The most common examples of freezing point depression referenced in GChem 1 or Introductory Chemistry (a survey course for non-majors or requirement/recommendation for allied health students such as nursing or radiology) are liquid automobile antifreeze, or road salting during winter. But I recently recall an episode on BBC's Frozen Planet showing footage of a supercooled brine solution that was freezing the salty ocean water immediately near it. I thought to myself, "Now this is a unique example of freezing point depression!" This phenomenon is informally known as the Icy Brinicles of Death because the supercooled brine solution formed also freezes any sea life that comes in contact with it as can be seen in the embedded BBC video below (run time = ~2 mins).
Knowledge about brinicles is not novel. They have been known about since the 1960s. However, they have been rarely observed forming in real time. Currently it is known that this unusual phenomenon only occurs in specific conditions under floating sea* ice in the frigid Arctic Ocean and the Southern Ocean of Antarctica. Unlike fresh water ice, sea surface ice is composed of H2O plus some salt, mostly NaCl. A sea ice crystal is relatively pure as water "pushes out" most of the salt while freezing. The exuded salt water (= brine) is higher in salt concentration (and hence lower in water concentration) than the surrounding salty sea water and remains as a liquid on account of its lowered freezing temperature due to its high concentration. The brine solution's freezing temperature is depressed compared to pure water or less concentrated saline sea water. Ice is not solid throughout its bulk but rather has varying degrees of porosity depending on how and how fast it was formed. There are highly concentrated brine channels within a porous mass of floating sea ice. When floating sea ice cracks the supercooled, dense brine solution leaks out to the surrounding and sinks as the liquid brine solution is more dense than the less salty sea water around it. On account of its greater density the supercooled brine solution sinks and being at a lower temperature than the surrounding cold sea water, it freezes the sea water (and sea life) that it comes into contact with. Since the brine is more dense than the salty sea water and accordingly sinks, the brinicle grows downward. Occasionally, a brinicle may reach the sea floor as recorded in the video. As a brinicle grows, it 'catches' various bottom-dwelling creatures such as sea urchins and starfish around it, enclosing them in an icy tomb and giving its informal moniker Icy Finger of Death.
If you like the video above and used it in your classroom, please provide in the comments how you used the video and maybe also include what your students thought of it. Thanks...Tom
*: In this post sea and ocean are used interchangeably and mean the same thing- salty water.
Note from ChemEd X Two-Year College Editor (Scott Donnelly): I took some liberty to include a small part (in italics below) of Samuel Taylor Coleridge's haunting poem, The Rime of the Ancient Mariner. Coleridge reminds us quite pointedly about how the seas- earth's largest geographical features- though comprised of vast amounts of water are not fit to drink- "Water, water, every where, Nor any drop to drink." To read the poem in its entirety click here. Idea- Perhaps English and chemistry faculty can collaborate. How so? Chemistry faculty give a guest lecture on the science of the poem.
The ship drove fast, loud roared the blast,
And now there came both mist and snow,
And through the drifts the snowy clifts
The ice was here, the ice was there,
Want to know more about this bizarre natural phenomenon? Check out the sources below.
Anthony, Franz. "Ice finger of death beneath the Antarctic ice". https://eartharchives.org/articles/brinicle-ice-finger-of-death-filmed-in-antarctic/index.html. Accessed June 14, 2021.
Cartwright, Julyan H.E.; Escribano, Bruno; Gonzalez, Diego L.; Sainz-Díaz, C. Ignatio; and Tuval, Idan. Brinicles as a Case of Inverse Chemical Gardens. Langmuir 2013 29(25), 7655-7660. DOI: https://pubs.acs.org/doi/10.1021/la4009703
Main, Douglas. "How Eerie Sea Ice 'Brinicles' Form". Scientific American LiveScience, April 26, 2013. https://www.scientificamerican.com/article/how-eerie-sea-ice-brinicles-form/. Accessed June 14, 2021.
Marlow, Jeffery. "Swimming Beneath the Brinicles, in Antarctica". WIRED, May 7, 2013. https://www.wired.com/2013/05/swimming-beneath-the-brinicles-in-antarctica/. Accessed June 14, 2021.
And for more cool (pun intended) ice formations found in the natural world check out the BBC Earth Unplugged video below. Accessed June 14, 2021.