Ocean Chemistry
Unit 5
Ocean Chemistry
 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.
Reflection
 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?
Properties of Water

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
Reflection
 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.
Seawater
 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, SO43-, and HCO3-
 Elements <1 ppm are called trace elements
Reflection
 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.
Sources of Ocean Salts

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
Chemical Equilibrium

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
Reflection
 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?
Water and Heat
 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.
Reflection
 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?
Colligative Properties of Seawater

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)
Reflection
 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?
Water Density
 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
Reflection
 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.
Pycnocline
 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.
Reflection
 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?
Dissolved Gases
•
•



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
Reflection
 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?
pH

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----------------------------14
acid
neutral
base
(alkaline)
Reflection
 26. Explain how pH works and in specific, the
pH of the ocean.
The End