Lecture 9. properties of water
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Transcript of Lecture 9. properties of water
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< 0o C - ice; 0o C - 100o C – liquid; > 100o C - steam
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• Water consists of an oxygen atom bound to two hydrogen atoms by two single covalent bonds.
– Oxygen has unpaired & paired electrons which gives it a slightly negative charge while Hydrogen has no unpaired electrons and shares all others with Oxygen
– Leaves molecule with positively and negative charged ends
Water is a Polar Molecule
-has oppositely charged
ends
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slightly positive
charge
slightly negative
charge
hydrogen bond
between (+) and (-)
areas of different
water molecules
Water molecules form Hydrogen
bonds
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Water’s Properties
• Cohesion
• Adhesion
• Capillarity
• High Specific Heat
• High Heat of Vaporization
• Density
• Solvent
• Transparent
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– Adhesion refers to attraction to other substances.
Water is adhesive to any substance with which it can form hydrogen bonds.
Adhesion
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Cohesion
• Water clings to polar
molecules through
hydrogen bonding
– Cohesion refers to
attraction to other
water molecules.
responsible for
surface tension
a measure of the
force necessary to
stretch or break
the surface of a
liquid
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Capillary actionwater evaporates from
leaves = transpiration
adhesion,
cohesion and
capillary action
water taken up by
roots
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•trees have specialized structures to transport water:
xylem and phloem “plumbing”
• water molecules are “dragged” from the roots to the
top of the tree by capillary action and cohesion:
hydrogen bonds help water molecules to each other
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High Specific Heat
– High specific heat
Amount of heat that must be
absorbed or expended to change
the temperature of 1g of a
substance 1o C.
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• A large body of water can absorb a
large amount of heat from the sun in
daytime and during the summer, while
warming only a few degrees.
• At night and during the winter, the
warm water will warm cooler air.
• Therefore, ocean temperatures and
coastal land areas have more stable
temperatures than inland areas.
• The water that dominates the
composition of biological organisms
moderates changes in temperature
better than if composed of a liquid
with a lower specific heat.
Impact of water’s high specific heat ranges from
the level of the whole environment of Earth to that
of individual organisms.
The Earth is over
75% water!
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High Heat of Vaporization
– High heat of vaporization
Amount of energy required to change
1g of liquid water into a gas (586
calories).
large number of hydrogen bonds
broken when heat energy is
applied
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• As a liquid evaporates, the
surface of the liquid that remains
behind cools - Evaporative
cooling.
• Evaporative cooling moderates
temperature in lakes and ponds
and prevents terrestrial
organisms from overheating.
• Evaporation of water from the
leaves of plants or the skin of
animals removes excess heat.
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“Universal” Solvent• A liquid that is a completely homogeneous
mixture of two or more substances is called a
solution.
– A sugar cube in a glass of water will eventually
dissolve to form a uniform mixture of sugar
and water.
• The dissolving agent is the solvent and the
substance that is dissolved is the solute.
– water is the solvent and sugar the solute.
• In an aqueous solution, water is the solvent.
• Water is not really a universal solvent, but it is
very versatile because of the polarity of water
molecules.
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• Water is an effective
solvent as it can form
hydrogen bonds.
– Water clings to
polar molecules
causing them to be
soluble in water.
Hydrophilic -
attracted to
water
– Water tends to
exclude nonpolar
molecules.
Hydrophobic -
repelled by water
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• Water transports molecules dissolved in it
– Blood, a water-based solution, transports molecules of nutrients and wastes organisms
– Nutrients dissolved in water get transported through plants
– Unicellular organisms that live in water absorb needed dissolved substances
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Solid water (ice) is less dense than liquid
same mass
but a larger
volume
• Ice is less dense than water: the molecules are
spread out to their maximum distance
Density = mass/volume
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water expands as it
solidifies
water reaches
maximum density at
4-degrees C
water freezes from
the top down
organisms can still
live in the water
underneath the ice
during winter
Oceans and lakes don’t freeze solid
because ice floats
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Water is Transparent
• The fact that water is clear allows light to
pass through it
– Aquatic plants can receive sunlight
– Light can pass through the eyeball to
receptor cells in the back
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