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1 Lecture 21 Properties of ocean Water

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Page 1: Lecture21 dec2-bb

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Lecture 21

Properties of ocean Water

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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?

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Distributions of Earth’s Water

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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

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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.

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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

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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.

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Hydrogen Bond -- Cohesioncohesion: water molecules stick to other water molecules by hydrogen bonding

Why they move together?

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Hydrogen Bond – CohesionHigh surface tension

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Hydrogen Bond -- Adhesionadhesion: water molecules stick to other polar molecules by hydrogen bonding

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Figure 3.3

Adhesion

Two types ofwater-conducting

cells

Cohesion

300 m

Directionof watermovement

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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

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Water thermal

properties

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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.

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The formation of ice in freshwater: Density of water

Hydrogen bonds are constantly being broken and reformed

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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).

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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.

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Why water has high latent heat?

Completely hydrogen bond control

No hydrogen bond

Above 4oC, kinetic energy

dominates

Below 4oC, hydrogen bond

dominates

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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.