Chapter 3 Presentation
Water and the Fitness of the
Environment
Essential Idea: Water is the medium of life.
TOK
Claims about the “memory of water”
have been categorized as
pseudoscientific.
What are the criteria that can be used
to distinguish scientific claims from
pseudoscientific claims?
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Properties of Water
There are 4 main properties of water
that make it an important molecule
which allows life to exist.
– 1. Cohesive behavior.
– 2. Water’s ability to moderate
temperature.
– 3. Expansion upon freezing.
– 4. Water is a good solvent.
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Attractive Forces
• Hydrogen bonds.
• Van der Waals interactions are weak
intermolecular forces that occur
between the atoms of compounds and
molecules.
– There are two different types:
• 1. Dispersion (London) forces.
• 2. Dipole interactions.
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Attractive Forces
• Van der Waals forces and H-bonds are
very weak individually, but in large
number they are very strong.
• The gecko example from the book and
H-bonding that creates surface tension
that allows water striders to walk on
water provide an example of the
collective strength of these forces.
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Hydrogen Bonds
• These are a special type of dipole
interaction.
• Hydrogen bonds are the strongest of
the dipole interactions. Hydrogen
bonds are seen in H-containing
compounds that are bonded to very
electronegative atoms.
• H 2O
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1. Dispersion Forces
• Dispersion forces are the weakest:
these forces generally increase as the
number of electrons increases.
• The halogens are an example.
• Chlorine is a gas, bromine is a liquid,
and iodine is a solid.
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2. Dipole Interactions
• Dipole interactions are a little stronger
and these result from the interactions of
polar molecules.
• SiO2 is a very polar molecule, they are
attracted to other SiO2 molecules.
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Water Structure
Water Structure
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Water Transport
Water Transport
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A comparison of the thermal properties of
water compared to methane.
1. Cohesion
Cohesion is basically the phenomenon
of the hydrogen bonds working
together.
This leads to the idea of surface
tension.
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1. Cohesion--Surface Tension
Surface tension is the thing you
observe when you see a water bug
walking on water or the beads of water
forming on a freshly waxed car.
The surface tension is created by the
hydrogen bonds that form between the
water molecules.
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QuickTime™ and a
decompressor
are needed to see this picture.
2. Moderation of Temperature
Water has a very high specific heat
capacity meaning that it takes a lot of
energy to raise and lower its
temperature.
– This high specific heat is due to hydrogen
bonding.
In order to break the bonds, heat must
be absorbed and in order for them to
form, heat must be released.
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2. Moderation of Temperature
Thus, adding heat to the water does
little more than disrupt H-bonds, it is
only when you add a lot of heat do you
begin to see a change in temperature.
When hydrogen bonds have been
disrupted and the H2O molecules begin
to move faster only then do we see a
change in temperature.
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2. Moderation of Temperature
These characteristics of water enable it
to moderate air temperatures in the
regions near water.
A large amount of heat can be
absorbed with only a small change in
temperature enabling life in the water to
enjoy a relatively stable climate.
Likewise, on cool nights “warm” water
can give off a lot of energy without itself
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cooling much. This warms the nearby
3. Expansion Upon Freezing
One of the unique properties of water is
that it expands when it freezes. Most
substance do not do this.
It expands because as the water
molecules lose energy, they align a
certain way according to their hydrogen
bonds.
This creates a small pocket and makes
the ice less dense than the liquid. 22
3. Expansion Upon Freezing
When the ice freezes at the top of the
body of water it acts to insulate the
remaining water preventing it from
freezing solid.
Because it is less dense and floats, it
prevents the body of water from
freezing from the bottom up. If this
were the case, only a small portion of
the ice would ever thaw making life23
4. Versatile Solvent
Water is a very versatile solvent.
Though not universal, water has an
amazing ability to dissolve many things
that are important to our existence.
Much of water’s ability to dissolve
things is due to its polarity. The polar
regions of the water molecule interact
with the positively and negatively
charged regions of the molecules with
which they are interacting.
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Hydrophobic Substances
These
substances do
not interact with
water.
Think oils.
Think non-polar.
http://caitlynmccartney.wordpress.com/
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Hydrophilic Substances
These substance mix
well with, or have an
affinity for water.
Think salt water.
Think vinegar.
http://hewittapbiology.files.wordpress.com/2012/08/salt.png
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http://www.bishopsnyder.org/blogs/bjsapbiology/2012/08/12/906/hydrophilic-properties-of-matter-water-and-vinegar/
Buffers
Buffers are important because they
minimize changes in pH by preventing
wild swings in H+ and OHconcentration.
A common buffer in the human body is
carbonic acid.
H2O + CO2 <-> H2CO3 <-> HCO3- + H+
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Le Chatlier’s Principle
When the pH of our blood changes, it
can have deadly consequences. So, in
accordance with Le Chatelier’s
principle, the above buffer system acts
to alter these changes in pH.
H2O + CO2 <-> H2CO3 <-> HCO3- + H+
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Le Chatlier’s Principle
When the pH increases, the reaction
shifts to the right to lower the pH back
to an acceptable level. When the pH
decreases, the reaction shifts left in an
attempt to normalize the pH.
H2O + CO2 <-> H2CO3 <-> HCO3- + H+
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