Oceanography 5 - Properties Of Water 67472e

MOLECULAR STRUCTURE OF WATER

PHYSICAL AND CHEMICAL PROPERTIES OF SEAWATER

H2O Two atoms of hydrogen (one proton +, one electron -)

One atom of oxygen (8 protons +, 8 electrons -) They are ed by COVALENT BONDS (the oxygen and hydrogen share electrons)

MOLECULAR STRUCTURE OF WATER Both hydrogen atoms are on one side of the water molecule The molecule’s electrons are clustered on the opposite side of the molecule This gives the water molecule POLARITY i.e. one side has a slight positive charge and one side a slight negative charge = DIPOLAR e.g. like a flashlight battery or bar magnet

HYDROGEN BONDS The positive side of the molecule is attracted to negative charges, e.g. the opposite sides of other water molecules The attraction forms a weak bond = HYDROGEN BOND NB A hydrogen bond is much weaker than a the bond between atoms (covalent bond)

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HYDROGEN BONDS Hydrogen bonds help give water some of its properties 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 a narrow tube (capillary) e.g. the xylem of plants

Capillarity

Surface Tension

THE UNIVERSAL SOLVENT Water molecules not only stick to other water molecules, but also other polar substances e.g. Atoms in salt (NaCl) molecules are held together by IONIC BONDS Positive Na+ is strongly attracted to negative Cl= ELECTROSTATIC ATTRACTION When NaCl is put in water the attraction between 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 molecules surround ions (positively or negatively charged particles)

= HYDRATION Because water molecules not only interact with other molecules, but also with polar substances water can dissolve almost anything

THERMAL PROPERTIES OF WATER Water, like other matter, can exist in three states: SOLID, LIQUID or GAS What need to happen to change the state of a substance? Bonds between molecules must be broken

= THE UNIVERSAL SOLVENT

This requires that molecules move faster and move further apart

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

This requires ENERGY

THERMAL PROPERTIES OF WATER

THERMAL PROPERTIES OF WATER

Bonds between molecules include VAN DER WAAL’S FORCES

The amount of energy needed to raise the temperature of water by 1oC

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

= THE SPECIFIC HEAT CAPACITY

In water – also Hydrogen Bonds

The SHC of water is very high – it takes more energy to increase the energy of hydrogen—bonded water molecules than molecules just kept together by Van Der Waal’s forces

THERMAL PROPERTIES OF WATER

THERMAL PROPERTIES OF WATER

Therefore water can absorb a lot of heat before its temperature rises.

SOLID: When water is solid (i.e. ICE) water has a rigid, regular structure (crystalline) and molecules are locked in place

Also a lot of energy has to be released to cause water to cool This means that water temperatures change more slowly compared to the land - for example e.g. California -

sea water:

6.3oC – 20oC

land:

3.6oC – 45.7oC

LIQUID: Most common state of water. Molecules move freely about, but still interact with each other (some bonds) GAS: Water molecules do not interact with each other except during random collisions and move freely

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THERMAL PROPERTIES OF WATER If enough energy is added to water, bonds break and it becomes liquid (@MELTING POINT) [0 C; 34 F] o

The heat energy needed to turn 1 g of a substance at the melting point temperature from solid to liquid=

o

If enough energy is then added it turns into gas (@BOILING POINT) [100 C; 212 F] o

LATENT HEATS

o

If energy is removed the gas reverts back into liquid (@CONDENSATION POINT) And more energy removes turns into a solid (@FREEZING POINT)

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

The heat energy needed to turn 1 g of a substance at the boiling point temperature liquid to gas= THE LATENT HEAT OF VAPOURIZATION

LATENT HEATS The heat energy needed to BE REMOVED to turn 1 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 molecules down and to allow bonds to reform) The heat energy needed to BE REMOVED to turn 1 g of a substance 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 than 20oC – nowhere near the boiling point Therefore – how does liquid turn to vapor at the ocean’s surface? To escape the liquid’s surface a water molecule must take enough energy from its neighboring molecules to become a gas Because energy is taken – when evaporation occurs it cools the molecules left behind = THE LATENT HEAT OF EVAPORATION NB: A greater amount of energy than the latent heat of vaporization

IMPORTANCE OF LATENT HEATS The huge amount of heat exchanged in the evaporation – condensation cycle allows life to be possible on the earth. Energy is moved from hotter regions to cooler regions . Water evaporates in warmer regions In the cooler regions the water is release as precipitation (releasing heat) Heat is also release when ice forms, warming higher latitudes = MODERATE CLIMATE

WATER DENSITY Density = Mass / Volume

The density of water increases as it becomes cooler

How heavy something is

e.g. cold water is heavier than warm water

Density of pure water =1g/cm3 Normally the density of a substance increases as it cools

BUT from 4oC to 0oC the density of water DECREASES

Molecules loose energy→

i.e. instead of contracting it expands

slow down→

Therefore ice is LESS DENSE than liquid water

closer together

→ICE FLOATS

= THERMAL CONTRACTION

Why?

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Below 4oC 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 crystal formation, so water can become colder before the density starts to increase THERFORE seawater becomes ice at temperature below 0oC

Density of water – with and without salt (g/cm3)

MARINE BIOLOGICAL SIGNIFICANCE OF WATER DENSITY In warmer water (i.e. not as dense) plankton have to be smaller 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 warmer water is often found under ice Fish etc. can live in this slightly warmer water layer

SALINITY Salinity = total amount of solid material dissolved in water (including gases) Salinity of seawater = 3.5% i.e. 96.5% pure water content

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/kg3

220 time saltier than “fresh water” NB salinity does not include particles and sediments FLOATING in water = TURBIDITY

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MAJOR COMPONENTS

MINOR COMPONENTS

SALINITY

TRACE COMPONENTS

SALINITY

Salinity varies in the oceans

However, in the Red Sea salinity

from 35 to 38 parts per thousand

= 42 parts per thousand

In coastal areas salinity can be much reduced

=HYPERSALINE WATER

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

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

=BRACKISH WATER (fresh water & seawater mixing)

THE DEAD SEA = 330 parts per thousand 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 October to 36.4 ppt in May & June i.e. when evaporation is high

e.g.2 Astoria, Oregon 0.3 ppt in April & May i.e. when the Colorado River is at the greatest flow rate

to 2.6 ppt in October i.e. the dry season

SOURCES OF SEAWATER COMPONENTS Largest source of seawater components is from streams and runoff But the composition of dissolved substances in stream water is not the same as seawater Why? Because some components have a high RESIDENCE TIME 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 and picks up minerals etc. The entire volume of the ocean may through hydrothermal vents every 3 million years

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)

= major influence on seawater composition

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REMOVAL OF SEAWATER COMPONENTS

PROCESSES THAT DECREASE SALINITY

In addition to some losses in tectonic boundaries/faults

Addition of water content:

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

Precipitation

Also living organisms extract minerals from seawater and when they die → biogenous sediment

Melting of sea ice, icebergs and glaciers Streams, rivers & runoff

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

ACIDITY AND ALKALINITY

An ACID is a compound that releases hydrogen ions (H+) when dissolved in water The resulting solution = ACIDIC

ACIDITY AND ALKALINITY H+ and OH- are always present in small quantities because water molecules dissociate and reform i.e. H2O ↔ H+ + OH-

In pure water

An ALKALINE or BASE releases OH- ions when dissolved in water

number of H+ ions = number of OH-

A stronger acid or alkaline releases more ions

Therefore the solution is neutral (pH =7) The pH scale measure acidity (pH<7) and alkalinity (pH>7)

ACIDITY AND ALKALINITY In the ocean CO2 reacts with water and releases hydrogen ions i.e. H2O + CO2→ H2CO3 → H+ + HCO3-

= 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 second hydrogen ion -

HCO3 →

H+

2-

+ CO3

THE CARBONATE BUFFERING SYSTEM

If the ocean becomes too acidic HCO3- + H+ → H2CO3

(acidity drops)

If the ocean becomes too alkaline/basic The carbonate ion can then react with calcium ions to form Calcium Carbonate

H2CO3 → H+ + HCO3-

(acidity increases)

(which precipitates onto the seafloor = hydrogenous sediment)

Ca2+ + CO32- → CaCO3

This balancing is called BUFFERING

ACIDITY IN DEEP WATER

In deep, cold waters more Carbon Dioxide dissolves in seawater (because gas at cooler temperatures dissolves more easily)

This should make the deep sea more acidic But when marine organisms that contain Calcium Carbonate (in shells or skeletal) die The Calcium Carbonate they release helps to buffer the acidic conditions

The Calcium Carbonate acts like an antacid

VARATION IN SALINITY

VARATION 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 surface waters

DEPTH VARATION IN SALINITY At a depth of 300m to 1000m there is a rapid change in salinity In low latitudes a decrease in salinity In 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

SEAWATER DENSITY

In the ocean seawater density varies

Several factors effect seawater density

1.022 - 1.030

g/cm3

(depending on salinity)

Density has important effects on ocean water Denser bodies of water will sink below less dense bodies of water Therefore – low density water is found at the surface, high density water found in the depths

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

SEAWATER DENSITY & DEPTH

Temperature has the greatest effect on density

In Low Latitudes (equator/tropics)

(temperature varies more than salinity)

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

Increases in temperature at higher temperatures cause 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

(due to good mixing of the surface waters)

Below 300m density increases rapidly until a depth of 1000m From 1000m the density remains relatively constant until the ocean floor

SEAWATER DENSITY & DEPTH 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 constant whether surface waters or deep waters

SEAWATER DENSITY & DEPTH 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 well mixed by currents/tides/waves

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SEAWATER DENSITY & DEPTH

Mixed surface layer Thermocline + Pycnocline = Upper water

Deep water

In High Latitudes thermoclines and pycnoclines rarely form (except during very hot sunny periods) The water column in high latitudes = ISOTHERMAL & ISOPYCNAL

= same temperature & density

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