Properties of Seawater

OEAS-604August 31, 2011

Outline:

1) Molecular structure of water2) Thermal characteristics of water3) Water as a solvent—Salinity4) Composition of seawater5) Density and the Equation of State6) Adiabatic Effects

Lecture 1

The Water Molecule Is Held Together by Chemical Bonds

A water molecule is composed of two hydrogen atoms and one oxygen atom. The water molecule has a positive and a negative side, and is referred to as a polar molecule.

A molecule is a group of atoms held together by chemical bonds.

Chemical bonds are formed when electrons are shared between atoms or moved from one atom to another.

Molecular Structure of Water Gives Rise to Hydrogen Bonds

Hydrogen bonds form when the positive end of one water molecule bonds to the negative end of another water molecule.

This gives water a number of unique properties:

1.Water becomes less dense when it freezes2.Water has unusually high boiling and melting points3.Water has very high heat capacity, latent heat of fusion, and latent heat of vaporization4.Water is a powerful solvent5.Water conducts electricity6.Water is slightly compressible

Lattice structure of ice makes it less dense than liquid water

Hydrogen Bonds Lead to Unusually High Freezing and Boiling Points for Water

Hydrogen Bonds give Water High Heat Capacity

Heat is energy produced by the random vibrations of atoms or molecules.Temperature is an object’s response to input or removal of heat.Heat Capacity is a measure of the heat required to raise the temperature of 1 g of a substance by 1C.

Water has a very high heat capacity, which means it resists changing temperature when heat is added or removed.

A calorie is the a measure of heat defined as: the amount of energy required to raise 1 gram of water 1 °C.

A joule is the SI unit for energy.

1 calorie = 4.186 joules

Hydrogen Bonds give Water High Latent Heat of Evaporation/Fusion

For water to evaporate, heat must be added to water in the liquid state. After water reaches 100C, an input of 540 cal/gram is required to break the hydrogen bonds and allow evaporation.

The amount of energy required to break the bonds is termed the latent heat of vaporization.

Water has the highest latent heat of vaporization of any known substance.

Latent Heat is the heat required to change state.Sensible Heat is the heat added/removed that changes the temperature

1. When water evaporates, it removes significant amounts of heat. The energy that is added to break the hydrogen bonds to allow evaporation is removed by the water vapor. Sweating cools you down on a hot day.

2. When ice melts, the ice absorbs large quantities of heat from the surrounding liquid. Adding ice to your drink cools it down quickly.

Water Is a Powerful Solvent

Salt in solution. When a salt such as NaCl is put in water, the positively charged hydrogen end of the polar water molecule is attracted to the negatively charged Cl- ion, and the negatively charged oxygen end is attracted to the positively charged Na+ ion. The ions are surrounded by water molecules that are attracted to them and become solute ions in the solvent.

A Few Ions Account for Most of the Ocean’s Salinity

A representation of the most abundant components of a kilogram of seawater at 35‰ salinity. Note that the specific ions are represented in

grams per kilogram, equivalent to parts per thousand (‰).

Why is Ocean Salty

Most obvious source of salt is the input by river water.

Rainwater contains dissolved gases (mainly CO2 and SO2) which form acidic solutions in water.

This leads to chemical weathering of continental rock:

CaCO3(s) + CO2(gas) + H2O = Ca2+(aq) + 2HCO3-(aq)

Calicite, common mineral in

sedimentary rocks

From rainwater In solution

Weathering of sedimentary rocks:

2NaAlSi3O8(s) + 2CO2(gas) + 3H2O = Al2Si2O5(OH)4(s) + 2Na+(aq) + 2HCO3-(aq) + 4SiO2(aq,s)

Albite, common mineral in igneous and

metamorphic rocks

From rainwater In solution

Weathering of igneous or metamorphic rocks:

Kaolinite, clay mineral

Silica, partly in solution

1) Materials are put in through river discharge, precipitation, and hydrothermal activity.

2) Materials are removed through sedimentation and biologic activity.

Residence Time = Amount of element in the ocean

Rate at which the element is added to (or removed from) the ocean

Mixing TimeTime it takes for a substance to become uniformly distributed.

Mixing time for the global ocean is estimated to be 1,600 years. This mixing is driven by the large scale circulation in the ocean.

When the residence time is much longer than the mixing time of the ocean, materials will behave conservatively and will maintain constant proportionality.

Short residence times lead to nonconservative behavior:Chemically and biologically reactive materials often exhibit nonconservative

behavior in the ocean.

Accomplished by molecular diffusion,

which is enhanced by turbulence

t = 0 t = tmix

Salinity May Vary

…but ratio of ions in seawater remains constant

Forchhammer’s principle or the principle of constant proportions

SalinityOld Definition: “The salinity of a sample of sea water represents the total mass of solid material dissolved in a sample of sea water divided by the mass of the sample, after all the carbonates have been converted into oxide, the bromine and iodine replaced by chlorine, and all organic matter completely oxidized.”

But this is difficult to measure exactly even in a laboratory ( some salts become gases when heated )1) Definition based on Chlorinity:

Salinity in parts per thousand = 1.80655 × Chlorinity in parts per thousand

2) Definition based on Conductivity:Electrically conductivity of seawater is proportional to amount of salt in solution.

3) Practical Salinity Scale (1978) – official definitionDefined Salinity based on the conductivity ratio relative to a KCl standard.Because it is a ratio, it is dimensionless. Reported as psu (practical salinity units)

Chlorinity is easier to measure and principle of constant proportions allows this to be converted directly to salinity.

Conductivity is a function of temperature, salinity, and pressure. So to measure salinity, you must measure Conductivity, Temperature and Depth (CTD).

1. Very important to oceanographers because it controls vertical stratification and baroclinic pressure gradients.

2. Measured in kilograms/cubic meter [kg/m3]

3. Range: 1020 at surface —> 1070 kg/m3 at depth

4. Density is a function of pressure, temperature, salinity

5. Because of the small range in density, (the first two digits never change) density is often reported as a density anomaly:

6. Some oceanographers still use specific volume (α), which is the inverse of density

Density (ρ)

S,T ,P 1000

1

Density of Seawater is calculated from the “Equation of State”Official equation of state from UNESCO (1983):

Density of Seawater is calculated from the “Equation of State”Official equation of state from UNESCO (1983):

This is way too complicated, so there are software programs that do all this for you:

i.e. MATLAB Seawater toolbox (sw_dens)

1027 0.15 T 10 0.78 S 35 4.510 3P Approximation of the Equation of State

Or in differential form:

T S KP

where = the coefficient of thermal expansion, = coefficient of saline expansion, and K= isothermal compressibility coefficient. All coefficients are functions of salinity, temperature and pressure. However both and K only change slightly with average values of = 7.8 10-4 per psu and K = 4.5 10-6 per decibar. In contrast ranges from 0 to 3.4 10-4 per degree C over the range of 0-30C.

1

T

1

S

As a general rule of thumb a change in density of 1 kg/m3 results from:

1) A temperature change of 5°C2) A salinity change of 1.2 ppt3) A pressure change of 200 decibars (200 meters depth)

90 % of Ocean Water

Mean T & S for World Ocean

Increased salinity:1.Increase density2.Lowers freezing point3.Lowers temperature of maximum density4.Lowers evaporation rate

Effects of Salinity on the Properties of Seawater

Adiabatic Temperature Changes

Definition: Changes in temperature that occur independently of any transfer of heat to or from the surrounding environment.

Adiabatic temperature changes occur because of the compressibility of fluids

1) As air rises, it experiences lower pressure so it expands.

2) Same # of molecules are now moving around in a much bigger volume (less energy).

3) So the temperature falls, even though no heat has been removed from the volume.

Potential Temperature (θ)

Definition: The temperature a fluid would attain if brought adiabatically to a pressure of 1000 millibars (i.e. approximately sea-level).

Potential temperature is different from the actual in-situ temperature you would measure with a thermometer!

In oceanography density is usually represented as the density anomaly:

1000

Three different definitions for density anomaly (σ):

S,T,P 1000

t S,T,0 1000

S,,0 1000

“Sigma”: The in-situ density anomaly.

“Sigma-t”: The density anomaly a parcel would have if it were brought

to the surface.

“Sigma-theta”: The density anomaly a parcel

would have if it were brought to the surface

adiabatically.

Difference between In-situ and Potential Temperature

In-situ Temperature Potential Temperature

In-situ temperature appears unstable with colder water over warmer.