Chemical and Physical Features of Sea Water and the World Ocean

Chapter 3

The Waters of the Ocean

• Earth is the only planet with “liquid” water.• Marine organisms cannot control the physical

and chemical nature of their environment so they have to “grin and bear it.”

• Organisms must adapt, live somewhere else, or die.

• Marine organisms consist of mostly water. Most are 80% water. Jellyfish are 95% water!

The Unique Nature of Pure Water

• Let’s take a minute to review:• Atoms• Molecules• Hydrogen bonds• Water is made of two hydrogen atoms bound

to an oxygen atom by two hydrogen bonds.

The Three States of Water

• Any substance can exist in three different states/phases:

• Solid• Liquid • Gas• Water is the only substance that “naturally”

occurs in all three states on earth.

Liquid Water

• Liquid water is held together by Hydrogen bonds. These bonds move, so they break and reform constantly.

• Temperature reflects the average speed of water. The higher the temperature the faster the water molecules are moving.

• When water molecules move fast enough and break free of hydrogen bonds it escapes to the gas/vapor phase. This process is called “evaporation.”

Water Vapor

• Water vapor molecules are not held together by hydrogen bonds. They are separate and much farther apart.

• As temperature increases the rate of evaporation increases.

• Water boils at 100C (212F) when nearly all the hydrogen bonds are broken and the molecules enter the vapor state.

Solid Water

• When liquid water cools, molecules move slower and pack closer together to take up less space.

• When the temperature of water decreases the volume of water decreases without changing the mass, so the water gets denser.

• Remember= D=m/V

Seawater

• As seawater gets colder, it gets more dense and sinks.

Freshwater

• As freshwater gets colder, it becomes less dense and floats.

• That is why a sheet of ice can cover the top of a lake or pond allowing aquatic life to remain alive below.

Water Crystals

• Crystals form when water molecules move slowly and the hydrogen bonds lock in a 3-D pattern.

• Ice crystals have molecules that are farther apart than liquid water.

• Ice expands as it freezes.

Heat and Water

• Ice is held with hydrogen bonds.• These bonds must be broken for ice to melt.• Ice melts at a much higher temperature than

similar substances.• If not for the hydrogen bonds ice would melt

at -90C (-130F) instead of 0C (32F)!

• Water absorbs a lot of heat when it melts.• The Latent Heat of Melting- is the amount of

heat to melt a substance.• Water has a higher latent heat of melting than

any other common substance.• A GREAT deal of heat must be removed to

freeze water. It takes a long time before complete body of water will freeze solid!

Drinks stay cold because any heat goes to melting the ICE, not raising the

temperature of the drink.

Heat Capacity

• Is the amount of heat need to raise a substances temperature by a given amount.

• Water has one of the highest heat capacities.• It can absorb a lot of heat and raise the

temperature very little.• This provides for utility as a coolant for

automobiles and allows marine life not to be subject to rapid changes in heating and cooling of their habitats.

Latent Heat of Evaporation

• Ability of water to absorb a great deal of heat when it evaporates.

• Evaporative cooling- when the fastest molecules leave the liquid phase take heat with them and those left behind on the organism, such as human skin, lower our body temperature.

Water as a Solvent

• Water can dissolve many more things than any other natural substance.

• Ex: Salts- which are made of ions. Na+ Cl-

Salt crystals in water attract water molecules with their charges like magnets. The ions dissociate (pull apart) and dissolve.

Seawater Composition

• Seawater is composed of particles that are dissolved in the water by different means.

• Weathering- produces some of the solids that are found in seawater.

• Hydrothermal vents- release sediments into seawater from underwater geothermal activity.

Salt Composition- made of mostly 6 ions in 99% of seawater.

• Sodium chloride accounts for 85% of the solids in seawater.

• When seawater evaporates salts are left behind.

• Salinity- is the total amount of salt dissolved in seawater.

• Ex: If 35g of salt are left from 1000g of seawater evaporation then the seawater’s salinity is 35%.

• Salinity is measured today with electrodes because salts are wonderful conductors.

Practical Salinity Units (psu)

• Units of salinity from conductivity measurements.

• Why is this important?• The degree of salinity affects organisms.• Not just the total amount of salt is important,

but also the composition of the salt.• Some organisms may tolerate NaCl, while

withstanding KCl.

• Chemist William Dittmar analyzed water samples from the Challenger and found that the major ions in seawater remained constant even though the total amount varied from place to place.

• For example, Cl was always at 55.03%.• This finding led to The Rule of Constant

Proportion.

• Water is removed from the ocean primarily by evaporation.

• When the water freezes the ions in the water do not freeze so frozen seawater becomes fresh!

• This frozen water makes up the polar ice caps.• The salinity of the water surrounding the ice

caps is very saline because of these remaining ions in the seawater.

• Water is added to the seas by precipitation and on a small scale glacial melting.

• The average ocean salinity is 35%. Most seawater varies between 33%-37%.

• The Red Sea has high evaporation and is 40% saline.

• The Baltic Sea has a great deal of runoff from fresh water sources and is 7% saline.

Salinity/Temperature/Density

• Temperature affects density.• The greater the temperature of the seawater

the more dense is so it?• Salinity affects density.• The saltier the seawater the more dense the

water is so it?

• Temperature in the open water varies from -2C to 30C (28F-86F).• In seawater temperatures can occur below 0C

(32F) because seawater freezes at a cooler temperature.

• This makes the ocean less prone to freeze then lakes and rivers.

• Temperature in the ocean is measured with specially designed bottles and thermometers lowered to a desired depth.

• Temperature Profiles- graphs generated from temperatures taken at different depths.

• They reflect a water column- vertical column of temperatures of water at their respective depth.

Temperature Profiles of Ocean

Tools for data collection.• Researchers can also plot salinity or density at

different depths using different probes.• Today CIDs are used to measure- Conductivity,

Temperature, and Depth of ocean water.• Bathythermographs (XBTs) are used to

measure temperature and are disposable.• Niksin bottles collect individual water sample

from different depths.• Satellites measure conditions near the surface

instantaneously.

Dissolved Gases in Water

• The most important gases in seawater are:• Oxygen• Carbon Dioxide• Nitrogen• These gases are dissolved in the water from

the atmosphere into the surface seawater and are released back into the atmosphere through evaporation (gas exchange.)

Water Cycle

• Gases dissolve better in cold water.• There are more dissolved gases found in polar

waters.• Oxygen gas is not very soluble only 0-8ml of

oxygen will dissolve in 1L of seawater. Usually about 4-6ml.

• In the atmosphere there is about 210ml of oxygen in 1L air.

Photosynthesis/Respiration

• Photosynthesis and respiration affect the amount of dissolved oxygen in seawater.

• Most of the oxygen produced in the ocean is by photosynthesizing organisms that release oxygen into the atmosphere as a product of this process.

• Seawater has little dissolved oxygen.

• Carbon dioxide is much more soluble.• It makes up 80% of the dissolved gas in the

ocean and stores 50 times as much CO2 as the atmosphere!

• This makes the ocean a critical component to controlling the greenhouse effect by absorbing a great deal of greenhouse gases that could be in the atmosphere increasing global

Transparency• Water is transparent• This is the ability for light to penetrate the water

and allows photosynthetic organisms to grow.• Remember, sunlight has all the colors of the

rainbow, and not all colors are going to penetrate equally.

• Clear ocean water is most transparent to blue.• Transparency can be affected by large quantities

of plankton (algae). They reduce transmittance of light from the surface.

Color Transparency of Ocean Water