Ocean ChemistryUnit 5

The chemical properties of the ocean are important to understand because the marine environment supports the greatest abundance of life on earth.

This life is largely made up of the same chemicals that comprise the ocean—water and salts.

Ocean Chemistry

1. Why is it important to understand the chemistry of the ocean?

2. What is the chemical make-up of the life in the oceans? What is it similar to?

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H20 is a compound of 2 hydrogen atoms and one oxygen atom in a fixed proportion.

Held together by covalent bonds (sharing of e-)

Molecular shape is bent into a 105° angle

Electrically unbalanced because of the angle and thus considered polar because of the (+) and (-) end

H bonds form between water molecules and other charged substances due to the polarity.

This allows water to stick to itself, a process known as cohesion

Adhesion is when water H bonds to other materials

Universal solvent: it will dissolve almost anything

Properties of Water

3. What makes up a water molecule?

4. How is it bonded? Give details.

5. Discuss the polarity of water.

6. Write about its adhesive, cohesive and solvent properties.

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NaCl dissolves in water because of it’s polarity

97.2% of Earth’s surface water is marine

Seawater is 96.5% water and 3.5% dissolved substances (mostly salts)

Earth has 5.5 trillion tons of salt

Nearly every element found in the crust and atmosphere is also present in the ocean

Major constituents of seawater H, O, Cl, Na, Mg, Ca, K, SO4

3-, and HCO3-

Elements <1 ppm are called trace elements

Seawater

7. Use the previous slide to describe marine water (saltwater) in detail. Include in your description all of the properties and composition that is given.

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Weathering running water dissolves crustal rock

Excess volatiles hydrothermal vents (underwater

volcanoes) on the ocean floor leak chemicals (C02,Cl, S, H, F, N) into the water

Sources of Ocean Salts

The ocean is in chemical equilibrium

For the most part, ions are added to the ocean at the same rate they are subtracted

Certain ions have longer residency times then others

Addition of salts from the mantle and weathering are balanced by the subtraction of minerals bound into sediments

Chemical Equilibrium

8. Where do the salts and other minerals in the ocean come from?

9. What is meant by equilibrium? How does the ocean maintain equilibrium?

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Heat is energy produced by random vibrations of atoms or molecules.

Four sources of heat in the ocean: solar energy radioactive decay heat from Earth formation artificial heat from humans

Temperature is an object’s response to input or removal of heat. 1°C = 1.8°F O°C is freezing 100°C is boiling

About 1m (3.3 ft) evaporates from the surface of the ocean every year.

Water and Heat

10. Define heat.

11. What are four sources of heat in the ocean?

12. Define temperature and note the freezing and boiling points for water.

13. How much water evaporates from the ocean each year?

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Heat Capacity heat required to raise 1 g of substance 1°C Heat capacity of water is among the highest of all

known substances. Water can absorb (or release) large amounts of

heat with little change in temp. The heat capacity of seawater decreases with

increasing salinity (saltwater is less able to hang on to heat)

Colligative Properties of Seawater

Salinity total quantity of dissolved inorganic solids in

water (NOT just salt!) Salinity is usually 3.3-3.7% depending on

evaporation, precipitation, and freshwater runoff

Proportion of Cl to salinity is constant: Salinity in % = 1.81 x Cl %

As salinity increases, freezing point decreases Gives seawater a natural “antifreeze” property

(saltwater freezes at a lower temp than fw) Salt water evaporates more slowly than fw

(salt hangs onto water)

Colligative Properties of Seawater

Osmotic Pressure O.P. of organisms increases with

increasing salinity (organisms lose more water when salinity is higher)

Colligative Properties of Seawater

14. What are the 3 given colligative properties of seawater?

15. Describe, in detail, the heat capacity of water.

16. Describe, in detail, the salinity of sea water.

17. How is the osmotic pressure of organisms affected by salinity?

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Density of water is a function of salinity and temperature.

Seawater density increases with increasing salinity, increasing pressure, and decreasing temperature.

Water Density

Freezing & Density During the transition from liquid to solid, water

expands This makes ice less dense than liquid water, and thus

floats. Density of ice is .917 g/cm3

Density of liquid water is .999 g/cm3. Density of water increases as seawater freezes.

Ice crystals are pure water because they exclude the salt.

The left over cold, salty water is very dense.

Water Density

Ocean Layers The ocean layers by density stratification.

Surface (mixing) zone 2% least dense zone Top of the sea can actually be fw

Pycnocline 18% density increases with depth

Deep zone 80% below 1000m, densest layer

Water Density

18. How does salinity and temperature affect the density of the ocean?

19. Describe in detail, the relationship of freezing temperatures and water.

20. Describe the result of density stratification and the three layers of the ocean associated with it.

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Thermocline + Halocline = Pycnocline Halocline - the area where the salinity changes

rapidly. Thermocline - the layer that changes in temp rapidly.

Can range in temp from 30.5-37.5°F Average temp of ocean being 38°F.

Water masses (having characteristic temp and salinity, density) get trapped at great depths. The pycnocline isolates 80% of the ocean from the

20% circulating on the surface.

Pycnocline

21. What are the two layers that make up the pycnocline?

22. What is special about each of these two layers?

23. How does the pycnocline isolate 80% of the water in the ocean?

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• Gases dissolve most readily in cold seawater• Plants and animals in the ocean require dissolved gases

in order to survive Nitrogen

48% of gas in ocean (atmosphere 78%) living organisms require N to build proteins, but bottom

dwelling bacteria must “fix” the N into a useable form for them

Oxygen 36% of gas in ocean (atmosphere has 100x more) Primary source of O2 in ocean is from plants most of the oxygen is near the surface and diffuses into

the atmosphere Carbon dioxide

15% of gas in ocean (60x more in ocean than atmosphere)

used by phytoplankton- low at surface

Dissolved Gases

24. Where does gas most readily diffuse in the ocean?

25. What are three major gasses found in the ocean? In what quantity is each found? How are they each used in the ocean?

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Acidity (release of H+) and alkalinity (release of OH-) is measured by pH

The ocean contains buffers to prevent large swings in pH when acids or bases are introduced

pH scale

0-----------------------------7----------------------------14acid neutral base

(alkaline)

pH

Pure w

ater

Seawater 7.8

26. Explain how pH works and in specific, the pH of the ocean.

The End

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