MAY 2011
Oceanography. Ch # 5.
Water and Sea Water.
Introduction
The unique thermal property of water (a result of the molecule geometry), causes the Mild
climate in coastal areas in contrast to harsh inland weather. The atomic arrangement (how
molecules stick together) gives water the ability to store and release heat slower than any
other substance on earth and makes it a perfect solvent.
Water expands as it approaches its freezing point, as opposed to every other liquid. Its chemical
properties are essential for sustaining life. It is the primary component of all living things (65 to
95%) except the Jelly fish which is 99% water. Blood is 83% water; it transports nutrients
through the body and removes waste.
5 – 1. UNUSUAL CHEMICAL PROPERTIES
The atomic structure is composed of a nucleus (a Proton+ and a Neutron) tightly bound
(Hydrogen bond) together. Electrons move around the nucleus, the number of which is equal
to the number of protons. The number of protons and neutrons are unique to any element . On
losing or gaining an electron the atom becomes an ion.
The Water Molecule. Two or more atoms bounded together by mutually shared electrons. The
atom is the smallest form of an element that can exist and retain the properties of the
substance.
Geometry. The bend in the geometry of the water molecule causes the unique properties. The
Oxygen atom bonds with two hydrogen atoms, share electrons and has a covalent bond.
Polarity. Due to the bend, there is a small neg charge on the side of the Oxygen atom and a
positive on the Hydrogen atom side. The separation of the charges make the molecule Di Polar.
Interconnections of Molecules. Similar to bar magnets, molecules orient themselves, forming a
hydrogen bond.
Co-valent bonds hold a molecule together and in between the molecules are hydrogen bonds.
These hydrogen bonds are strong enough to cause water molecules to stick together –
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cohesion, which causes water to bead up on a polished surface. Its surface tension is greater
than any other substance except Mercury.
The Universal solvent. When water is added to any chemical compound, the ionic bond
between the molecules is weakened and the ions of the compound attach themselves to the
water molecule. Hence the name Universal Solvent. Given enough time water dissolves
everything. When water surrounds an ion completely, the result is hydration.
5 – 2. Other Important Properties of Water.
Thermal Properties. Water exist as solid liquid and gas and has the ability to store and release
great amounts of heat, responsible for the weather. Heat, Temp and Changes of state – Energy
must be added to change the state of matter to overcome Van der Waals Forces.
Heat is the energy of moving molecules or Kinetic Energy. A Calorie is the amount of heat
required to raise the temp of 1 gm of water by 1 degree F. Calorie in terms of food is actually a
KC or 1000 calories.
Temp is the direct measurement of the average Kinetic energy of the molecule that makes up a
substance. Measured in Degree F or C. Solids do not conform with the shape of a container. The
molecules vibrate , but remain relatively fixed. Liquids have enough energy for molecules to
flow past each other, and take the shape of the container. Gas molecules no longer interact
with each other, except for random collisions. They fill up any container.
Water Freezing and Boiling Points. The temp at which melting occurs is the Melting Point. If
enough energy is removed, the substance refreezes. – the Freezing Point. Fresh water melting
and freezing points is 0 Deg C (32 Deg F.)
Boiling Point and Condensation is 100 Deg C (212 Deg F)
Water Heat Capacity. The amount of heat required to raise the Temp by 1 Deg C of a gram of
any substance is its heat capacity. Example: Cu has a heat capacity of 10X less than water.
Water Latent Heat. When water undergoes a change of state, a large amount of heat is
absorbed or released due to the water’s high latent heat.
Latent heat of melting (see p. 134)
Latent heat of Vaporization
Cal/gm.
540 Cal/gm. (Temp Deg C). Latent heat of melting 80
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Latent heat of Evaporation. The conversion of water to a gas below its boiling point is called
evaporation.
At the ocean surface temp, 20 Deg C, individual water molecules converted from the liquid to
gas have less energy than the water molecules at 100 Deg C. The individual molecules capture
energy from surrounding molecules, hence those left behind are cooler.
Latent Heat of Condensation. When vapor is cooled sufficiently, it condenses and latent heat
of evaporation is released. The heat released is enough to cook food, enough to cause thunder
storms and hurricanes.
Latent heat of Freezing. When water freezes, heat (latent heat of melting ) is released.
Global Thermostatic Effects. The properties that act to moderate temp changes and Earth’s
climate. Evaporation takes heat from the ocean into the atm. Vapor condenses into clouds/ ppt.
Latent heat of Condensation is released.
Difference in Ocean Temp between Day and Night are minor, but continental mass differences
may be large. That reflects the higher heat capacity of water.
Water Density. Pure water 1.0 gm/cub cm. (Mass per unit vol). As air cools, it shrinks and sinks,
while the warm air rises. As a consequence of molecular packing, water density increases as it
cools to 4 Deg C. It decreases from 4 Deg to 0 Deg C and ice floats. Sea water never freezes
except near the poles because of the dissolved salt content.
5 – 3. How Salty is Sea Water?
Difference from fresh water, Sea water contains dissolved salts and taste salty. Salinity is the
total amount of solids dissolved in water, including gases, but not particles in suspension.
Salinity is equal to the ratio of mass of dissolved substance to the mass of the water. Sea water
salinity is typically 35%o approx 220 X saltier than fresh water. Sea water is 96.5% pure water.
Chlorine, Na, Sulfur, Magnesium, Ca, and K make up 99% of solids, more than 80 other
chemicals are identified.
Salinity is expressed in parts / thousand. 1% is one part per 100. 1%o is 0ne part/1000. Sea
water is 35%o.
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Methods used to determine salinity

Evaporation – time consuming and not necessarily accurate

Principle of Constant proportions. Based on the fact that the major salts remain
constant. By measuring the Chlorine concentration, the total salinity can be
determined by a formula: Salinity %o = 1.80655 X Chlorinity %o.

Salinometer. Very accurate electrical conductivity.
Comparing pure to sea water

Colors are identical

Dissolved substances change freezing and boiling points. Sea water freezes @ 1.9
Deg C. Lower than Pure water, boils at 0.6 Deg C higher than fresh water.

pH is 7.0 for fresh water. Sea water is slightly alkaline. Density of sea water is 1.028
gms/cub cm.
5 – 4. Why Does Sea Water Salinity vary?
33 to 38%o. Extremes include the Red Sea 42%o; Baltic Sea 10%o; Great Salt Lake 280%o.
Hypersaline lakes are usually found on land bodies with high evaporation rates and limited
circulation. Example: Dead Sea is 330%o. Tap water is 0.6 to 0.8%o and Bottled water 0.3%o.
Processes that affect sea water salinity:
# The amount or composition of dissolved substances. Salinity is decreased by ppt and
run-off or melting ice. Salinity is increased by formation of sea ice, and evaporation.
Hydrologic Cycle – Ocean stores 97.2%; Glaciers 2.15%; GW 0.62% Streams and lakes 0.02%
and atm 0.001%.
# Dissolved components are added or removed by Volcanic eruptions, Biologic
interactions and sea spray 3.3 Billion tons/year.
Residence time of Al is about 100 years, Na about 260 MY.
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5 – 5. Sea Water, Acidic or Basic.
Acid releases H+. Sea Water releases OH- and has a pH 8 to 8.3
Carbonate Buffering System protects the ocean from becoming too alkaline or too acid. Carbon
Dioxide combines with water and forms carbonic acid which can lose H+ to form a bicarbonate,
combines with Ca allowing CaCO3 to precipitate.
Deep water has more CO2 than surface water, because of its cooler temp. At great depths, tests
neutralize the acid.
Recent increases in Ocean acidity
A pH decrease of 0.1 unit (since pre- industrialization) may be affecting all CaCO3 secreting
organisms.
5 – 6. How does Salinity vary at the surface and with Depth?
The average salinity 35%o varies from place to place and depth.
Surface Salinity Variation. Lowest at low latitudes (Tropics). @ high latitudes, there is melting
ice and run off. Evaporation is limited, except very high at the equator.
Salinity Variation with Depth. Varies widely on the surface but very little at depth. Processes
at surface have very little influence at depth. The layer of rapidly changing salinity with depth is
called a Halocline.
5 – 7. How does sea water density vary with depth?
Density is an important property, because it determines the vertical position of a layer in the
water column. Creates Deep water Currents.
Factors affecting Sea Water Density. As temp increases, density decreases; When Salinity
increases, Density increases; Pressure increase causes density to increase.
Pycnocline and Thermocline. Rapidly changing density – pycnocline and a rapidly changing
temp is a thermocline.
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5 – 8. Methods of Desalinating Sea water.
Difficult at best and expensive due to high energy requirements and environmental issues.
There are now more than 12,500 plants worldwide.

Distillation – efficient, much better than bottled-water, but taste worst.

Solar Humidification - solar energy has to be concentrated

Membrane Processes 1. Electrolysis –energy needs; 2. Reverse Osmosis –
pressurized filters are flimsy

Freeze Separation – freeze and thaw several times . Energy requirements

Melt Sea Ice naturally

Salt eating bacteria

Demineralization using a chemical catalysis.
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