CHEMISTRY OF WATER NOTES

Why is the ocean salty?

Sea salts come mostly from the weathering of rocks on the land.

Sea salts can also be formed from water moving through underwater fissures (hydrothermal vents).

Sea salts can also come from underwater volcanoes.

How does weathering form sea salts?

Rock is slowly dissolved by running water.

Fresh water has a pH value of about 7, which is neutral.

It would take 200-300 million years to replace all of the salts in the ocean by river intake.

How do hydrothermal vents create sea salts?

Hydrothermal Vents have only been discovered about 20 years ago.

These cracks allow water to seep into the crust, where it is heated from magma, close to the surface. This hot water dissolves more of the minerals trapped in the rocks.

It could replace all of the oceans salt in about 10 million years.

How do volcanoes replace sea salts?

The salts from volcanoes fall into the ocean and dissolve.

In underwater volcanoes, the minerals dissolve in the seawater directly, including the salts.

Chilean Volcano

How are the salts removed from the ocean?

by evaporation deposits left behind when shallow seas dry up

by the burial of sediments with water trapped in the pore spaces

biological processes and chemical reactions with other ions can remove salts

Are the oceans getting saltier?

The oceans are in a “steady state” because the concentration of salts have been about the same for about 1.5 billion years.

It is the result of the removal rate of salts being equal to the input rate.

What is salty?

fresh water is less than 1 ppt

brackish water is about 5-25 ppt

Salt water is anything above_____

the average salinity of seawater is 35 ppt

Is salt only “salt”?

NO – eleven major ions make up more than 99% of the total dissolved elements found in seawater.

The salty taste of seawater is actually the taste from all of the dissolved elements found there.

Chemical Composition of Sea Water

Probably every known naturally occurring element can be found.

So many they must be divided into subgroups.

Only a few in large quantities.

Components of Sea Water Water – 96.5% by weight.

Dissolved substances – 3.5% by weight.

Major Ions

11 elements that form these ions account for 99% of dissolved ions (salts).

The “Big Six Ions” are the top 6 ions.

Minor ions and other constituents

All other known elements - 1%

Dissolved gasses

Nutrients

Organic compounds

Radioactive elements

Top 10 Salts (Ions)

Cl (Chloride) Na (Sodium)

Mg (Magnesium) S (Sulfide)

Ca (Calcium) K (Potassium)

Br (Bromide) C (Carbon)

Sr (Strontium) B (Boron)

Chemistry of Water

Water is a “polar” molecule: meaning, there is an uneven distribution of electron density.

The ability of ions and other molecules to dissolve in water is due to its polarity.

The unique properties of water are due to the hydrogen bonds.

ice floats

water’s high heat of vaporization

strong surface tension

high specific heat

nearly a universal solvent

The Gases in Seawater

The most abundant gases in the atmosphere and ocean:

1. Nitrogen (N) 2. Oxygen (02) 3. Carbon dioxide (CO2) They are very important because they

are necessary to life, and biological activities modify their concentration at different depths.

The Gases in Seawater

Colder water holds more dissolved gas than warmer water.

Less salty water holds more dissolved gas than saltier water.

Water under pressure holds more dissolved gas than water under less pressure.

Photosynthesis plants (plankton, kelp, etc.) use

CO2, lets off O2

mainly occurs at the top 100-200 meters

Respiration

animals use O2, lets off CO2

occurs at all depths

Decomposition

uses O2, lets off CO2

**Oxygen can only be added at the surface, from exchange with the atmosphere or as a waste product of photosynthesis.

**Carbon Dioxide enters the water through the exchange with the atmosphere at the surface as well as all depths in the water.

Oxygen in the Oceans

Oxygen maximum (top 200 meters)

Oxygen minimum (700-1000 meters)

Oxygen is concentrated in deep water because cold water holds more dissolved gas.

The distribution of O2 and CO2 with depth.

Density of Water Notes

Density - Affected by salinity (‰ or ppt), Temperature (˚), & pressure (atmospheres).

Conductivity - Ability for water to conduct heat or electricity Increases with higher dissolved salts

Density

Affected by:

1. Salinity (‰ or ppt)

2. Temperature (˚)

3. Pressure (atmospheres)

Layers of the Ocean Thermocline - A layer in the ocean where the temperature rapidly decreases with depth. Halocline - A layer of water in which the salinity increases rapidly with depth. Pycnocline - A layer of water in which the density increases rapidly with depth. Usually corresponds closely to the halocline & thermocline.

Modifiers

*Seasonally The depth of these layers can be affected by changes in evaporation, precipitation & solar energy.

Modifiers

*Globally The latitude of the water can affect these layers.

Tropics Sunlight penetrates deeper in lower latitudes, so the tropics generally have deeper thermoclines.

Poles Polar waters do not usually have a thermocline b/c surface water is as cold as deeper water.