Chapter 3: Chapter 3: Chemical and Physical Chemical and Physical Features of Seawater and Features of Seawater and the World Oceanthe World Ocean

Properties of SeawaterProperties of Seawater

Review: Properties of Pure WaterReview: Properties of Pure Water

Physical PropertiesPhysical Properties Chemical PropertiesChemical PropertiesCohesion/Surface tensionCohesion/Surface tension

Found in all three states on the Found in all three states on the earthearth

Universal Solvent Universal Solvent – dissolves more – dissolves more substances than any other substances than any other common liquidcommon liquid

Conduction of heatConduction of heat – highest of all – highest of all liquids (except for mercury)liquids (except for mercury)

pHpH – water dissociates into anions – water dissociates into anions (OH(OH--) and cations (H) and cations (H++))

Latent heat of vaporizationLatent heat of vaporization – – highest of all common substanceshighest of all common substances

Polarity Polarity – water has positive and – water has positive and negative ‘ends’negative ‘ends’

Latent heat of fusionLatent heat of fusion – high for a – high for a molecule of its sizemolecule of its size

Hydrophobic effectHydrophobic effect

Heat capacityHeat capacity – highest of all – highest of all common solids & liquidscommon solids & liquids

DensityDensity – max at 4ºC for pure – max at 4ºC for pure waterwater

ViscosityViscosity –relatively low for a liquid –relatively low for a liquid

The Water MoleculeThe Water Molecule

• Two hydrogen atoms, one oxygen atom

• H atoms form 105º angle• This angle produces an

asymmetrical dipole. Slight (+) charge on the H atoms and slight (-) charge on the O atoms.

O

HH

105º

The Water MoleculeThe Water Molecule

• These slight charges cause the (+) H atoms of one water molecule to attract the (-) O atoms of other water molecules.

• These weak bonds are called hydrogen bonds.

• Water can hydrogen bond with other substances aside from itself.

O

H

H

O

HH

O

H

H

Why does ice float? Why does ice float? Density-Temperature Relationship

Background

• Temperature is a measure of kinetic energy (KE). As KE decreases, hydrogen bonds stay formed and break less. Water molecules stay closer together until…

Explanation• As the temperature approaches 4ºC, less dense ice clusters

begin to form in the liquid.• At 0ºC when all water molecules become locked in the

rigid ice lattice, the hydrogen bonds actually hold molecules farther apart than at 4ºC. This creates spaces making the water less dense.

Why does ice float? Why does ice float? Density-Temperature Relationship Graph

Why do fish not get electrocuted Why do fish not get electrocuted when lightning strikes the ocean? when lightning strikes the ocean? Conductivity

Explanation

• Conductivity is a property that measures the ability of a substance to transmit heat, electricity, or sound.

• Pure water is not a good conductor of electricity. Its conductivity is about 20 dS/m. (Compare with silver – the highest conductivity with 63 x 106 S/m)

• In addition, the electrical charge of lightning usually spreads instantaneously along the surface of the water from the location of the strike and to a lesser degree below the surface at the strike site. Fish in other areas are not affected.

Background

• Heat – energy of molecular motion

• Water can absorb or give up heat by conduction (molecular exchange of heat) or convection (mixing)

Explanation

• Water can hold heat longer and release heat more slowly than land.

• Temperature differential between land and ocean will cause uneven heating of air masses which drive winds and moderate any drastic temperature changes.

Why do coastal areas have slower Why do coastal areas have slower temperature changes than inland areas?temperature changes than inland areas?Heat capacity

Off-shore breeze

On-shore breeze

• Fats and oils are nonpolar molecules.– These compounds do not have slight regions

of charge like water does.

• Therefore water molecules are not attracted to nonpolar molecules and actually can be repelled by them.– Basis for cell membranes and the water

repellency of marine mammals and birds.

Why don’t fats and oils dissolve in water?Why don’t fats and oils dissolve in water?Polarity

• The cohesion or mutual attraction of water molecules creates a flexible barrier on the surface of water.

• This helps support aquatic insects such as water striders (Halobates sp.)

How can water bugs “stride” across water How can water bugs “stride” across water without breaking the surface?without breaking the surface?Surface tension and cohesion

www.nps.gov/olym/insect/gerridae.jpg

• Because water is polar, it dissolves most substances, especially other polar molecules and compounds composed of ions, atoms or molecules that carry an electrical charge.

• These ionic compounds are often called salts.

• NaCl (salt) most common dissolved salt in ocean. There are many others.

• Seawater is a solution of these salts.

Why does water dissolve more substances Why does water dissolve more substances than any other common liquid?than any other common liquid?The Universal Solvent

Salt crystal

SeawaterSeawaterSources of salts and dissolved solids:• Erosion of rocks and soil• Breakdown of organisms• Condensation of rain from the atmosphere• Releases from hydrothermal vents

Seawater is• 96.5% water• 3.5% dissolved compounds

SalinitySalinity

• Measured in parts per thousand (ppt)• Average value 35 ppt or 35‰• Range from 0‰ near river mouths to 40% in the dead sea.• Globally, seawater salinity remains constant. Rule of

constant proportions states that the percentage of various ions in seawater remains constant.

• Total amount of dissolved salts in seawater.

Surface Salinities of the OceansSurface Salinities of the Oceans

Dissolved compounds in seawaterDissolved compounds in seawater

• Inorganic substances (salts, nutrients)

• Dissolved gases

• Organic compounds (fats, oils, vitamins, amino acids, proteins)

• Nitrates and phosphates (usually in excess as pollution)

• Pollution (DDT, PCBs, chlorinated hydrocarbons that are synthetic)

Dissolved GasesDissolved Gases

• Primarily nitrogen, carbon dioxide, oxygen

• N2 biologically inert.

• CO2 needed for photosynthesis and pH buffering.

• O2 required for respiration.

              

                                

www.abc.net.au/science/news/ stories/s269960.htm

Factors that Affect SalinityFactors that Affect Salinity

• Salinity increases due to…– freezing of seawater.

– evaporation.

• Salinity decreases due to…– melting of icebergs and sea ice.

– precipitation.

– run-off from rivers.

Seawater pHSeawater pH• The carbonic acid – bicarbonate – carbonate system keeps

seawater at a pH value between 7.5 and 8.4.• The oceans are an enormous “sink” for atmosphere CO2

The Carbon The Carbon Buffering SystemBuffering System

• Seawater has an unusually large capacity to absorb CO2.CO2 + H2O H2CO3 (carbonic acid)

H2CO3 H+ + HCO3 – (bicarbonate ion)

HCO3 – H+ + CO32- (carbonate ion)

pHpH

• The amount of hydrogen ions in a substance is referred to as pH.– pH = (pondus hydrogeni or “power of

hydrogen”– pH = -log10 [H+]– Scales ranges from 0 to 14.

• Therefore, a pH of 14 means that the water is very alkaline (basic) while pH 1 means it is acidic. A pH of 7 is neutral.

Work CitedWork Cited

Kreger, Chris. "Acid Mine Drainage: Alkalinity." Exploring the Environment: Water Quality. 2004. Wheeling Jesuit University. 2 Oct 2008 <http://www.cotf.edu/ete/modules/waterq/wqalkalinity.html>.

Lower, Stephen. "A gentle introduction to water and its structure." H2O. 2008. 1 Oct 2008 <http://www.chem1.com/acad/sci/aboutwater.html>.

"October 2006 Archives." [Weblog The Marine Electronics Weblog] Oct 2006. Panbo . 1 Oct 2008 <http://www.panbo.com/archives/2006_10.html>.

Water Conductivity. 2008. Lenntech Water Treatment & Purification B.V.. 1 Oct 2008 <http://www.lenntech.com/water-conductivity.htm>.

Woodruff, Steve W.. "Water & Weather." Los Angeles Pierce College Weather Station. 1 Oct 2008 <http://data.piercecollege.edu/weather/water.html>.