Seawater Chemistry70% of the Earth is covered by ocean water!

Seawater CompositionOcean 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)

ReviewWhy is water such a good

solvent?Answer: Because it is a polar

molecule.

Seawater CompostionThere 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

SalinitySalinity: 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 PPTFresh water is <1 PPTHypersaline water (Dead Sea) can be over

40 PPT). ◦ Use Salinometer to measure salinity

Principle of Constant ProportionsStates 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.

Determining SalinityDue 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%.

4 Colligative Properties of SeawaterHeat capacity of water decreases

with increasing salinityAs 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).

Factors that Affect Salinity1.) 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.

ApplicationDuring 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.

Sources of SaltThree Major Sources

◦Volcanic Eruptions◦Chemical reactions between sea

waters and hot, newly formed crustal rocks.

◦Weathering of continental crust and erosion due to rivers.

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.

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

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.

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.

Seawater pH

Closer to 1 means more AcidicCloser to 14 means more Basic (a base)7 is neutral

Seawater pHpH: 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.