Water :
A Molecule Essential for Life
Thoughts to Ponder……
• Why can’t we survive more than a week
without water?
• How does water rise up 300 feet from the
roots of a redwood tree without a pump?
• Why does water take so long to boil?
• Can it really boil faster if salt is added?
• Why do oil and water not mix?
3
And how does this Spider Walk on Water?
Source: http://static.flickr.com/82/237875014_4d579d57c5.jpg
Life is dependent on Water
Why?
Source: http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-earth.html
Water has unique
properties that support life
due to its structure
http://www.lenntech.com/images/Water%20molecule.jpg
Water is a Covalently bonded molecule –
Hydrogen and Oxygen share electrons
The electrons are not shared equally – the
Oxygen atom pulls the shared electrons closer to
itself.
This “pulling force” is calledNegative
electronegativity.
Pole
Positive Pole
http://rst.gsfc.nasa.gov/Sect20/water_molecules_con_c_la_784.jpg
Electrons that are shared by hydrogen and oxygen
stay closer to the Oxygen atom because oxygen has
a greater electronegativity than hydrogen.
Because the e- are closer to the oxygen atom,
it gets a slight/partial negative charge
Because the e- are away from the hydrogen atom,
it gets a slight/partial positive charge
A molecule that has different charges on
opposite sides is called a Polar Molecule
Water is one example of a Polar Molecule.
Negative Pole/ Side
Positive Pole/Side
http://rst.gsfc.nasa.gov/Sect20/water_molecules_con_c_la_784.jpg
What else do you know of that has
opposite poles or sides?
A polar molecule behaves a like a
magnet due to having different
charges on either side
Water’s polarity gives it the ability to form
hydrogen bonds with other charged molecules or
ions.
A hydrogen bond is an attractive force between a
positively charged hydrogen atom in one
molecule and an negatively charged atom or ion
in a different molecule.
One hydrogen bond is not very strong - it doesn’t
require much energy to break it.
But when there are many hydrogen bonds
working together A LOT of energy is needed to
break the bonds.
Strength in Numbers!
Let’s be water molecules and try this out!
Source: www.personal.psu.edu/.../ bonddiagram.gif.
This is why it takes so much energy to melt and
evaporate water – billions of hydrogen bonds
must first be broken before the individual
molecules can be separated.
Hydrogen Bond Formation
Intermolecular force
Between molecules
Intramolecular force
Within molecule
http://www.biology.arizona.edu/biochemistry/tutorials/chemistry/page3.html
• Intramolecular bonds –
form between atoms within the same molecule
Ex. Covalent and Ionic bonds
• Intermolecular bonds –
form between atoms in different molecules
Ex. Hydrogen bonds
Oxygen’s electronegativity ->
Water’s polar structure ->
Hydrogen bonding
These characteristics of water molecules give rise
to the properties that support life.
It is an important example of how a molecule’s
structure determines how it functions /what it can
do.
Remember a common theme is biology:
Structure and Function are related.
Water’s ability to Hydrogen Bond
gives it Properties which Support Life
•
•
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Cohesion / Surface Tension
Adhesion
High heat capacity
Dissolves polar and ionic compounds
Hydrophobic effect on nonpolar
substances
• Expands and less dense as it freezes
•Because water is a polar
molecule it is attracted to itself
•Cohesion – the attractive force
between Water molecules
Water molecules “stick” together
•Creates surface tension
•Water molecules are attracted
to other charged molecules.
(Polar molecules/Ions)_
• Capillarity – ability of
water to move upwards
in small tubes due to its
attraction for each other
(cohesion) AND the solid
surface it’s in contact
with (adhesion).
Ex.’s of Capillarity
Water exists in all three states (liquid,
solid, gas) over a wide range of
temperatures
• 32 – 212 F
• 0 – 100 C
•Water can absorb and
release large quantities of
heat energy without
changing its temperature.
•Ex. It takes a long time for
water to boil and to cool
down.
http://i.telegraph.co.uk/multimedia/archive/01549/perspiration_1549996c.jpg
When’s the water the warmest at the beach?
High Heat Holding Capacity
• Cities located near large
bodies of water benefit
from this property
• The slow release of heat
the water absorbed in the
summer warms their air in
the winter and the
absorption of heat energy
cools their air in the
summer.
• Water helps to maintain
stable temperatures
• Our bodies are made of
70% water which helps
keep our temp. stable
• Large amounts of heat E are
needed to break the
hydrogen bonds between
liquid water molecules so
they can evaporate into gas.
•Sweating is a homeostatic
mechanism designed to cool
your body down as heat from
your body is used to
evaporate the sweat
•Water’s ability to
hydrogen bond lets it
dissolve many polar and
ionic compounds which
also have a charge.
2 Parts of a Solution
• Solvent
1. Substance that’s present in the greater amount
2. Dissolves another substance
• Solute
1. Substance that’s present in lesser amount
2. Gets dissolved
Water “the Great Dissolver”
• “Like Dissolves Like”
• Water dissolves many ions and molecules that
can be carried throughout the body in blood
and urine (both of which are mostly water)
• Most chemical reactions must occur in water
STAY HYDRATED – YOUR HEALTH DEPENDS ON IT!
• Water cannot dissolve nonpolar
substances like fats, waxes and
oils – they have No Charge to
attract water with.
•“Oil and Water Don’t Mix”
•As Water molecules hydrogen
bond with each other they exclude
nonpolar substances Hydrophobic effect
• Causes molecules like DNA and
proteins to fold into their specific
shapes
Why Water and Oil Don’t Mix
• Water is NOT able to dissolve nonpolar
molecules (molecules that don’t have a
charge)
• Examples of nonpolar molecules:
*Fats
*Waxes
*Oils
• When fat molecules and water molecules are
mixed, water molecules are attracted to each
other by hydrogen bond and push the fat
molecules away because they can’t hydrogen
bond with them
• YouTube: Why Oil and Water Don't Mix
•The space between
water molecules
expands as water
freezes making ice less
dense than liquid water
- ice floats
•Water bodies freeze
from the top down with
the ice insulating the
water and organisms
below.
Biology Crash Course Videos
• Water - Liquid Awesome
https://www.youtube.com/watch?v=HVT3Y3_g
HGg
• Polar and Nonpolar molecules
https://www.youtube.com/watch?v=PVL24HAes
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