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Seawater Chemistry

Seawater Chemistry. 70% of the Earth is covered by o cean water!. Seawater Composition. Ocean water is made up of three things: 1.) Dissolved Gases 2.) Organic Molecules 3.) Salts Seawater is a solution. Solution: Made of a solute and a solvent. Solute: a dissolved substance (salt)

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Seawater Chemistry

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  1. Seawater Chemistry 70% of the Earth is covered by ocean water!

  2. Seawater Composition • Ocean water is made up of three things: • 1.) Dissolved Gases • 2.) Organic Molecules • 3.) Salts • Seawater is a solution. • Solution: Made of a solute and a solvent. • Solute: a dissolved substance (salt) • Solvent: the dissolver (water)

  3. Review • Why is water such a good solvent? • Answer: Because it is a polar molecule.

  4. Seawater Compostion • There are 1,370 million cubic kilometers of water with 5,000 Trillion kilograms of salt! (Enough to cover entire planet with 45 meters of salt!!!!) • Seawater is: • 3.5% dissolved solids (salts) • 96.5% H2O

  5. Salinity • Salinity: the total quantity of dissolved solids in a solution. • Measured in Parts Per Thousand (PPT). • i.e. 2 PPT of salt would mean there are 2 grams of salt for every 1,000 grams of water. • Ocean water has a salinity of 34-35 PPT. • Brackish water (like the Chesapeake Bay) is about 15-20 PPT • Fresh water is <1 PPT • Hypersaline water (Dead Sea) can be over 40 PPT). • Use Salinometer to measure salinity

  6. Principle of Constant Proportions • States that: Seawater is constantly mixed, which means the ionic composition of open-ocean seawater is the same at every place and depth, and the ratio of one major constituent to another is the same. • Amount of salts can vary but ratio will always be the same. • For example, a high salinity sample from the North Atlantic and a low salinity sample from the Antarctic will both have 55.04% Chloride Ions but they may have a different amount.

  7. Determining Salinity • Due to the previous principle, we can use an easy formula. • Salinity in PPT = 1.80655 x Chlorinity in PPT. • Chlorinity-A measure of the total mass of halogen ions (flourine, chlorine, bromine, and iodine) which is always 55.04%.

  8. 4 Colligative Properties of Seawater • Heat capacity of water decreases with increasing salinity • As salinity increases, the freezing point lowers (freezes at -2 degrees C) • Seawater evaporates more slowly than fresh water • Osmotic pressure rises with increasing salinity (pressure on cell membranes is higher).

  9. Factors that Affect Salinity • 1.) Water Temperature. Colder water dissolves less salt. • 2.) Evaporation. Warmer waters near the equator are saltier because of evaporation. • 3.) Precipitation. More rainfall dilutes the saltwater making it less salty. • 4.) Surface runoff from rivers. Again, makes water less salty. • 5.) Melting Glaciers and Sea Ice. Makes water less salty.

  10. Application • During what season would sea surface salinity be less saline? Spring or Winter? • Why? • Answer: Spring. More rain means more runoff which would dilute the salt content.

  11. Sources of Salt • Three Major Sources • Volcanic Eruptions • Chemical reactions between sea waters and hot, newly formed crustal rocks. • Weathering of continental crust and erosion due to rivers.

  12. Chemical Equilibrium and Residence Time • Chemical Equilibrium: Proportion and amounts of dissolved salts per unit volume of ocean water are constant. What is put in also comes out in equal amounts. • Residence Time: The average length of time an atom of an element spends in the ocean. • Residence Time = Amount of element in the ocean/Rate at which the element is added (or removed) from the ocean.

  13. Dissolved Gases • Atmospheric gases dissolve into the sea at the ocean surface and is facilitated by wind and waves. • Major gases include: • Nitrogen: 78% in Atmosphere, 48% dissolved in seawater • Oxygen: 21% in Atmosphere, 36% dissolved in seawater • Carbon Dioxide: 0.035% in Atmosphere, 15% dissolved in seawater

  14. Amount of dissolved gas is affected by: • 1.) Temperature: Colder water can dissolve more gas. (A warm soda gets flat because you lose Carbon Dioxide). • 2.) Salinity: Higher Salinity = less dissolved gas. • 3.) Pressure of water: Lower pressure = less dissolved gas.

  15. Distribution of gases with depth depends on: • Photosynthesis: More carbon dioxide dissolved. Oxygen output. Important in first 100 meters (photic zone). • Respiration: Oxygen dissolved, carbon dioxide output. • Decomposition: Requires oxygen, produces carbon dioxide. Important for consuming oxygen at greater depths. • Compensation Depth: Depth at which photosynthesis balances rate of respiration. • Oxygen minimum at 500-800 meters.

  16. Seawater pH Closer to 1 means more Acidic Closer to 14 means more Basic (a base) 7 is neutral

  17. Seawater pH • pH: Concentration of hydrogen ions in a solution. • Seawater has an average pH of 7.8 so it is slightly basic or alkaline. • Due to formation of bicarbonate which gives more OH- ions.

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