oceanography 5 - properties of water

7/28/2019 Oceanography 5 - Properties of Water

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PHYSICAL AND CHEMICALPROPERTIES OF SEAWATER

MOLECULAR STRUCTURE OF WATER

H 2OTwo atoms of hydrogen

(one proton +, one electron -)

One atom of oxygen

(8 protons +, 8 electrons -)

They are joine d by COVALENT BONDS

(the oxygen and hydrogen share electrons)

MOLECULAR STRUCTURE OF WATER

Both hydrogen atoms are on one side of thewater molecule

The molecules electrons are clustered on theopposite side of the molecule

This gives the water molecule POLARITY

i.e. one side has a slight positive charge and oneside a slight negative charge = DIPOLAR

e.g. like a flashlight battery or bar magnet

HYDROGEN BONDS

The positive side of the molecule is attractedto negative charges,

e.g. the opposite sides of other watermolecules

The attraction forms a weak bond =HYDROGEN BOND

NB A hydrogen bond is much weaker than athe bond between atoms (covalent bond)


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HYDROGEN BONDS

Hydrogen bonds help give water some of itsproperties

Including SURFACE TENSION

the skin on glass of water or a pond

COHESION

.molecules pulling together to form droplets

CAPILARITY

The ability to pull molecules behind it up anarrow tube (capillary) e.g. the xylem of plants

Surface Tension

Capillarity

THE UNIVERSAL SOLVENT

Water molecules not only stick to other watermolecules, but also other polar substances

e.g. Atoms in salt (NaCl) molecules are heldtogether by IONIC BONDS

Positive Na + is strongly attracted to negative Cl -

= ELECTROSTATIC ATTRACTION

When NaCl is put in water the attractionbetween Na + and Cl - is reduced (80 times)

Na + and Cl - become separated

and the salt crystals dissolve


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THE UNIVERSAL SOLVENT

The process by which water moleculessurround ions

(positively or negatively charged particles)= HYDRATION

Because water molecules not only interact withother molecules, but also with polar substances

water can dissolve almost anything

= THE UNIVERSAL SOLVENT

If water cannot dissolve a substance it isbecause it is not polar (e.g. oil)

THERMAL PROPERTIES OF WATER

Water, like other matter, can exist in threestates: SOLID, LIQUID or GAS

What need to happen to change the state of asubstance?

Bonds between molecules must be broken

This requires that molecules move faster andmove further apart

This requires ENERGY

Bonds between molecules includeVAN DER WAALS FORCES

Weak forces of attraction, but can be importantwhen molecules are close together (e.g. solid)

In water also Hydrogen Bonds

THERMAL PROPERTIES OF WATER THERMAL PROPERTIES OF WATER

The amount of energy needed to raise thetemperature of water by 1oC

= THE SPECIFIC HEAT CAPACITY

The SHC of water is very high

it takes more energy to increase the energy of hydrogenbonded water molecules than

molecules just kept together by Van Der Waalsforces


Therefore water can absorb a lot of heat before itstemperature rises.

Also a lot of energy has to be released to cause

water to cool

This means that water temperatures change moreslowly compared to the land - for example

e.g. California - sea water: 6.3 oC 20 oC

land: 3.6 oC 45.7 oC

SOLID:When water is solid (i.e. ICE) water has a rigid,

regular structure (crystalline) and moleculesare locked in place

LIQUID:Most common state of water. Molecules movefreely about, but still interact with each other

(some bonds)

GAS:Water molecules do not interact with each other

except during random collisions and movefreely



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If enough energy is added to water, bondsbreak and it becomes liquid

(@MELTING POINT) [0 oC; 34 oF]

If enough energy is then addedit turns into gas ( @BOILING POINT) [100oC; 2 12oF]

If energy is removed the gas reverts back intoliquid ( @CONDENSATION POINT)

And more energy removes turns into a solid(@FREEZING POINT)


The heat energy needed to turn 1 g of asubstance at the melting point temperature

from solid to liquid=

THE LATENT HEAT OF MELTING(i.e. the energy require to break bonds)

The heat energy needed to turn 1 g of asubstance at the boiling point temperature

liquid to gas=

THE LATENT HEAT OF VAPOURIZATION

LATENT HEATS

LATENT HEATS

The heat energy needed to BE REMOVED to turn1 g of a substance at the boiling point temperature

from gas to liquid=

THE LATENT HEAT OF CONDENSATION

(i.e. the energy required be removed to slow moleculesdown and to allow bonds to reform)

The heat energy nee ded to BE REMOVED to turn 1 g of asubstance at the melting point temperature from liquid to

solid=

THE LATENT HEAT OF FREEZING / FUSION


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Water Phase Changes

Sea surface temperatures are usually less than20oC nowhere near the boiling point

Therefore how does liquid turn to vapor at theoceans surface?

To escape the liquids surface a water moleculemust take enough energy from its neighboring

molecules to become a gas

Because energy is taken when evaporationoccurs it cools the molecules left behind

= THE LATENT HEAT OF EVAPORATION

NB: A greater amount of energy than the latentheat of vaporization

IMPORTANCE OF LATENT HEATS

The huge amount of heat exchanged in theevaporation condensation cycle allows life to be

possible on the earth.

Energy is moved from hotter regions to coolerregions .

Water evaporates in warmer regions

In the cooler regions the water is release asprecipitation (releasing heat)

Heat is also release when ice forms, warminghigher latitudes

= MODERATE CLIMATE

WATER DENSITY

Density = Mass / Volume

How heavy something is

Density of pure water = 1g/cm 3

Normally the density of a substance increases as itcools

Molecules loose energy

slow down

closer together

= THERMAL CONTRACTION

The density of water increases as it becomes cooler

e.g. cold water is heavier than warm water

BUT from 4 oC to 0 oC the density of waterDECREASES

i.e. instead of contracting it expands

Therefore ice is LESS DENSE than liquid water

ICE FLOATS

Why?


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Below 4 oC ice crystals start forming

These crystals are large and bulky

They take up more volume than liquid water

Therefore they are less dense than liquid water

When water freezes its volume increase by 9%

(this is why pipes burst in winter etc)

BUT adding dissolved substances INHIBITS ice crystalformation, so water can become colder before the

density starts to increase

THERFORE seawater becomes ice at temperaturebelow 0 oC

Density of water with and without salt (g/cm 3) MARINE BIOLOGICAL SIGNIFICANCE OFWATER DENSITY

In warmer water (i.e. not as dense) plankton have to besmaller in order to float (higher surface area : volume ratio )

Or have structure on their surface to help floatation

Ice floats when it freezes

In the polar regions a layer of unfrozen, slightly warmerwater is often found under ice

Fish etc. can live in this slightly warmer water layer

SALINITY

Salinity = total amount of solid material dissolved inwater (including gases)

Salinity of seawater = 3.5%

i.e. 96.5% pure water content

220 time saltier than fresh water

NB salinity does not include particles and sedimentsFLOATING in water

= TURBIDITY

SALINITY

Salinity is usually measured in parts per thousand

3.5% = 3.5 parts per hundred

= 35 parts per thousand

= 35 o/oo or 35 ppt or 35 g/kg 3


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MAJOR COMPONENTS

MINOR COMPONENTS TRACE COMPONENTS

SALINITY

Salinity varies in the oceans

from 35 to 38 parts per thousand

In coastal areas salinity can be much reduced

e.g. only 10 parts per thousand in river estuaries

=BRACKISH WATER

(fresh water & seawater mixing)

SALINITY

However, in the Red Sea salinity


=HYPERSALINE WATER

Occurs in areas with high evaporation and limited connection to / circulation with the open ocean

THE DEAD SEA


10 times saltier than sea water


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SALINITY

Salinity can also vary seasonally

e.g. 1 Miami Beach

34.8 ppt in Octoberto 36.4 ppt in May & June

i.e. when evaporation is high

e.g. 2 Astoria, Oregon

0.3 ppt in April & Mayi.e. when the Colorado River is at the greatest flow rate

to 2.6 ppt in Octoberi.e. the dry season

SOURCES OF SEAWATER COMPONENTS

Largest source of seawater components is from streamsand runoff

But the composition of dissolved substances in streamwater is not the same as seawater

Why?

Because some components have a high RESIDENCETIME and accumulate high concentrations over years

e.g. Na + has a residence time of 260 million years

BUT on average rate of salt added = rate of salt removal

SOURCES OF SEAWATER COMPONENTS

Sea also enters the crust near hydrothermal vents andpicks up minerals etc.

The entire volume of the ocean may pass throughhydrothermal vents every 3 million years

= major influence on seawater composition

PROCESSES THAT INCREASE SALINITY

Removal of water content:

Evaporation

Formation of sea ice(which has only a salinity of 10ppt i.e. mostly fresh water )


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REMOVAL OF SEAWATER COMPONENTS

In addition to some losses in tectonic boundaries/faults

Some salts etc are lost when sea spray etc. hits the land

Also living organisms extract minerals from seawaterand when they die biogenous sediment

Finally, some components may be absorbed (attach tothe surface of) sediments

PROCESSES THAT DECREASE SALINITY

Addition of water content:

Precipitation

Melting of sea ice, icebergs and glaciers

Streams, rivers & runoff

ACIDITY AND ALKALINITY

An ACID is a compound that releases hydrogen ions(H +) when dissolved in water

The resulting solution = ACIDIC

An ALKALINE or BASE releases OH - ions whendissolved in water

A stronger acid or alkaline releases more ions


H + and OH - are always present in small quantitiesbecause water molecules dissociate and reform

i.e. H 2O H + + OH -

In pure water

number of H + ions = number of OH -

Therefore the solution is neutral (pH =7)

The pH scale measure acidity (pH7)


In the ocean CO 2 reacts with water and releaseshydrogen ions

i.e. H 2O + CO 2 H 2CO 3 H + + HCO 3-

= CARBONIC ACID

So, theoretically the ocean should be acidic

BUT this is prevented by the

CARBONATE BUFFERING SYSTEM


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THE CARBONATE BUFFERING SYSTEM

The bicarbonate ion can also loose a secondhydrogen ion

HCO 3- H + + CO 32-

The carbonate ion can then react with calcium ions toform Calcium Carbonate

(which precipitates onto the seafloor = hydrogenous sediment)

Ca 2+ + CO 32- CaCO 3

THE CARBONATE BUFFERING SYSTEM

If the ocean becomes too acidic

HCO 3-

+ H+

H 2CO 3 (acidity drops)

If the ocean becomes too alkaline/basic

H 2CO 3 H+ + HCO 3

- (acidity increases)

This balancing is called BUFFERING

ACIDITY IN DEEP WATER

In deep, cold waters more Carbon Dioxidedissolves in seawater

(because gas at cooler temperatures dissolves more easily)

This should make the deep sea more acidic

But when marine organisms that contain CalciumCarbonate (in shells or skeletal) die

The Calcium Carbonate they release helps to buffer theacidic conditions

The Calcium Carbonate acts like an antacid

VARATION IN SALINITYVARATION IN SALINITY

Why are there variations in oceanic salinity?

High latitudes increase precipitation & runoff

Polar regions melting of ice

Cooler temperature - Less evaporation Tropical regions little precipitation

Higher temperature more evaporation

At the Equator High evaporation

but high precipitation balances out


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DEPTH VARATION IN SALINITY

Salinity also varies with depth

In low latitudes (near the tropics equator)

Salinity at the surface is HIGH

As depth increases salinity decreases

In high latitudes (temperate & polar)

Salinity at the surface is LOW

As depth increases salinity increases

Most of the factors affecting salinity only alter surfacewaters

DEPTH VARATION IN SALINITY

At a depth of 300m to 1000m there is a rapid changein salinity

In low latitudes a decrease in salinityIn high latitudes an increase in salinity

This area of rapid change is called the HALOCLINE

Haloclines separate layers of different salinity in the ocean

SEAWATER DENSITY

In the ocean seawater density varies

1.022 - 1.030 g/cm 3 (depending on salinity )

Density has important effects on ocean water

Denser bodies of water will sink below less densebodies of water

Therefore low density water is found at the surface, high density water found in the depths

SEAWATER DENSITY

Several factors effect seawater density

As temperature increases density decreases(thermal expansion)

As salinity increases density increases(addition of more dissolved material)

As pressure increases density increases(pressure compresses materials)

Only temperature and salinity effect surface waters

Pressure only effects very deep waters (e.g. trenches)

(density in greatest depths only 5% more than surface)


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SEAWATER DENSITY

Temperature has the greatest effect on density

(temperature varies more than salinity)

Increases in temperature at higher temperaturescause greater decreases in density than the same

number of degrees increased at lower temperatures.

Increases in water temperature in low latitudes(tropics and equator) have three times the effect on

water density than in high latitudes

SEAWATER DENSITY & DEPTH

In Low Latitudes (equator/tropics)

Seawater density in surface stays the same until adepth of about 300m

(due to good mixing of the surface waters)

Below 300m density increases rapidly until a depth of 1000m

From 1000m the density remains relatively constantuntil the ocean floor


In High Latitudes (temperate etc.)

Density is high at the surface

(because temperature is low)

The density below the surface is also high

(due to cool temperatures)

Therefore the density remains relatively constantwhether surface waters or deep waters


In Low Latitudes the layer of rapid changing density

= THE PYCNOCLINE

The layer of rapid changing temperature

= THE THERMOCLINE

They occur between 300m & 1000m

The pycnocline acts as a barrier between deep and surface waters preventing mixing

Although above the pycnocline the surface waters are wellmixed by currents/tides/waves


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Mixed surface layer

Thermocline + Pycnocline = Upper water

Deep water


In High Latitudes thermoclines and pycnoclinesrarely form

(except during very hot sunny periods)

The water column in high latitudes

= ISOTHERMAL

& ISOPYCNAL

= same temperature & density

oceanography 5 - properties of water

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