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Seawater Chemistry: Key Ideas

Water is a polar molecule with the remarkable ability to dissolvemore 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 dissolvedgases.

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The Water Molecule

Water is a polar molecule with a positive and anegative side.

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Asymmetry of a watermolecule anddistribution ofelectrons result in adipole structure withthe oxygen end ofthe moleculenegatively chargedand the hydrogenend of the moleculepositively charged.

Water Molecule

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The Water Molecule

Hydrogen bonds formwhen the positive end ofone water molecule bondsto the negative end ofanother water molecule.

Dipole structure of watermolecule produces anelectrostatic bond(hydrogen bond)between watermolecules.

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Figure 4.1

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As solid sodium chloride dissolves, the positive and negative ionsare attracted to the positive and negative ends of the polar watermolecules.

The Dissolving Power of Water

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Formation of Hydrated Ions

Water dissolves salts by surrounding theatoms in the salt crystal and neutralizing the

ionic bond holding the atoms together.

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Important Property of Water:Heat Capacity

Amount of heat to raise T of 1 g by 1oCWater has high heat capacity - 1 calorieRocks and minerals have low HC ~ 0.2 cal.Significance: For a given addition of heat

Water T increases less than rock TThat’s why sand at the beach feels hotter than water

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Water And Heat

Note the high heatcapacity of water.

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Energy absorbed and released during phasechanges of water

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The energy input or output associated with water inthe three states of matter.

The Three States of Matter

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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 anyknown substance.

Evaporating Water

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Unique properties of water include:Higher melting and boiling point than other

hydrogen compounds.High heat capacityGreater solvent power than any other

substance.

The Unique Water Molecule

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Ice floats in water because all of themolecules in ice are held in hexagons andthe center of the hexagon is open space,making ice 8% less dense than water.

Water Molecule

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The arrangement ofwater molecules inan ice crystal

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Water reaches its maximum density at 3.98oC.

Below this temperature water moleculesbecome structured and density decreases.

Above this temperature water moleculesare increasingly energetic, move fartherapart, and density decreases.

Hydrogen bonding is responsible for many ofthe unique properties of water becauseenergy is required to break the hydrogenbonds and separate the water molecules.

Water Molecule

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19The relationship of density to temperature for pure water.

Water Temperature And Density

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Addition of salt modifies the properties of water

Pure water freezes at 0oC. Adding salt increasinglylowers the freezing point because salt ionsinterfere with the formation of the hexagonalstructure of ice.

Boiling point increases because ions retardevaporation.

Density of water increases as salinity increases.

Salinity

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Seawater as a Solvent

Salinity is the total amount of salts dissolved inthe 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 temperaturedecreases and as salinity and pressureincrease.

Pressure increases regularly with depth,but temperature and salinity are morevariable.

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Sound and light both travel in waves.

Refraction is the bending of waves,which occurs when waves travel fromone medium to another.

The refractive index is a ratio thatexpresses how much light is refractedfrom one medium to another.

Refraction, Light, and Sound

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Sunlight does not travel well in the ocean.Scattering and absorption weaken light.

Scattering occurs when light is bouncedbetween air and water molecules, dust andother objects.

Absorption occurs when light’selectromagnetic energy is converted to heatin the molecules of seawater.

Light in the Ocean

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Amount of light entering the ocean dependsupon the height of the sun above the horizonand the smoothness of the sea surface.

65% of light entering the ocean is absorbedwithin the first meter and converted into heat.Only 1% of light entering the ocean reaches100 m.

Water displays selective absorption of lightwith long wavelengths absorbed first andshort wavelengths absorbed last.

In the open ocean, blue light penetrates thedeepest.

Light Penetration

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Figure 4.8

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The speed of sound in water increases assalinity, temperature and pressure increase,but in the ocean speed of sound is mainly afunction of temperature and pressure.

Near the surface, as pressure increases thespeed of sound increases despite the gradualdecrease in temperature.

At mid-depths, the speed of sound decreasesbecause of the decrease in temperature andonly slight increase in pressure.

Sound in Seawater

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In the deepest part the speed of soundgradually increases because pressureincreases, but temperature declines onlyslightly.

SOFAR Channel is located where soundspeed is at a minimum.

Refraction of sound waves within the channelprevents dispersion of the sound energy

Sound waves travel for 1000s of kilometerswithin the channel.

Sound in the Ocean

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The relationship between water depth and sound velocity.

Sound in the Ocean

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Sea Ice

As ice forms, the salt remains in solution,increasing salinity and further lowering thefreezing 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 inorganicsolids 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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Salinity is the total mass, expressed ingrams, of all substances dissolved in onekilogram of sea water when all carbonatehas been converted to oxide, all bromineand iodine has been replaced by chlorineand all organic compounds have beenoxidized at a temperature of 480oC.

Definition of Salinity

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Dissolved Salts in Sea Water

99% of salt: Na+, Cl-, SO4=, Mg+2, Ca+2

and K+

Sodium and chloride comprise about86% of the salt

Major chemical constituents display littlevariation over time - conservativeproperty of sea water

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Chlorinity is the amount of halogens(chlorinity, bromine, iodine and fluorine)in seawater and is expressed asgrams/kg or o/oo.

Salinity - 1.8065 times chlorinity

Salinity determined from the electricalconductivity of seawater produced bythe dissolved salts - salinometer

Salinity and Chlorinity

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The Components of Salinity

The most abundant components of a kilogram of seawater

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Principle of constant proportion states thatthe absolute amount of salt in sea watervaries, but the relative proportions of theions is constant.

• Because of this principle, it is necessary totest for only one salt ion, usually chloride, todetermine the total amount of salt present.

Principle of Constant Proportions

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Determining Salinity

Salinity can be determined by measuring thechlorinity of the sample. Since the chlorinity iseasy to measure, and the principle of constantproportions applies to all seawater, the followingformula 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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Salt sinks include the following:

Precipitation of evaporites -evaporation removes only watermolecules

Wind-blown spray - minute droplets ofsaltwater carried inland

Adsorption onto clays and authigenicminerals

Formation of shells by organisms

Removal of Salts

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Circulation of SeawaterThrough Oceanic Ridges