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I had a sprain in my leg a few days back. The doctor recommended dipping my foot alternately in ice-cold and hot water to aid blood circulation. It is here that I discovered something interesting.

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The picture above shows the piece of ice that was put in the bucket. The above picture shows the ice cube from above.

If you look at the large piece of cube from the side (see below), you can see that the upper part of the cube, that was near the open surface of the container in which the ice froze, seems to be almost transparent and has a crystalline appearance. The lower part does not have this appearance, and it is white and opaque.

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Why is there a difference in the layers of ice in the large cube? Is it because the water was from tap and not completely pure? The water was put in the refrigerator for a period greater than 12 hours, so the ice has frozen properly. Can anyone explain this unique structure of ice? I've never seen this before.

Update:

This update is to simply demonstrate the bubble formation in the ice,which causes the cloudiness. Out of the two answers, I had accepted the one by @IliaSmilga . Today, the ice formed demonstrated this idea clearly. Given below are the pictures in which the bubbles of dissolved gas are clearly visible.

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The last two pictures are the best.

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    $\begingroup$ Just google "What Makes The Center Of Ice Cubes White?". Plenty of articles on this. :) Here's one relevant link $\endgroup$
    – user139621
    Nov 5, 2017 at 9:36
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    $\begingroup$ @Blue I didn't know that. Anyways, a sprain can sometimes help in learning new things. :-D $\endgroup$
    – Wrichik Basu
    Nov 5, 2017 at 10:09
  • $\begingroup$ Feel the learn! $\endgroup$
    – Vendetta
    Nov 8, 2017 at 15:14
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    $\begingroup$ @Vendetta yes :-) $\endgroup$
    – Wrichik Basu
    Nov 8, 2017 at 15:16

2 Answers 2

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As @pr1268 explained, tap water is not pure: it contains dissolved gases (basically air) and dissolved minerals.

However, I do not think stratification causes this phenomenon: I think that as long as water remains unfrozen, the dissolved gas concentration remains approximately constant throughout the solution (basically equal to the saturation concentration). Here is an alternative explanation.

When ice crystals start to form, they naturally tend to exclude the impurities; so the impurities are "squeezed out" into the part of the water that remains liquid. Eventually the impurity concentration exceeds the saturation threshold, and they start to precipitate out of the solution. But by this time, the ice has already formed an airtight enclosure around the liquid water which prevents gas bubbles from escaping to the surface; and the mineral crystals stay no matter what.

For this reason, on a big lake where only a small portion of the total water freezes, you can get crystal clear ice (unless the water surface has snow, slush or other impurities to begin with).

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  • $\begingroup$ This is the correct explanation. The stratification concept is not very correct. It simply has to do with dussolution of gases. $\endgroup$
    – Wrichik Basu
    Nov 6, 2017 at 5:28
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    $\begingroup$ @WrichikBasu I'm wondering though. You asked for citations on the other answer but accept this one without citations. This is an interesting question and I think a more absolute answer is in order, instead of 2 well-informed opinions. $\endgroup$
    – Mast
    Nov 6, 2017 at 13:36
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    $\begingroup$ @LateralTerminal At home, boiling is probably the simplest method; Google has other suggestions. $\endgroup$
    – Andrew Morton
    Nov 6, 2017 at 16:53
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    $\begingroup$ @AndrewMorton I bodied water, and then froze it. It became crystal clear. $\endgroup$
    – Wrichik Basu
    Nov 6, 2017 at 18:11
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    $\begingroup$ @LateralTerminal you can get clear ice by putting water in a cooler then putting that in the freezer without a lid. This causes it to freeze top down, like a lake, and all the impurities will be pushed to the bottom. You can either take the ice out before it freezes all the way to the bottom, or cut/chop off the ugly bottom part. Then cut up the clear ice into cubes the size you'd like (it's much easier to do that you might guess). I learned this from a book called Liquid Intelligence by Dave Arnold. $\endgroup$
    – Kat
    Nov 6, 2017 at 22:27
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The cloudiness is caused by dissolved air bubbles. Plus, tap water is notoriously rich in mineral particles (giving nucleation sites).

Seeing how you placed the tap water in the freezer, the cold temperature kept the dissolved gases and minerals in solution, and only partial stratification occurred before the water froze. The expanding ice1 "magnified" the dissolved gas bubbles, causing the cloudy appearance. The clear section is that portion of the water from which the gas bubbles and solute had precipitated (sinking in the container, so to speak).

1 Water is one of the very few substances that expands when transitioning from a liquid to a solid state.

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  • $\begingroup$ @Ilia Smilga has given a bit of different explanation. He has denied the stratification. Can you provide some citation to your answer? Till then, I'm forced to unaccept your answer, as both answers seem correct, and I need a fourth party intervention through citation. Please don't mind, this is a courtesy gesture. Both answers are equal in my eyes. $\endgroup$
    – Wrichik Basu
    Nov 5, 2017 at 14:43
  • $\begingroup$ My "stratification" theory is more about how the tap water you originally poured into the container separated (slightly) into a lower layer with more air bubbles than the upper layer - my rationale is based on personal experience with noting how when you shut off the faucet, the flow rate decreases noticeably (until it stops completely), and thus the water on "top" has less dissolved gas (but not necessarily less mineral content). I, too, agree with Ilia Smilga's point about how the water freezes outside-in (so the majority of the gas bubbles are visible in the "center" of the block). $\endgroup$
    – pr1268
    Nov 5, 2017 at 15:12
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    $\begingroup$ I don't understand. When you fill a container with water from the tap, the water in the container is well mixed because the falling water sets up a circulation in the container. So how would this stratification occur? And why would the water containing air bubbles be at the bottom? Water-plus-air is less dense than water, so it would surely float to the top? And why would water from a slowly-flowing tap contain less dissolved air than water from a fast-flowing tap? It might contain less entrained air but even that's not clear. $\endgroup$
    – David Richerby
    Nov 5, 2017 at 16:52

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