lecture21 dec2-bb
TRANSCRIPT
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Lecture 21
Properties of ocean Water
Why ocean is a major player in Earth’s climate system?
1. A source of most atmospheric water vapor;
2. A source/sink for carbon dioxide;3. A source/sink of heat energy;4. With high specific heat, the Ocean has a
high thermal inertia and can store a tremendous amount of heat energy.
How much water do we use?
Distributions of Earth’s Water
What are the properties of water?CohesionAdhesionCapillarityHigh freezing pointHigh boiling pointHigh surface tensionHigh specific heatHigh latent heatDensity – greatest at 4oCSolid water (ice) is less dense than liquidUniversal solvent
Why are the properties of water unique?
The structure of water is the basis for its unique properties.
The most important property of water is the ability to form hydrogen bonds.
Chemical Bonds
Ionic bonds are formed by the attraction of oppositely charged ions.
Covalent bonds form when atoms share 2 or more valence electrons.Nonpolar covalent bonds = equal
sharing of electronsPolar covalent bonds = unequal
sharing of electrons
Water molecule structure--polar covalent bond
Water consists of an oxygen atom bound to two hydrogen atoms by two single covalent bonds.
If the two charges have the same sign, the electrostatic force between them is repulsive
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Water molecule structure--polar covalent bond
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Hydrogen BondsHydrogen bonds are weak attractions
between the partially negative oxygen of one water molecule and the partially positive hydrogen of a different water molecule.
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What is the difference between hydrogen bond and covalent bond?
Covalent bondWithin a water molecule the bonds between oxygen and hydrogen are highly polar.- oxygen is partially negative- hydrogen is partially positive
Hydrogen bondHydrogen bonds can form between water molecules or between water and another charged molecule.
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Hydrogen BondHydrogen bond
between water molecules
Hydrogen bondbetween water and another charged molecule.
Hydrogen Bond -- Cohesioncohesion: water molecules stick to other water molecules by hydrogen bonding
Why they move together?
Hydrogen Bond – CohesionHigh surface tension
Hydrogen Bond -- Adhesionadhesion: water molecules stick to other polar molecules by hydrogen bonding
Figure 3.3
Adhesion
Two types ofwater-conducting
cells
Cohesion
300 m
Directionof watermovement
Hydrogen Bond -- Adhesion
• 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
Water thermal
properties
This heat works to move the molecules faster. But there's something holding the water molecules from moving freely -- they are "stuck" together to a certain extent because of their hydrogen bonds.
Why water has high specific heat?
Recall definitions of
specific heat and temperature
Some of heat that is absorbed goes to break hydrogen bonds, some of them goes to speed up molecules.
The formation of ice in freshwater: Density of water
Hydrogen bonds are constantly being broken and reformed
Most liquids have a quite simple behavior when they are cooled (at a fixed pressure): they shrink.
Why does water expand when it freezes?
When liquid water is cooled, it contracts like one would expect until a temperature of approximately 4 degrees Celsius is reached. After that, it expands slightly until it reaches the freezing point, and then when it freezes it expands by approximately 9%..
This hydrogen bonding tendency gets stronger as the temperature gets lower (because there is less thermal energy to shake the hydrogen bonds out of position).
Hydrogen bonding -- creates the orderly arrangement in crystal lattice and hexagonal structure of ice crystals.-- The open network in ice’s framework creates space so it is less dense than liquid water.
Why ice has low density?
Each water molecule is bound tightly to its neighbors but intermolecular bonds are elastic so that molecules vibrate about fixed locations in the lattice.
Why water has high latent heat?
Completely hydrogen bond control
No hydrogen bond
Above 4oC, kinetic energy
dominates
Below 4oC, hydrogen bond
dominates
Why is density of water important?
1. Prevents water from freezing from the bottom up.
2. Ice forms on the surface first—the freezing of the water releases heat to the water below creating insulation.
3. Makes transition between season less abrupt.