Seawater Chemistry: Key Ideas

Water is a polar molecule with the remarkable ability to dissolve
more substances than any other natural solvent.

Salinity is the measure of dissolved inorganic solids in water.

The most abundant ions dissolved in seawater are chloride,
sodium, sulfate, and magnesium.

The ocean is in steady state (approx. equilibrium).

Water density is greatly affected by temperature and salinity

Light and sound travel differently in water than they do in air.

Oxygen and carbon dioxide are the most important dissolved
gases.
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The Water Molecule
Water is a polar molecule with a positive and a
negative side.
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1
Water Molecule
Asymmetry of a water
molecule and
distribution of
electrons result in a
dipole structure with
the oxygen end of
the molecule
negatively charged
and the hydrogen
end of the molecule
positively charged.
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The Water Molecule
Dipole structure of water
molecule produces an
electrostatic bond
(hydrogen bond)
between water
molecules.
Hydrogen bonds form
when the positive end of
one water molecule bonds
to the negative end of
another water molecule.
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2
Figure 4.1
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The Dissolving Power of Water
As solid sodium chloride dissolves, the positive and negative ions
are attracted to the positive and negative ends of the polar water
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molecules.
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Formation of Hydrated Ions
Water dissolves salts by surrounding the
atoms in the salt crystal and neutralizing the
ionic bond holding the atoms together.
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Important Property of Water:
Heat Capacity
of heat to raise T of 1 g by 1oC
Water has high heat capacity - 1 calorie
Rocks and minerals have low HC ~ 0.2 cal.
Significance: For a given addition of heat
Water T increases less than rock T
Amount
That’s why sand at the beach feels
hotter than water
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4
Water And Heat
Note the high heat
capacity of water.
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Energy absorbed and released during phase
changes of water
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The Three States of Matter
The energy input or output associated with water in
the three states of matter.
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Evaporating Water
For
water to evaporate, heat must be added to water in
the liquid state. After water reaches 100°C, an
input of 540 cal/g is required to break the
hydrogen bonds and allow evaporation.
The
amount of energy required to break the bonds is
the latent heat of vaporization.
Water
has the highest latent heat of vaporization of any
known substance.
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The Unique Water Molecule
 Unique
properties of water include:
 Higher melting and boiling point than other
hydrogen compounds.
 High heat capacity
 Greater solvent power than any other
substance.
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15
Water Molecule
Ice floats in water because all of the
molecules in ice are held in hexagons and
the center of the hexagon is open space,
making ice 8% less dense than water.
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8
The arrangement of
water molecules in
an ice crystal
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Water Molecule


Water reaches its maximum density at 3.98oC.
 Below this temperature water molecules
become structured and density decreases.
 Above this temperature water molecules
are increasingly energetic, move farther
apart, and density decreases.
Hydrogen bonding is responsible for many of
the unique properties of water because
energy is required to break the hydrogen
bonds and separate the water molecules.
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9
Water Temperature And Density
The relationship of density to temperature for pure water.
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Salinity
Addition of salt modifies the properties of water
 Pure
water freezes at 0oC. Adding salt increasingly
lowers the freezing point because salt ions
interfere with the formation of the hexagonal
structure of ice.
 Boiling
point increases because ions retard
evaporation.
 Density
of water increases as salinity increases.
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10
Seawater as a Solvent
 Salinity
is the total amount of salts dissolved in
the water.
 It is measured in parts of salt per thousand
parts of salt water and is expressed as ppt
(parts per thousand) or abbreviated as o/oo.
 Average salinity of the ocean is about 35 o/oo.
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Table 4.4
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Density of Seawater: Function of
Temperature, Salinity and Pressure.
 Density
increases as temperature
decreases and as salinity and pressure
increase.
 Pressure increases regularly with depth,
but temperature and salinity are more
variable.
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12
Refraction, Light, and Sound
Sound and light both travel in waves.
Refraction
is the bending of waves,
which occurs when waves travel from
one medium to another.
The
refractive index is a ratio that
expresses how much light is refracted
from one medium to another.
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Light in the Ocean
Sunlight
does not travel well in the ocean.
Scattering and absorption weaken light.
Scattering
occurs when light is bounced
between air and water molecules, dust and
other objects.
Absorption
occurs when light’s
electromagnetic energy is converted to heat
in the molecules of seawater.
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Light Penetration
Amount of light entering the ocean depends
upon the height of the sun above the horizon
and the smoothness of the sea surface.



65% of light entering the ocean is absorbed
within the first meter and converted into heat.
Only 1% of light entering the ocean reaches
100 m.
Water displays selective absorption of light
with long wavelengths absorbed first and
short wavelengths absorbed last.
In the open ocean, blue light penetrates the
deepest.
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14
Figure 4.8
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15
Sound in Seawater
The speed of sound in water increases as
salinity, temperature and pressure increase,
but in the ocean speed of sound is mainly a
function of temperature and pressure.


Near the surface, as pressure increases the
speed of sound increases despite the gradual
decrease in temperature.
At mid-depths, the speed of sound decreases
because of the decrease in temperature and
only slight increase in pressure.
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Sound in the Ocean

In the deepest part the speed of sound
gradually increases because pressure
increases, but temperature declines only
slightly.

SOFAR Channel is located where sound
speed is at a minimum.


Refraction of sound waves within the channel
prevents dispersion of the sound energy
Sound waves travel for 1000s of kilometers
within the channel.
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16
Sound in the Ocean
The relationship between water depth and sound velocity.
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35
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Sea Ice
 As
ice forms, the salt remains in solution,
increasing salinity and further lowering the
freezing point of the water.
 Depending upon how quickly the ice freezes,
some salt may be trapped within the ice mass,
but is gradually released.
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Salinity
Salinity is the total quantity of dissolved inorganic
solids in water.
As salinity increases:
 Heat capacity of water decreases
 Freezing point decreases
 Evaporation slows (i.e., Boiling Pt.
increases)
 Osmotic pressure increases
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Definition of Salinity
Salinity is the total mass, expressed in
grams, of all substances dissolved in one
kilogram of sea water when all carbonate
has been converted to oxide, all bromine
and iodine has been replaced by chlorine
and all organic compounds have been
oxidized at a temperature of 480oC.
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Dissolved Salts in Sea Water
of salt: Na+, Cl-, SO4=, Mg+2, Ca+2
and K+
 Sodium and chloride comprise about
86% of the salt
 Major chemical constituents display little
variation over time - conservative
property of sea water
 99%
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Salinity and Chlorinity
 Chlorinity
is the amount of halogens
(chlorinity, bromine, iodine and fluorine)
in seawater and is expressed as
grams/kg or o/oo.
 Salinity - 1.8065 times chlorinity
 Salinity determined from the electrical
conductivity of seawater produced by
the dissolved salts - salinometer
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The Components of Salinity
The most abundant components of a kilogram of seawater
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Principle of Constant Proportions
Principle of constant proportion states that
the absolute amount of salt in sea water
varies, but the relative proportions of the
ions is constant.
• Because of this principle, it is necessary to
test for only one salt ion, usually chloride, to
determine the total amount of salt present.
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Determining Salinity
Salinity can be determined by measuring the
chlorinity of the sample. Since the chlorinity is
easy to measure, and the principle of constant
proportions applies to all seawater, the following
formula is used to determine salinity:
Salinity (in parts per thousand) =
1.80655 × chlorinity (in ppt)
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Sources of Sea Salt



Weathering of continents
Volcanic eruptions
Hydrothermal vents
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Removal of Salts
 Salt




sinks include the following:
Precipitation of evaporites evaporation removes only water
molecules
Wind-blown spray - minute droplets of
saltwater carried inland
Adsorption onto clays and authigenic
minerals
Formation of shells by organisms
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Circulation of Seawater
Through Oceanic Ridges
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