Properties of Water
WJEC GCE BIOLOGY
Water is important to living organisms because it is a medium
for:
•Metabolic processes
•Transport
•Aquatic organisms to live in
Properties of Water
WJEC GCE BIOLOGY
•
Water consists of an
oxygen atom bound to
two hydrogen atoms by
two single covalent
bonds.
–
The shared electrons
are not shared equally.
The oxygen atom pulls
the electrons closer to it
–
This leaves the water
molecule with positively
and negative charged
ends and is therefore
called a polar molecule.
Water is a Polar Molecule
-has oppositely charged
ends
Water molecules form hydrogen bonds
Properties of Water
WJEC GCE BIOLOGY
slightly positive
charge
hydrogen bond
between (+) and (-)
areas of different
water molecules
slightly negative
charge
Surface
Tension
Colour
Cohesion
and adhesion
Properties of Water
WJEC GCE BIOLOGY
Latent
heat
State at
room
temp.
Specific
heat
capacity
Density
Next
Surface Tension
•
•
Properties of Water
WJEC GCE BIOLOGY
•
•
•
Water has the highest surface
tension of any liquid except mercury
The surface of water can behave
like an elastic sheet, this is due to
cohesion between water molecules.
Those molecules in the surface are
not effected by molecules above
them, and therefore ‘pull’ together
more strongly, effectively resembling
a stretched membrane
A habitat can therefore be produced
on the surface of the water
A pond skater can walk across the
surface of the water
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Colour – transmission of light
• Water is colourless and is
therefore transparent to light
• This means that sunlight can
reach the cells of water plants so
that photosynthesis may occur
Reflected by leaves
53%
10%
13%
Properties of Water
WJEC GCE BIOLOGY
Sun light
Light absorbed by leaves
(typical values):
Near IR 15%
Red 82%
Green 80%
Blue 84%
9%
8%
7%
7%
32%
Transmitted through
leaves
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Cohesion and adhesion
•
•
•
Water molecules show a strong
attraction to one another due to
hydrogen bonding, this is called
cohesion.
Water molecules can also show
attraction to other polar molecules called
adhesion
The forces of cohesion and adhesion
help water travel up the xylem vessels of
plants.
Properties of Water
WJEC GCE BIOLOGY
Droplets of water
form spheres due to
cohesion between
water molecules
Adhesion and
cohesion help
water move up
xylem
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Latent heat
•
While changing state, a substance will take
in heat energy (solid  liquid  gas), or
expel heat energy (gas  liquid  solid)
without a change in temperature. This is
termed Latent Heat.
temperature
evaporate
GAS
Properties of Water
WJEC GCE BIOLOGY
condense
•
Water has a large latent heat
of vaporisation, and therefore
absorbs a large amount of
heat energy while changing
from water to vapour. Lots of
energy is needed to break
hydrogen bonds
•
Organisms use this to cool
down by the process of
sweating.
melt
solidify
time
Back
State at room temperature
•
Water is a liquid at room temperature it can therefore be used as a
solvent.
– This allows chemical reactions to take place in solution
– Water can acts as a transport medium carrying dissolved molecules e.g.
blood is a transport medium in animals, in plants water transports mineral
ions in the xylem.
Properties of Water
WJEC GCE BIOLOGY
•
Water is an effective solvent
as it can form hydrogen bonds
with ions. e.g. Na Cl.
– The positive end of the water
molecule attracts the
negative ion and the negative
end of the water molecule
attracts the positive ion.
– The water molecules
surround the ions and they
therefore dissolve.
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Specific Heat Capacity
•
The heat needed to raise the
temperature of 1kg of water by 1°C is
termed the Specific Heat Capacity.
• Water has a large specific heat
capacity, and therefore can
absorb large amounts of heat
energy before its temperature
raises a significant amount.
Again it takes a lot of energy to
break hydrogen bonds for
molecules to move around.
Properties of Water
WJEC GCE BIOLOGY
thermometer
1kg of water
• This prevents large fluctuations
in the temperature of water in
the environment e.g ensures a
stable environment for aquatic
organisms
Back
Density of water
•
Unlike other substances, water
expands as it freezes. It has it’s
H
H
maximum density at 40C
O
Ice
Water H H H H
H
H
O
O
O
H
H
O H H
O
H
Properties of Water
WJEC GCE BIOLOGY
Water molecules are able to
approach one another quite
closely
O
H
H
H
O
O
Water molecules in ice form
a rigid structure so that there
is more space between
them, this is seen as
expansion as a whole
H
H
H
H
O
O
H
H
• As water expands when it freezes, its
density (mass per unit volume) will
decrease.
• This means that ice is less dense than
water, and will therefore float on top of
it.
• In aquatic environments, ice forms an
insulatory layer and prevents the entire
water column from freezing
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Properties of Water
WJEC GCE BIOLOGY
Match the property of water with
the corresponding significance
for life and correctly fill in the
table
START
Property of water
Ice is less dense than water
The high surface tension of water
means that it can form a habitat on
the surface of the water
High surface tension
Ice forms an insulating layer over
water
Strong cohesive properties and high
tensile strength
Colourless with a high transmission
Properties of Water
Liquid at room temperature
WJEC GCE BIOLOGY
Significance for life
Answers
In order to evaporate it must absorb a
large amount of energy (high latent
heat of vaporisation)
Water can absorb a lot of energy for
only a small rise in temperature (high
specific heat capacity)
Can be used for cooling organisms
by evaporation of for example sweat
Conditions are stable in cells and
aquatic environments
Can be used for transport and a
medium for reactions
Light can pass through cells for
photosynthesis
Water can be pulled through plants
in a column as the water molecules
are held together by H-bonds
Property of water
Ice is less dense than water
Ice forms an insulating layer over
water
High surface tension
The high surface tension of water
means that it can form a habitat on
the surface of the water
Properties of Water
Strong cohesive properties and high
tensile strength
WJEC GCE BIOLOGY
Significance for life
Water can be pulled through plants
in a column as the water molecules
are held together by H-bonds
Colourless with a high transmission
Light can pass through cells for
photosynthesis
Liquid at room temperature
Can be used for transport and a
medium for reactions
In order to evaporate it must absorb a
large amount of energy (high latent
heat of vaporisation)
Can be used for cooling organisms
by evaporation of for example sweat
Water can absorb a lot of energy for
only a small rise in temperature (high
specific heat capacity)
Conditions are stable in cells and
aquatic environments
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