Chemistry of Water

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Chemistry of Water
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I. Water

• H20 is a compound of 2 hydrogen atoms and one
  oxygen atom in a fixed proportion.
• H20 is held together by covalent bonds (sharing
  of e-)
• H20 is bent into a 105 angle
• Water is electrically unbalanced because of the
  angle and thus considered polar because of the
  () and (-) end
• H bonds form between water molecules and other
  charged substances due to the polarity. This
  allows water to itself, a process known as
  cohesion
• Adhesion is when water H bonds to other materials
• Water is a powerful solvent it will dissolve
  almost anything

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II. Seawater

• A. NaCl (salt) dissolves in water because of
  its polarity (Na Cl-)
• B. 97.2 of Earths surface water is marine
• C. Seawater is 96.5 water and 3.5 dissolved
  substances (mostly salts)
• D. Earth has 5.5 trillion tons of salt
• E. Salinity is the total quantity of dissolved
  inorganic solids in water (NOT just salt!)
• F. Salinty is usually 3.3-3.7 depending on
  evaporation, precipitation, and fw runoff
• H. Nearly every element found in the crust and
  atmosphere is also present in the ocean
• I. Major constituents of seawater include H, O,
  Cl, Na, Mg, Ca, K, Sulfate, and bicarbonate
• J. Elements lt1 ppm are called trace elements

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III. Sources of ocean salts

• Weathering- running water dissolves crustal rock
• Excess volatiles- hydrothermal vents (underwater
  volcanoes) on the ocean floor leak chemicals
  (C02,Cl, S, H, F, N) into the water

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IV. Determining salinity

• Proportion of Cl to salinity is constant
• Salinity in 1.81 x Cl
• Equipment
• Nansen bottle- bottle is sent down upside down on
  a string, flipped open at a desired depth, and a
  sample of water is taken
• Salinometer- electronic electricity conductor
• Refractometer- measures the degree the light is
  bent (refraction)

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V. Chemical equilibrium

• The ocean is in chemical equilibrium
• For the most part, ions are added to the ocean at
  the same rate they are subtracted
• Certain ions have longer residency times then
  others
• Addition of salts from the mantle and weathering
  are balanced by the subtraction of minerals bound
  into sediments

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VI. Colligative properties of seawater

• The heat capacity of seawater decreases with
  increasing salinity (saltwater is less able to
  hang on to heat)
• As salinity increases, freezing point decreases
  giving seawater a natural antifreeze property
  (saltwater freezes at a lower temp than fw)
• Salt water evaporates more slowly than fw (salt
  hangs onto water)
• The osmotic pressure of organisms increases with
  increasing salinity (organisms lose more water
  when salinity is higher)

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VI. Dissolved gases

• Gases dissolve most readily in cold seawater
• Plants and animals in the ocean require dissolved
  gases in order to survive
• A. Nitrogen
• 48 of gas in ocean (atmosphere 78)
• living organisms require N to build proteins, but
  bottom dwelling bacteria must fix the N into a
  useable form for them
• B. Oxygen
• 36 of gas in ocean (atmosphere has 100x more)
• Primary source of O in ocean is from plants
• most of the oxygen is near the surface and
  diffuses into the atmosphere
• Carbon dioxide
• 15 of gas in ocean (60x more in ocean than
  atmosphere)
• used by marine plants- low at surface

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VII. pH

• Acidity (release of H) and alkalinity (release
  of OH-) is measured by pH
• pH scale 0-------------------7----------------
  --14
• acid neutral base
• Pure water is neutral (7)
• Seawater is slightly alkaline (avg. 7.8)
• The ocean contains buffers to prevent large
  swings in pH when acids or bases are introduced