CHAPTER 2
Water and Life
http://www.youtube.com/watch?feature=player_embedded&v=ASLUY2U1M-8
Water and Life
– Life on Earth began in water and evolved
there for 3 billion years.
• Modern life still remains tied to water.
• Your cells are composed of 70%–95% water.
– The abundance of water is a major reason
Earth is habitable.
• Like no other common substance, water
exists in nature in all three physical
states:
– as a solid
– as a liquid
– as a gas
Figure 2.10B
Figure 19.29d
The Structure of a Water Molecule
• What atoms make up a water molecule?
• How are these atoms joined together to
make a water molecule?
• What type of bond joins the atoms
together?
• In a water molecule, oxygen exerts a stronger
pull on the shared electrons than hydrogen
– This makes the
oxygen end of the
molecule slightly
negatively charged
– The hydrogen end of
the molecule is slightly
positively charged
– Water is therefore a
polar molecule
(–)
(–)
O
H
(+)
H
(+)
Figure 2.9
The Properties of Water
• Atoms in a molecule that form
covalent bonds may share electrons
equally, creating a nonpolar molecule
• If electrons are shared unequally, a
polar molecule is created
Water’s polarity leads to hydrogen bonding
and other unusual properties
• The charged
regions on water
molecules are
attracted to the
oppositely charged
regions on nearby
molecules
Hydrogen bond
– This attraction forms
weak bonds called
hydrogen bonds
– Water model activity
Figure 2.10A
Essential Question
• How does water
rise from the
roots of a
redwood tree to
the very top?
•
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The Cohesion of Water
– Water molecules stick together as a result
of hydrogen bonding.
• This is called cohesion.
• Due to cohesion water molecules form a
continuous column of water in the
conducting tubes of a plant.
• Cohesion is vital for water transport in
plants.
The Adhesion of Water
– Water molecules stick to other polar
(charged) molecules by hydrogen bonding
• This is called adhesion.
• Due to adhesion water molecules stick
to the charged walls of the conducting
tubes keeping the column of water from
falling due to gravity
• Cohesion is vital for water transport in
plants.
Cohesion and Adhesion in Your Life
• Can you think of an example of
cohesion and adhesion in your everyday
life?
• Explain how these properties of water
explain this example.
Water’s Life-Supporting Properties
– The polarity of water molecules and the
hydrogen bonding that results explain most of
water’s life-supporting properties:
•
•
•
•
Water’s cohesive and adhesive nature
Water’s ability to moderate temperature
Floating ice
Versatility of water as a solvent
Essential Question
• How do insects
walk on water?
•
http://www.youtube.com/watch?v=VcXzZt2iVk&feature=player_embedded
Surface Tension
Surface tension is the measure of how
difficult it is to stretch or break the
surface of a liquid.
• Hydrogen bonds give water an
unusually high surface tension.
• It is as if a film is formed on the
surface of the water and this allows
insects to walk on the surface
Surface Tension in Your Life
• What is an example of surface
tension in your life?
Essential Question
• Why do people
sweat and some
animals pant to
reduce their body
temperature
•
http://www.youtube.com/watch?feature=play
er_embedded&v=lqwPS6wJN-c
How Water Moderates Temperature
• It takes a lot of energy to disrupt hydrogen
bonds
– Therefore water is able to absorb a great
deal of heat energy without a large increase
in temperature
– As water cools, a slight drop in temperature
releases a large amount of heat
– Because of hydrogen bonding, water has a
strong resistance to temperature change.
–
http://oceanservice.noaa.gov/education/pd/oceans_weather_climate/media/specific_heat.swf
– A water molecule takes a large amount of
energy with it when it evaporates
– This leads to evaporative cooling
– This is why people sweat and some animals
pant
Figure 2.12
Essential Question
•
http://www.sumanasinc.com/webcontent/animations
/content/propertiesofwater/water.html
• Why does ice float
rather than sink?
• How would life in a
lake be affected if
ice sank and lakes
froze from the
bottom up?
The Biological Significance of Ice Floating
• Due to hydrogen bonds molecules in ice
are farther apart than those in liquid
water
– The density of ice is lower than liquid
water.
• This is why ice floats.
– Since ice floats, ponds, lakes, and even
the oceans do not freeze solid.
– How would life in a lake be affected if ice
sank and lakes froze from the bottom up?
Water as the Solvent of Life
– A solution is a liquid consisting of two or
more substances evenly mixed.
• The dissolving agent is called the
solvent.
• The dissolved substance is called the
solute.
Water as the Solvent of Life
• Solutes whose
charges or polarity
allow them to stick
to water molecules
dissolve in water
– They form
aqueous
solutions
Essential Question
•
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• Why is your blood
mostly water?
• Why is the inside
and outside of your
cells mostly water?
Acids, Bases, and pH
- Acid
• A chemical compound that donates H+ ions to
solutions.
- Base
• A compound that accepts H+ ions and
removes them from solution.
- To describe the acidity of a solution, we use the
pH scale.
Figure 2.17
– Cells are kept close to pH 7 by buffers.
http://www.youtube.com/watch?feature=player_embedded&v=NJyAme5GVF8
– Buffers are substances that resist pH
change.
• They accept H+ ions when they are in
excess.
• They donate H+ ions when they are
depleted.
– Buffering is not foolproof.
• Example: acid precipitation
Acid precipitation threatens the environment
• Some ecosystems are threatened by acid
precipitation
•
http://www.natgeoeducationvideo.com/film/1233/acid-rain
• Acid precipitation is formed when air
pollutants from burning fossil fuels combine
with water vapor in the air to form sulfuric
and nitric acids
Figure 2.16A
– These acids can kill fish, damage buildings, and
injure trees
– Regulations, new technology, and energy
conservation may help us reduce acid
precipitation
Figure 2.16B