Rationale
Students will learn that through experimentation disaster could have been
avoided. They will also learn about the physical chemistry of sea ice and its
formation.
Grade Level/Discipline
Middle School/ Earth Science/ Physical Science
Objectives
Students will use inquiry and good experimental technique to get the answer to
a problem, an answer which may surprise them.
National Standards
UCP1,2,5:: A1,2; B 1,2,3; C 4, 5; E 1,2; F 3,5; G 1,2,3.
Teacher Preparation for
Activity
Materials
Cake pans in which water will be frozen, beakers, salt, water, balances,
graduated cylinders, freezer space, world map.
Optional are Seatest Salinity measuring devices available at tropical fish stores,
CBL devices, salinity titration equipment.
Pre-activity set-up
The teacher should assemble the lab materials.
Time Frame
Two periods and internet research time.
Teaching Sequence
Engagement and Exploration (Student Inquiry Activity)
Teacher should use a map for the little lecture on the Jeannette Expedition
below.
Read or recite the history of the Jeannette below:
The Jeannette (http://www.uss-salem.org/danfs/steamers/jeannett.txt) left San
Francisco in July,1879. Her mission was to sail through the Bering Sea to the
Arctic Ocean and then to sail to the North Pole. Her captain, U.S. Navy Lt.
George Washington DeLong, believed that this was possible thinking that a
warm water part of the Kuroshio current should keep the pack ice from forming
along his route. He was wrong, and after twenty months frozen in the ice pack,
the Jeannette was crushed by sea ice . Later her wreckage was found 135
degrees to the east in Baffin Bay. This astounding find caused Nansen to
believe that if he had a ship able to withstand the crushing effects of moving sea
ice, and which could be fitted out for a crew for three years, that he could
perhaps drift with the ice to or near the North Pole, the goal of goals of the
Heroic Age. Being a biologist by training, and a generalist by intellectual bent,
Nansen saw the potential of the scientific research that could be done on such
an exploration. He then designed and had Archer build the FRAM.
But what of the Jeannette. What is her story? Why did most of the
survivors nearly die on their walk back to land? Their diaries relate that they
had no water to drink. But isn't sea ice fresh water? Why did they find it too
salty to drink?
Divide the class into small groups. Students will prepare salt water
solutions, 35 parts per thousand. Have them relate this to percent (3.5 % salt by
weight, perhaps 35 grams of salt dissolved in one liter of water.)
Have the students taste the water. Test the salinity of the water. You may use
CBL ion probes to test the salinity, or determine salinity through titration, or
better and much less expensive, use Seatest density/salinity monitors available
in most home hobbyist aquarium shops.
Tell your students that we are going to freeze the water so that there's a
crust of ice on top and that we will taste it to see if it's fresh water ice.
Before you begin, have the students fill out the following worksheet.
Student Name(s) ______________ Period _____
Jeannette Salinity Predictions
Draw diagrams of your set-up.
1) at outset
2) halfway frozen
3) fully frozen
On each diagram predict where you think you would find salty water, fresh
water, salty ice, freshwater ice.
After tasting the ice and water in the halfway frozen state (and or measuring its
salinity) , write a few words about how successful your predictions were.
Now have the students freeze the water until there's an ice crust, a thin layer of
ice on top. Don't use glass containers. As in the Lesson "FRAM", cake pans are
ideal freezing vessels. Also a large clean plastic basin will work well. A clear
plastic container will allow the successive freezing to be observed.
The students should now test the salinity of the unfrozen water and also the
salinity of the ice crust after melting it. Have them record their results in a small
table.
Explanation (Discussing)
Sea ice may be up to 2 meters thick. It freezes from the top down. TEA Peter
Amatti relates that on video taken under the pack ice in the
Antarctic he saw cones of ice forming at the bottom of the ice as sea water was
freezing. These cones coalesce into the next layer of ice. Out of these cones,
which are the ends of "brine channels" forming in the ice comes streaming
hypersaline (very salty) water . This is easily observable because of the
diffraction they cause in the light surrounding them. Peter mentions, by the way,
that visibility under the ice is extremely good during the Antarctic summer (
austral summer) objects being brightly visible over 200 meters away. So here
is the mechanism for the removal of salt from the sea ice. How then can one
explain that natural sea ice is too salty to drink?
Have your students think of mechanisms to explain this. Have them design an
experiment (full lab report form or a descriptive set of hypotheses to test)
showing that in the natural environment the ice remains too salty to drink.
Elaboration (Polar Applications)
TEA Peter Amatti says that the reasons are two-fold. First, sea spray and wind
carries salt onto the surface of the already frozen ice making it salty, and,
secondly, sometimes the ice freezes so quickly that pockets of salt water are
encapsulated in the fresh ice later to freeze making the whole body of the ice
salty to taste.- see what your students come up with. TEA Charlotte Kelchner
adds that when ice forms it is slightly concave towards the center of the floe thus
trapping and freezing and spray that comes onto the floe. Also, she continues,
sea ice is constantly moving causing pressure ridges and depressions which
scoop up or splash up large quantities of sea water onto the floes.
Exchange (Students Draw Conclusions)
In a class discussion, brainstorm to consensus why natural sea ice has so much
salt in it (occasionally or always?? Is sea ice ever fresh, pure? ) Discriminate
between sea ice and icebergs. Icebergs are the floating remnants of glaciers
which have calved into the sea and they contain fresh water only having formed
on the continent.
Evaluation (Assessing Student Performance)
Teacher can assess the quality of work on the worksheet and the lab report. A
first person report on the Jeannette expedition can be included in a portfolio
document which may also include the lab report, the worksheet and other art
work . Some writing about the Heroic Age is requisite in the portfolio.
Additional work and research that may be included in an authentic
assessment mode might include an essay with maps about why DeLong may
have thought that the Kuroshio current would have carried him to the pole. Map
work showing the extent of the Arctic and Antarctic ice sheets , including data
from this year and last would be of interest.
Authors
Larry Rose, Science Teacher, TEA Associate, Pleasanton Middle School,
Pleasanton, California
.
Background
Students can explore the journals of TEA Besse Dawson who worked on the poor areal development of the ice pack during the
El Nino year of 1997-98, and how this negatively affected the development of
the phytoplankton in Antarctic seas. At high latitudes, phytoplankton are frozen
in the ice and survive in that fashion. Icebreakers churning through pack ice
leave a wake of brown ice behind them, the brown color due to masses of
phytoplankton embedded in the ice. If they are not frozen into the pack ice they
sink and die. Phytoplankton are the base of the food chain in the oceans. The
El Nino year's poor development of sea ice bodes poorly for the krill population
and thus for the populations of krill-eating penguins and cetaceans and all
throughout the food chain.
Phytoplankton which depend on the development of the pack ice for their
survival also, perhaps, are the most efficient way that nature removes carbon
dioxide from the atmosphere. Student research on the carbon cycle, the
greenhouse effect and global warming and its intimate relationship with pack
ice development should provide your students with many avenues for research
into the global ecological importance of polar studies.
Resources
Some web sites to start with include:
Some fine photos of the arctic pack along with personal descriptions at
http://www.voicenet.com/~jstewart/nwt
A paper on the reduction of the size of ice packs overall since 1978 is at
http://www.greenpeace.org/~comms/97/arctic/library/climate/packice.html
Japanese research in the Sea of Ohkotsk on pack ice physics and distribution is
at http://clim.lowtem.hokudai.ac.jp/teion/sirl-e.html.
Student Reproducible Masters
We
look forward to hearing from you! Please review this activity.
Return to top of
page
Back to: TEA
Activities Page
