Unique Properties of
Water
Polar Covalent Bonds
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Polar covalent bond: unequal sharing of
electrons
Hydrogen Bond
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Attraction between a
hydrogen atom and an
atom such as oxygen or
nitrogen that is either part
of another molecule or
located at a distant site on
the same molecule.
Both the hydrogen atom
and the other atom must
be involved in a polar
covalent bond
Unique Properties of Water
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This is the chemical basis for the unique
properties of water.
Life as we know it would not exist without
these properties
Before exploring more of the unique
properties of water, please take the
following Water Quiz to test how much you
understand already.
Adhesion & Cohesion
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Cohesion: attraction of water for itself
Adhesion: attraction for water to other polar or
charged materials
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Surface tension:
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Because of the hydrogen-bonding, water acts as if it were
coated with a film. This surface tension (the tough "skin"
formed on the surface of water) causes water to "bead-up"
on a hard surface. Surface tension allows us to skip rocks on
water and it allows small organisms like the water strider to
"walk on water".
High specific heat

Specific heat: the amount of heat required to raise or lower
the temperature of 1 gram of substance 1 degree C.
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a lot of heat energy is required to break hydrogen bonds, water
resists temperature change.
When water is heated, most of the heat is used to break
hydrogen bonds and not much is left over to raise the
temperature of the water.
Because water can absorb a lot of heat with little change in
temperature, it acts as a thermal buffer.
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On a small scale, water in a cell can absorb much heat with little
change in temperature.
On a larger scale, the water in the ocean acts as a thermal
buffer for the earth, resisting temperature change and creating a
hospitable environment for life.
High heat of vaporization

Heat of vaporization: The amount of heat required for 1 gram of a
substance to be converted from a liquid to a gas.
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Hydrogen bonds make it difficult for water molecules to escape the liquid
state and are responsible for water's high heat of vaporization.
The hydrogen bonds must be broken before water can evaporate and this
requires considerable energy.
Because water has a high heat of vaporization
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Helps our bodies and our planet to maintain our temperature within a
tolerable range.
When we get hot and sweat, water evaporates from our skin and cools us.
Since the evaporation of water requires a considerable amount of energy, it
is very effective in cooling us.
Water also helps moderate global climate by absorbing solar radiation and
dissipating the heat by evaporation of surface water.
Density
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Water is also unique in the way temperature affects its
density.
Most substances increase in density as temperature
decreases because the molecules making up the substance
begin to move more slowly and get closer together.
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The density of water increases as it is cooled to 4 degrees C
and then its density begins to decrease as the temperature
decreases to 0 degrees C, the freezing point of water.
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As the freezing point is approached, hydrogen bonds relax and
form a crystal lattice that keeps molecules further apart than they
are in liquid water.
This is the reason why ice floats and ponds don't freeze solid.