Download Chemical and Physical Features of Seawater and the World Ocean

Chapter 3:
Chemical and Physical
Features of Seawater and
the World Ocean
Properties of Seawater
Review: Properties of Pure Water
Physical Properties
Chemical Properties
Cohesion/Surface tension
Found in all three states on the
earth
Universal Solvent – dissolves more
substances than any other
common liquid
Conduction of heat – highest of all
liquids (except for mercury)
pH – water dissociates into anions
(OH-) and cations (H+)
Latent heat of vaporization –
highest of all common substances
Polarity – water has positive and
negative ‘ends’
Latent heat of fusion – high for a
molecule of its size
Hydrophobic effect
Heat capacity – highest of all
common solids & liquids
Density – max at 4ºC for pure
water
Viscosity –relatively low for a liquid
The Water Molecule
• Two hydrogen atoms, one
oxygen atom
• H atoms form 105º angle
• This angle produces an
asymmetrical dipole. Slight
(+) charge on the H atoms
and slight (-) charge on the
O atoms.
O
H
H
105º
The Water Molecule
O
H
H
• These slight charges cause the
(+) H atoms of one water
molecule to attract the (-) O
atoms of other water molecules.
• These weak bonds are called
hydrogen bonds.
• Water can hydrogen bond with
other substances aside from
itself.
Why does ice float?
Density-Temperature Relationship
Background
• Temperature is a measure of kinetic energy (KE). As KE
decreases, hydrogen bonds stay formed and break less.
Water molecules stay closer together until…
Explanation
• As the temperature approaches 4ºC, less dense ice clusters
begin to form in the liquid.
• At 0ºC when all water molecules become locked in the
rigid ice lattice, the hydrogen bonds actually hold
molecules farther apart than at 4ºC. This creates spaces
making the water less dense.
Why does ice float?
Density-Temperature Relationship Graph
Why do fish not get electrocuted
when lightning strikes the ocean?
Conductivity
Explanation
• Conductivity is a property that measures the ability of a
substance to transmit heat, electricity, or sound.
• Pure water is not a good conductor of electricity. Its
conductivity is about 20 dS/m.
(Compare with silver – the highest conductivity with 63 x 106 S/m)
• In addition, the electrical charge of lightning usually
spreads instantaneously along the surface of the water from
the location of the strike and to a lesser degree below the
surface at the strike site. Fish in other areas are not
affected.
Why do coastal areas have slower
temperature changes than inland areas?
Heat capacity
Background
• Heat – energy of molecular motion
• Water can absorb or give up heat by
conduction (molecular exchange of heat)
or convection (mixing)
Explanation
• Water can hold heat longer and release
heat more slowly than land.
• Temperature differential between land and
ocean will cause uneven heating of air
masses which drive winds and moderate
any drastic temperature changes.
Off-shore breeze
On-shore breeze
Why don’t fats and oils dissolve in water?
Polarity
• Fats and oils are nonpolar molecules.
– These compounds do not have slight regions
of charge like water does.
• Therefore water molecules are not
attracted to nonpolar molecules and
actually can be repelled by them.
– Basis for cell membranes and the water
repellency of marine mammals and birds.
How can water bugs “stride” across water
without breaking the surface?
Surface tension and cohesion
• The cohesion or mutual attraction of
water molecules creates a flexible barrier
on the surface of water.
www.nps.gov/olym/insect/gerridae.jpg
• This helps support aquatic insects such as
water striders (Halobates sp.)
Why does water dissolve more substances
than any other common liquid?
The Universal Solvent
• Because water is polar, it dissolves most
substances, especially other polar
molecules and compounds composed of
ions, atoms or molecules that carry an
electrical charge.
• These ionic compounds are often called
salts.
• NaCl (salt) most common dissolved salt
in ocean. There are many others.
• Seawater is a solution of these salts.
Salt crystal
Seawater
Sources of salts and dissolved solids:
• Erosion of rocks and soil
• Breakdown of organisms
• Condensation of rain from the atmosphere
• Releases from hydrothermal vents
Seawater is
• 96.5% water
• 3.5% dissolved compounds
Salinity
• Total amount of
dissolved salts in
seawater.
• Measured in parts per thousand (ppt)
• Average value 35 ppt or 35‰
• Range from 0‰ near river mouths to 40% in the dead
sea.
• Globally, seawater salinity remains constant. Rule of
constant proportions states that the percentage of
various ions in seawater remains constant.
Surface Salinities of the Oceans
Dissolved compounds in seawater
• Inorganic substances (salts, nutrients)
• Dissolved gases
• Organic compounds (fats, oils, vitamins,
amino acids, proteins)
• Nitrates and phosphates (usually in excess
as pollution)
• Pollution (DDT, PCBs, chlorinated
hydrocarbons that are synthetic)
Dissolved Gases
• Primarily nitrogen, carbon dioxide, oxygen
• N2 biologically inert.
• CO2 needed for photosynthesis and pH
buffering.
• O2 required for respiration.
www.abc.net.au/science/news/
stories/s269960.htm
Factors that Affect Salinity
• Salinity increases due to…
– freezing of seawater.
– evaporation.
• Salinity decreases due to…
– melting of icebergs and sea ice.
– precipitation.
– run-off from rivers.
Seawater pH
• The carbonic acid – bicarbonate – carbonate system keeps
seawater at a pH value between 7.5 and 8.4.
• The oceans are an enormous “sink” for atmosphere CO2
The Carbon
Buffering System
• Seawater has an unusually large capacity to
absorb CO2.
CO2 + H2O  H2CO3 (carbonic acid)
H2CO3  H+ + HCO3 – (bicarbonate ion)
HCO3 –  H+ + CO32-
(carbonate ion)
pH
• The amount of hydrogen ions in a substance
is referred to as pH.
– pH = (pondus hydrogeni or “power of
hydrogen”
– pH = -log10 [H+]
– Scales ranges from 0 to 14.
• Therefore, a pH of 14 means that the water
is very alkaline (basic) while pH 1 means it
is acidic. A pH of 7 is neutral.
Work Cited
Kreger, Chris. "Acid Mine Drainage: Alkalinity." Exploring the Environment: Water
Quality. 2004. Wheeling Jesuit University. 2 Oct 2008
<http://www.cotf.edu/ete/modules/waterq/wqalkalinity.html>.
Lower, Stephen. "A gentle introduction to water and its structure." H2O. 2008. 1 Oct 2008
<http://www.chem1.com/acad/sci/aboutwater.html>.
"October 2006 Archives." [Weblog The Marine Electronics Weblog] Oct 2006. Panbo . 1
Oct 2008 <http://www.panbo.com/archives/2006_10.html>.
Water Conductivity. 2008. Lenntech Water Treatment & Purification B.V.. 1 Oct 2008
<http://www.lenntech.com/water-conductivity.htm>.
Woodruff, Steve W.. "Water & Weather." Los Angeles Pierce College Weather Station. 1
Oct 2008 <http://data.piercecollege.edu/weather/water.html>.