On how ice freezes from salt water
I’ve been wondering how to best show how sea ice freezes for quite a while. Not just that it freezes, but how brine is rejected. By comparing the structure of fresh water and salt water ice, one can get an idea of how that is happening (and I’ll write a post on that after we have done this experiment in class). But I accidentally stumbled upon a great visualization when preparing dyed ice cubes for the melting ice cube experiment (see this post) when all my ice cubes came out like this:
Ice cubes made from colored water.
Instead of being nicely homogeneously colored, the color had concentrated in the middle of the ice cubes! And since the dye acts in similar ways to salt in the ocean (after all, it IS a salt dissolved in water, even though not the same as in sea water), this is a great analogy. It is even more visible when the ice cubes have started to melt and the surface has become smooth:
The dye has frozen out of most of the ice and been concentrated in the middle of the ice cube.
Clearly, when forming, the ice crystals have been rejecting the dye! In the ocean, due to cooling happening from above, ice would freeze downward from the surface, under the influence of gravity the brine channels would be vertical, and brine would be released in the water underneath. In my freezer, however, cooling is happening from all sides at once. There is a tendency for the dye to be rejected towards the bottom of the ice cube tray under gravity, but as ice starts forming from all sides, the dye becomes trapped and concentrated in the middle of the forming ice cube. Can you see the little brine channel leading to the blob of color in the middle?
I must say, when I first took the ice cubes out of the freezer I was pretty annoyed because they weren’t homogeneously colored. But now I appreciate the beauty of the structure in the ice, and you can bet I’ll try this again with bigger ice cubes!
Melting ice cubes reloaded | Adventures in Oceanography and Teaching says:
[…] food dyes are the devil. My whole kitchen is red. Plus the ice cubes didn’t freeze nicely (for a post on ice cubes freezing from salt water click here), the ice chipped when I tried to get the cubes out of the ice cube tray. I definitely can’t […]
The icy elevator | says:
[…] So what is going on there? I think the solution to this riddle lies in me forcing ice to freeze even though it contains more salt (or in this case, red food dye) than it is happy with. Remember how dyed ice cubes look? […]
Properties of sea ice and fresh water ice | says:
[…] “on how ice freezes from salt water” I talked a bit about how dye was rejected when I tried to produce colored ice cubes for […]
Treibholz podcast, my final episode in 2020 - Adventures in Oceanography and Teaching says:
[…] no, it’s already my last Treibholz podcast episode this year! We are looking at ice cubes frozen from fresh and salt water, and are having a lot of fun, as always when we do #KitchenOceanography! Thanks for having me, […]