrogen atoms.
– The average electron
density around the
oxygen atom in a
water molecule is
about 10 times that
around the hydrogen
atoms.
Hydrogen Bonds
• Polar water
molecules act like
magnets and attract
each other
• Hydrogen Bonds
– The attraction of the
Hydrogen end (+) of
one molecule for the
Oxygen end (-) of
another water
molecule.
– These are the
strongest bonds that
can form between
molecules
• Cohesion is the
attraction between
molecules of the same
substance (water is
attracted to water).
• Cohesion allows some
insects and spiders to
walk on water.
• The property of the
surface of a liquid that
allows it to resist an
external force is called
surface tension
Cohesion
• Adhesion is the
attraction between
molecules of
different substances
• Water is attracted to
any substance that
has a charge (either
+ or -)
Adhesion
Transpiration
• Evaporation from the
surface of plants. It
uses adhesion and
cohesion to move
water from the roots to
the leaves of a plant
– Water adheres to the
vascular tissue in the
roots (adhesion)
– Water molecules stick to
each other like a chain
(cohesion)
– Water is moved by
evaporation from the
leaves
Unique
Properties of
Water
• Water is considered
the universal solvent
• It exists in nature as a
solid, liquid or gas
• The density of ice is
less than liquid water
• Water has a high
surface tension
• Water has a high heat
capacity
Liquid vs. Ice
• Ice is less dense than
liquid water.
Water is one of the few
compounds that is a liquid at
the temperatures found over
most of Earth’s surfaces.
Unlike most other substances,
water expands when it
freezes. Liquid water is more
dense than ice, so ice freezes.
Lakes freeze at the top, but
fish and other aquatic
organisms can survive
beneath the surface.
Mixtures and Solutions
• A mixture is a material composed of two
or more compounds that are physically
mixed together but not chemically
combined.
• One type of mixture is a solution.
• Solutions are evenly distributed
• Two parts of a solution
– SOLUTE - Substance that is
being dissolved
– SOLVENT- Substance into
which the solute dissolves
Water
Solution
• Water can dissolve a
substance by
surrounding it, and
forming hydrogen bonds
with any +/- charged
particles.
Water is the Universal Solvent
• Water can dissolve almost
any substance.
• It can do this because of its
polarity—it is attracted to any
substance that has a charge
and will eventually dissolve
it.
• The only exceptions are
substances such as fats and
oils that do not have a
charge.
Acids, Bases & pH
• 1 water molecule in 550 million
naturally dissociates into a
Hydrogen Ion and a Hydroxide
Ion
H2O 
Hydrogen Ion
Acid
H+
+ OH
-
Hydroxide Ion
Base
The pH Scale
• Indicates the concentration
of H+ ions
• Ranges from 0 – 14
• pH of 7 is neutral
• pH 0 – 6.99 is acid … more
H+
• pH 7.01 – 14 is basic… less
H+ and more OH• Each pH unit represents a
factor of 10 times a change
in H+ concentration
Acids
• Any substance
that has a higher
concentration of
H+ ions or
releases H+ ions
into solution.
• Strong Acid = pH
0-3
H+ ions
Bases
• Any
substances
that has
fewer H+
ions or
more OHions.
• Strong
Base = pH
11 – 14
OH-ions
Buffers
• Weak acids or bases that react with
strong acids or bases to prevent sharp,
sudden changes in pH.
• Blood contains chemicals that act as
buffers to keep blood pH levels healthy.