Why does the salinity of seawater in the Polar Regions increase during winter months? Why is this process called Brine Rejection?

Why does the salinity of seawater in the Polar Regions increase

during winter months?

Why is this process calledBrine Rejection?

You can develop an understanding You can develop an understanding of brine rejection as sea ice forms if you: of brine rejection as sea ice forms if you:

• Make a saltwater solution. Make a saltwater solution. • Determine the salinity of the saltwater solution.Determine the salinity of the saltwater solution.• Form ice on the surface of the saltwater solution.Form ice on the surface of the saltwater solution.• Separate the surface ice from the unfrozen Separate the surface ice from the unfrozen

saltwater.saltwater.• Let the surface ice melt.Let the surface ice melt.• Determine the salinity of the melted surface ice.Determine the salinity of the melted surface ice.• Determine the salinity of the saltwater that did not Determine the salinity of the saltwater that did not

freeze.freeze.• Hypothesize why the process of forming sea ice Hypothesize why the process of forming sea ice

called brine rejection.called brine rejection.

Salinity of Seawater Salinity of seawater is the total number of grams of

“salts” dissolved in one kilogram of a saltwater solution when:– all carbonates have been converted to oxides – bromide and iodide ions are replaced by chloride

ions – all organic matter is oxidized – Salinity is expressed either as grams of solute per

kilogram of solution rather then grams of solute per liter of solution.

– Salinity is also expressed as parts per thousand: for example: 35‰

Salinity can be determined by an evaporation method.

• Measure the mass of a saltwater sample.

• Evaporate water from the saltwater sample.

• Let the salts dry completely.• Measure the mass of the dry salts.• Use a ratio to calculate the number of

grams of salts that would have been in one kilogram of saltwater.

Oceanographer use a CTD device to measure the salinity of seawater.

CTD instruments have

become integral in measuring water characteristics such as salinity, temperature, pressure, depth and density.

A CTD recorder, which stands for Conductivity-Temperature-Depth recorder, measures some of these characteristics directly and others indirectly.

http://www.windows.ucar.edu/tour/link=/earth/Water/CTD.html&edu=high

Simple multimeters are not sophisticated enough to detect changes in the salinity.

You can use a hydrometer to detect changes in the specific gravity of saltwater as surface ice forms.

• You will lower a hydrometer into a tall tube containing tap water and saltwater a tall tube and it will sink to a certain depth.

• If the hydrometer is in pure water at 4º Celsius, it will indicate that the specific gravity of that water is 1.0.

• If the hydrometer is in a saltwater solution, it will float higher in the liquid and indicate that the specific gravity is greater than 1.0.

• The higher the salinity of the saltwater, the higher the hydrometer will float in the saltwater.

• You goal is to observe the changes that occur in the salinity of saltwater as surface ice forms.

What is Specific Gravity/ Salinity Connection?

• Specific gravity is defined as the ratio of the density of a given solid or liquid to the density of water at a specific temperature and pressure, typically at 4 °Celsius and 1 atmosphere of air pressure (760.00 mm Hg).

• Because the mass of water 1 cm3 (at that temperature and pressure) has been arbitrarily established as 1.0 gram of mass, that means that specific gravity is an indication of the density of a substance.

• As more salt is dissolved in water, the saltwater solution becomes more dense.

• Therefore, a hydrometer can be used to make an indirect measurement of the changes in the density of saltwater as surface ice forms.

Begin the brine rejection activity.

• Measure the temperature and specific gravity of tap water.

• Make a saltwater solution.• Measure the temperature and specific gravity

of the saltwater solution.• Put the saltwater into triple insulated foam

cups.• Put the cups in a freezer until tomorrow

morning.

Tomorrow we will:• Remove the foam cups from the freezer.• Separate the surface ice from the unfrozen

saltwater.• Let the surface ice melt and warm up to the

appropriate temperature.• Let the saltwater that did not freeze also

warm up to the appropriate temperature.• Measure the specific gravity of the melted

surface ice and the saltwater that did not freeze.

• Analyze the changes in the specific gravity of the saltwater as surface ice formed.

Why is the change in the salinity of the saltwater referred to as brine rejection? How is brine rejection (also referred to as brine exclusion) one of the driving forces of the Thermohaline Circulation?

What might be other factors that influence the Thermohaline Circulation?

Brine Rejection in a Soda Bottle

Brine Rejection in a Mason Jar

Brine Exclusion AnimationBrine Exclusion AnimationThe University Corporation for The University Corporation for Atmospheric Research (UCAR) Atmospheric Research (UCAR)

provides a learning module provides a learning module about ocean currents that includes an about ocean currents that includes an

animation of brine exclusion.animation of brine exclusion.

http://www.meted.ucar.edu/oceans/ocean_models/brine_exclusion.html

Brine rejection creates a downward plume of more dense water.

A comparison of freshwater ice and saltwater ice

Additional hands-on activities relating to Additional hands-on activities relating to the formation of Sea Ice Include:the formation of Sea Ice Include:

• Calculate the Heat of Fusion of Water.Calculate the Heat of Fusion of Water.• Construct a diagram that reveals the rate Construct a diagram that reveals the rate

of change in temperature as water of change in temperature as water changes from a solid to a liquid.changes from a solid to a liquid.

• Determine the relationship between the Determine the relationship between the temperature and the maximum salinity of temperature and the maximum salinity of sea water.sea water.

• Determine the relationship between the Determine the relationship between the salinity of sea water and the freezing point salinity of sea water and the freezing point of sea water.of sea water.