Question for a Physicist or Chemist

I have an ice cube tray that lives in my freezer. Earlier today, I used the last of my ice cubes, so I filled it with water and stuck it in the freezer again. Just now (maybe an hour later? It’s been a few hours at most), I open up the freezer again, and see this:

(taken out of the freezer and photographed with a Kleenex box as a backdrop) The tops of the ice cubes are frozen, but there is still liquid water underneath. The interesting thing, though, is that one spot has risen up about half a centimeter higher than the rest. What caused this to happen? Why isn’t the surface totally flat, like it was when I put it in?

Details that may or may not be important:

  • I know there is liquid water underneath because I thought there wasn’t and flexed the tray to crack the cubes loose, and some water came out and then there were air bubbles floating around under the cubes.
  • I kinda over-filled the tray, so that all the cubes are connected by a thin sheet of ice that goes over the plastic dividers in the tray.
  • When I filled the tray with water, there were some small bits of ice stuck in the bottom of the tray, maybe a third the size of a normal ice cube. These were left over from cubes that had cracked in two, where I used the top part but couldn’t get the bottom part out. I expect they’ll become part of the new batch of ice cubes, but I haven’t checked on that yet.
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  1. MMartin says:

    Magic. That’s the only possible explanation.

  2. Shannon says:

    My friend suggested an experiment: freeze distilled water and see if the same thing happens. If it does not then maybe it has to do with impurities in the water affecting how it freezes. My thought was that it might be something like growing rock candy where the stuff kind of grows on itself.

    • Alan says:

      Unfortunately, I can’t reproduce this phenomenon. I’ve been making ice cubes for years, and I don’t recall this ever happening before. So, if I freeze distilled water and nothing happens, I can’t confidently say that this is caused by a difference between distilled and tap water. …Also, how would this be caused by impurities in the water? I don’t see where your friend is coming from here.

      Your idea hadn’t occurred to me before; thanks! Water has crazy surface tension and cohesion properties, so this seems very plausible to me (though I’m much less informed than I’d like to be here).

      My current guess is that the whole surface froze except for that one point, and then water below the surface froze, which expanded its volume and pushed water up out of the hole, which froze as it rose. I wonder if the water on the bottom froze more quickly than usual because of the bits of ice that were already in the troughs of the tray, which leads back towards your crystal-growing idea…

      • Shannon says:

        His thought was that there might be some sort of non-uniform freezing … kind of like your idea that that point froze last.

        My mom’s suggestion was that it might be something like an iceberg: a bit of ice broke off from what was already there and floated to the top before the water froze around it.

        • Alan says:

          Your mom might be onto something! It fits perfectly with the small bits of ice left over in the tray already, which could explain why I haven’t noticed this with previous batches of ice cubes. I could try to reproduce by filling the tray with tiny ice cubes, then overfilling it and seeing if it happens again.

  3. MMartin says:

    Water expands when it freezes. If there already is a thin sheet of surface ice over the body of water, further freezing can force water out and upwards through a crack or weak point in the sheet. This can produce a tube-like structure where water emerges at the tip, progressively lengthening the tube.[1] Tube formation stops when the tip freezes and seals.

    • Alan says:

      You, sir, win today’s internet. Thank you! :-D

      and it sounds like both my theory (it’s formed by pushing water out a small hole by water expanding as it freezes) and Shannon’s friend’s idea (distilled vs. nondistilled water is significant) were both partially right. I can’t tell if Shannon’s idea (formed like rock candy) is related or not; this is done through supercooling the water, rather than supersaturating it, but both involve phase changes due to symmetry breaking in an unstable equilibrium.

      It’s neat how many people were all dancing around the right idea from so many different angles!

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