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A Weddell Seal at a breathing hole. Courtesy NOAA Corps Collection
Photographer: Giuseppe Zibordi. Credit: Michael Van Woert, NOAA NESDIS, OAR
“Antarctica is the coldest, driest, windiest continent on the planet. Temperatures can plummet to -58°F, which is 90°F below freezing…Antarctica is so cold that most of the ice there never melts; the continent is permanently covered in ice. Yet, Weddell seals can live there…because some water remains unfrozen, and they can dive and re-surface through these holes in the ice. How do these holes stay open?…The answer is, it’s a joint effort between the seals and the properties of the water.”
From the Web site of Dr. Terrie M. Williams, Professor, Ecology and Evolutionary Biology, University of California Santa Cruz https://williams.eeb.ucsc.edu/education/for-teachers/580-2/freezing-introduction/
What is the most obvious property of seawater? It’s salty! But if you mix salt from your kitchen into a glass of water, it doesn’t taste exactly like seawater. That’s because seawater contains many other chemicals in addition to sodium chloride (which is ordinary kitchen salt), such as magnesium sulfate, magnesium chloride, and calcium carbonate. Scientists call the content of all dissolved salts in seawater “salinity,” and measure it in parts per thousand (abbreviated ppt or ‰), which is equivalent to grams per kilogram. Freshwater has a salinity of 0‰; normal seawater has a salinity of about 35‰.
Salinity makes seawater very different from freshwater. Most animals have a specific range of salinities that they can tolerate, and cannot survive if the salinity is above or below their tolerance range. Changes in salinity can affect the circulation of the oceans, and may even affect climate. Because salinity influences our environment in many ways, NOAA keeps track of salinity in many places along the U.S. coasts and around the world. Here are some experiments you can do to discover some of the most important properties of seawater.
An iceberg in Gerlache Strait, West Antarctica
Photographer: Rear Admiral Harley D. Nygren, NOAA Corps (ret.) Courtesy NOAA.
Pure water freezes at 32°F (0°C), but adding salt lowers the freezing point of pure water. This is why salt is sometimes used to keep ice from forming on sidewalks. When water freezes, it forms crystal-like structures. When salt water freezes, only the water forms these structures; the salt is left out in unfrozen water. So as salt water freezes, the water that is not frozen becomes saltier.
After 24 hours in your freezer, the cup containing fresh water should be frozen solid (if it isn’t, your freezer isn’t working!). The salt solution probably contains some ice, but it is not frozen solid. It may appear slushy, and you should have no trouble sticking your finger through whatever ice is in the cup.
Salt water is more dense than freshwater, so freshwater floats on top of salt water. The greater density of salt water also means that objects float more easily in salt water than in freshwater. Remember Archimedes’ Principle, which says that an object in a fluid is buoyed up by a force equal to the WEIGHT of the fluid displaced by the object (Boat Building Challenge Activity Boat Building Challenge - Discover Your World) One cup of salt water weighs more than one cup of fresh water, so its buoyant force is greater. So your egg (if it was fresh) sank in freshwater, but was buoyed up by the salt water.
NOAA wildlife biologists Mike Goebel and Birgitte McDonald get a Southern elephant seal ready for measurement and tagging, as part of the U.S. Antarctic Marine Living Resources research program that provides scientific information needed to conserve and manage marine living resources in the oceans around Antarctica. Courtesy NOAA/Scott Seganti.
