Seawater Chemistry

Seawater overview  The characteristics of seawater are due both to the nature of pure water and to the materials dissolved in it.  The Salinity of seawater is the concentration of solutes, mostly salts, dissolved in the water.

Seawater  The ocean’s salinity varies from about 3.3% to 3.7% by weight.  The average salinity is usually given as 3.5% or 35 parts per thousand ( 0/00 ).  Most of the dissolved salts in the ocean have been separated into ions. The most abundant of these are sodium and chloride.

Seawater: Heat Capacity  The Heat capacity is the amount of energy needed to raise the temperature of a substance 1 degree Celsius.  The salinity of seawater will raise the Heat Capacity, so more energy will be needed to raise the water temperature.

Seawater  As the salinity increases, the freezing point of water becomes lower; the salts act as a sort of antifreeze. Therefore sea ice forms at a lower temperature than freshwater lakes.  Dissolved salts tend to attract water molecules. Therefore seawater evaporates more slowly than freshwater.

Seawater composition  3.5% of seawater consists of dissolved substances.  If we were to boil away 100 kilograms of seawater we would be left with a residue weighing 3.5 kilograms.  Oceanographers prefer to use parts per thousand notation rather than percent (parts per hundred).

Seawater

 The constancy of the ocean seems to suggest that ions are being added to the ocean at the same rate they are removed.  Ions are removed by precipitating out and becoming part of the sediments on the ocean bottom or by being extracted from seawater by animals and plants and used to form skeletal materials as well as being used in metabolic processes.

Seawater: Dissolved Gases  Most gases in the air readily dissolve in seawater and the ocean’s surface.  Plants and animals living in the ocean need these gases for photosynthesis and respiration.  No marine animal has the ability to break down water molecules to obtain oxygen directly.

Seawater: Dissolved gases  The most important gases in the ocean are Oxygen, Carbon dioxide, and Nitrogen.  They dissolve better in cold than in warm water, so dissolved gas concentrations are higher in polar waters than in the tropics.  Most of the Oxygen produced by organisms in photosynthesis is released to the atmosphere. Carbon Dioxide makes up more than 80% of the dissolved gas in the ocean.

Seawater pH  Seawater is slightly alkaline with an average pH of about 8.  At first this sounds odd because there are large amounts of carbon dioxide dissolved in the ocean. Dissolved carbon dioxide combines with water to form carbonic acid.

Seawater  Some carbonic acid breaks down to produce hydrogen ions, bicarbonate ions, and carbonate ions.  The net effect of the bicarbonate and carbonate ions is to buffer seawater, so it remains close to the neutral pH.

Seawater: Density  Seawater is denser than regular water due to the presence of solutes such as NaCl.  The density of seawater is affected by temperature and salinity: it gets denser as it gets saltier, colder, or both

Seawater: Temperature  Temperature in the ocean varies between -2° and 30°C. Temperatures can be lower than 0°C because seawater freezes at a lower temperature than fresh water.  The deeper, the colder is the water.

Seawater: Transparency  Transparency is one of the most important factors for living things, since it determines the amount of sunlight that reaches the organisms. This is vital because all photosynthetic organisms need light to grow, and the whole ecosystem usually depends on them as producers.

Seawater: Light penetration  Sunlight has all the colors of the rainbow, but not all of them penetrate the water equally: in clear waters, blue penetrates better than the other colors.  The different radiations are absorbed as light penetrates deeper, and the last one is always the ultraviolet. That’s why at certain depths no red color can be seen.

Seawater: Pressure  Pressure is the weight of the air and/or the water over an organism. Water is heavier than air, so pressure increases faster in the ocean than on land.  The pressure on land and on the surface of the water is 1 atmosphere. In the ocean, it increases 1 atm every 10 m of depth.

Seawater: Pressure  As the pressure increases, gas-filled structures (bladders, lungs, etc) sink or collapse. This limits the depth range for many marine organisms.

Ocean Circulation  The water of the oceans is constantly moved by currents, waves, and tides. These forces mix waters and transport heat, nutrients, pollutants, and organisms.  The Coriolis Effect affects all the movements on the surface of the Earth, tending to turn movements to the right on the North Hemisphere and to the left on the South Hemisphere.

Wind Patterns  The winds are driven by the heat energy coming from the sun: in the Equator, the air is warmer, becomes less dense, and rises. Air from surrounding areas gets sucked in to replace the rising air, creating wind.

Wind Patterns  The winds that move towards the Equator are called Trade Winds, and are the most steadiest. They are affected by the Coriolis Effect, hitting the Equator at an angle of 45º.  At middle latitudes lie the Westerlies winds, and the Polar Easterlies at high latitudes. These are not as steady as the Trade Winds.

Wind Patterns  The winds of the atmosphere push the sea surface, creating currents.  The Coriolis effect causes surface currents to deviate from the wind that drives them.  Lower water layers move at progressively greater angles from the wind in a pattern known as Ekman Spiral.

Surface currents  The result of the Ekman Spiral is the Ekman transport, in which the surface currents move perpendicular to the wind direction, to the right in the North Hemisphere and to the left in the South Hemisphere.

Surface currents  The surface currents are warmer in the western sides of the ocean basins and colder in the eastern sides. When they combine, they form circular systems called gyres that act like a giant thermostat in the planet: warming the poles, cooling the tropics, and regulating the climate.

Quiz  1. When an atom gains an additional electron it becomes:  a. An atomb. An ion  c. A moleculed. A quark

Answer  1. Ion

Quiz  2.The oxygen and hydrogen atoms in the water molecule are held together by:  a. Covalent bonds. Ionic bonds  c. Ester bondsd. Hydrogen bonds  3.When hydrogen bonds form between adjacent water molecules they form a “skin” which allows the water to support a razor blade. This is called:  a. adhesionb. capillary action  c. cohesiond. tension

Answers  3 a. Covalent bonds  4. C. cohesion

Quiz  5.Water is:  a. A good solvent  b. A poor solvent  c. A moderately good solvent  d. A very poor solvent  6.Pure water is:  a. A good conductor of electricity  b. A poor conductor of electricity

Answers  5. a. good solvent  6. b. A poor conductor

Quiz  7.The average salinity of the open ocean is:  a. 10 ppt.b. 25 ppt.  c. 35 ppt.d. 55 ppt.  8.True or false. Ice forms at a lower temperature in fresh water than in salt water.

Answers  7.C. 35 ppt.  8.False

Quiz  9.The most abundant ion in seawater is:  a. Magnesiumb. Chloride  c. Sodiumd. Sulfate  10.The average pH of seawater is:  a. 6.2b. 7  c. 7.5d. 8.2

Answers  9. b. Chloride  10. D. 8.2

 The End