The temperature of the Southern Ocean rarely rises above 2°C. McMurdo Sound averages -1.87 degrees, and ranges between -1.4 and -2.15°C. Arthur Harbor is relatively similar in size of temperature fluctuation. From December through February, temperatures don't increase much, ranging from -1.9°C to -1.8°C. In such sub-freezing conditions, why don't fish freeze when their blood is much like fresh water? Is it possible for fish to be cooled below the freezing point of water, and yet for their bodily fluids to remain liquid?
Under special experimental conditions, fish have been observed functioning in ice-free cold salt water at a temperature of -6°C! Research (largely pioneered by Art DeVries, who appeared in the first Live From Antarctica series), has found that these fish have eight types of anti-freeze molecules which bathe the interior surface of their skin, acting as a barrier to ice propagating in from outside. When the anti-freeze molecules are not present, ice filters through their skin at these temperatures and crystallizes (freezes) their blood and tissues.
Objective
Students will experiment with lowering the freezing point of a substance, thus causing it to remain liquid at a temperature when it is normally solid. Students will compare their findings with facts about Antarctic ice-fish, which have bodily fluids that remain liquid at temperatures below freezing.
Materials
Procedure
The chemically super-saturated solution begins to crystallize around the "seed" (the sodium thiosulfate crystal dropped in, step 8) immediately and continues to do so until all the chemical-which is normally solid at room temperature-has changed state. The test tube was heated to dissolve the solid (touch the test tube, to feel heat being released). This is very similar to what happens to water. Heat is added to melt ice, and heat is given off when water solidifies back into ice. The reaction takes place below our body temperature so we cannot feel it.
Fish, with blood containing sugars and salts, have a freezing plateau below that of fresh water (probably app. -0.8°C.) Antarctic fish have the ability to supercool (-2.2°C) under ice in McMurdo Sound without crystallizing (freezing). Thus, they can live under pack ice where salt water is below the freezing temperature of fresh water, but only so long as no ice enters their body. They crystallize if they cool by as little as 0.1°C when an ice crystal penetrates the skin and seeds the reaction.
Post a "Super Scientist Challenge Question" each day for extra credit. Students can respond in their Logbooks or turn in their written explanation at the end of the class.
Why do we put salt on roads in winter? Salt on roads mixes with ice and water on roads. More heat must be removed from the mixture of ice and water for it to reach its freezing temperature than from pure water. This happens because the molecules have to overcome adhesion of water to the salt molecules in order to arrange themselves in the ice crystal lattice. We put salt on roads because the water remains liquid at a lower temperature. Below -5° or so, everything freezes.
Why do we put salt in ice when making old fashioned ice cream? And do we put the salt in, or outside, the container? The cream inside the container is not pure water-thus its freezing temperature is lower than the freezing temperature of water. If we just pack ice around the metal canister of cream, the ice may be above the freezing temperature of the cream. In order to make the cream freeze, liquid on the outside of the container must be colder than the cream inside the container. The ice outside the container must melt so that the salt can be added. (Since salt lowers the freezing temperature to -5° or so, the ice at zero degrees is now above the freezing temperature of the water-salt mixture). To melt the ice, heat has to be absorbed from somewhere. The ice cream maker is well insulated so the only place where the heat needed to melt the ice can come from is the metal cream container. Thus the melting of the ice (caused by the salt) draws heat from the metal canister holding the cream and lowers the temperature of the cream, so it freezes. Notice, when the ice is all melted, the cream is unable to freeze any more.
Ed. note: OK, the next question is not about salt...but we think it should provoke some warm discussion amid all this talk of freezing!
Why did pioneer cooks not make fudge in rainy weather? Surprisingly enough, wet air is lighter in weight than dry air. H2O weighs less than N2-given a uniform distribution of gas molecules. Since wet air weighs less, it does not take as much energy for water molecules to jump out of the beaker (boiling) so water boils at a lower temperature on rainy days. The glucose molecules take longer to cook at this lower temperature-and since water evaporates more easily (trying to jump out of the beaker against less pressure), dry spots appear more easily. These dry spots tend to crystallize the fudge making it grainy rather than smooth.
Ice. When there is so much of one thing, how can one word possibly do it justice? As students work through the Live From Antarctica 2 Activities, have them look for, list and characterize all the different types of ice they encounter.
Read Kurt Vonnegut's science fiction novel, Cat's Cradle, where a new kind of ice, "Ice-nine", ends up destroying life on Earth in a fantastic extension of the seed/crystallization phenomena.
Illustrate the life cycle of krill in relation to the ice pack ecosystem.
In Antarctica ice freezes at the rate of 2.2 square miles per minute. How great an area would this cover in one hour? (132 sq. miles) Twelve hours? (1584 sq. miles) Twenty-four hours? (38,016 sq. miles) On a state or national map, chart this area starting from your own town as the center point.
Track the changes in the Antarctic ice sheet using NIH imaging software.
http://octopus.gma.org/surfing/antarctica/ice.html
Lessons and resources created by ICAIR's LEARNZ
(Linking Education with Antarctic Research in New Zealand).
http://icair.iac.org.nz/~psommerv/web/lessonpl/learnz/learnz96/seaice.htm
University of Colorado National Snow and Ice Data Center. Education resources
for teachers with links to studies on Antarctic ice and useful information
about the cryosphere, climate and global change, and remote sensing.
http://www-nsidc.colorado.edu/NSIDC/coldlinks.html#EDUCATE
The Teel Family of Alaska shares their web site full of fun snow activities!
http://www.teelfamily.com/activities/snow/
