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

Review: Properties of Pure Water Physical Properties Chemical Properties Cohesion/Surface tension Found in all three states on the earth Universal Solvent – dissolves more substances than any other common liquid Conduction of heat – highest of all liquids (except for mercury) pH – water dissociates into anions (OH-) and cations (H+) Latent heat of vaporization – highest of all common substances Polarity – water has positive and negative ‘ends’ Latent heat of fusion – high for a molecule of its size Hydrophobic effect Heat capacity – highest of all common solids & liquids Density – max at 4ºC for pure water Viscosity –relatively low for a liquid

The 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 H 105º

O H The 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 O H

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? Density-Temperature Relationship Graph

Why do fish not get electrocuted when lightning strikes the ocean Why do fish not get electrocuted 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.

Why do coastal areas have slower temperature changes than inland areas Why do coastal areas have slower temperature changes than inland areas? Heat capacity 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. Off-shore breeze On-shore breeze

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

How can water bugs “stride” across water without breaking the surface How can water bugs “stride” across water without breaking the surface? Surface tension and cohesion 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.) www.nps.gov/olym/insect/gerridae.jpg

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

Seawater Seawater is 96.5% water 3.5% dissolved compounds Sources 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

Salinity Total amount of dissolved salts in seawater. 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.

Surface Salinities of the Oceans

Dissolved 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 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 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 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 Buffering 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)

pH 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 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>.