# Why does seawater in the Polar Regions become more dense as sea ice forms? and Why is this process called “Brine Rejection”

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Why does seawater in the Polar Regions become more dense as sea ice forms? and Why is this process called “Brine Rejection”

Does water reach its maximum density at its freezing point?

Water molecules move further apart as the temperature of water falls below 4º C. The hexagonal crystals of ice are approximately 9% less dense than liquid water There must be another reason why polar seawater becomes more dense as sea ice forms.

Three closely related scientific measurements can be made to determine why seawater becomes more dense as sea ice forms. The Density of a solution is defined as its mass per unit volume. As an example, the density of pure water has a density of gram per cubic centimeter at 4º Celsius. Salinity is defined as the total number of grams of “salts” dissolved in one kilogram of a saltwater solution. There are 1000 grams of water in a kilogram of water. Therefore, salinity is sometimes expressed as parts per thousand: for example: 35‰. Specific Gravity is the ratio of the density of a substance to the density of a given reference material. Specific gravity usually means relative density with respect to water. The term "relative density" is often preferred in modern scientific usage.

The Salinity, Density, and Specific Gravity Connection
As the amount of solute dissolved in a solvent increases, the density of the solution increases. As the density of a solution increases, the specific gravity of the solution also increases.

Measurements are made both directly and indirectly.
Oceanographers use a CTD device to analyze changes in the density of seawater. A Conductivity-Temperature-Depth recorder, measures the salinity, temperature, pressure, depth and density of the seas and oceans. Measurements are made both directly and indirectly.

Hydrometers will be used to make indirect measurements of changes in the salinity of saltwater.
If you lower a hydrometer into a tall tube containing a liquid or a solution, it will sink to a certain depth. If a hydrometer is in pure water at 4º Celsius, the scale on the hydrometer should indicate that the specific gravity of the water is 1.0. Saltwater contains dissolved “salts” and is more dense than pure water. 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, and vice versa. You goal is to observe changes that occur in the salinity of saltwater as ice forms on the surface of the saltwater..

Today, you will begin to explore why sea water become more dense as sea ice forms as you:
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 and let surface ice form overnight.

Tomorrow you 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.

Lets evaluate the hydrometer data.
How does the specific gravity of melted “sea ice” compare with the specific gravity of saltwater that did not freeze? Why might this process be referred to as “brine rejection”? How might brine rejection (also referred to as brine exclusion) be one of the driving forces of the Thermohaline Circulation?

Brine Exclusion Animation The University Corporation for Atmospheric Research (UCAR) provides a learning module about ocean currents that includes an animation of brine exclusion.

Additional hands-on activities relating to the formation of Sea Ice Include:
Using an evaporation method to determine the salinity of saltwater. Calculating the Heat of Fusion of Water. Collecting data to construct a diagram that reveals the rate of change in temperature as water changes from a solid to a liquid to a vapor. Determining the relationship between the temperature and the maximum salinity of sea water. Determining the relationship between the salinity of sea water and the freezing point of sea water.

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