Fancy Ice Redux

Classical Mechanics Level 1

I was once on a Canadian lakeshore in late autumn, and the frozen water appeared perfectly clear.

I haven't seen a mountain lake freeze since, and I've never been successful at making clear ice cubes at home, so I've decided that the environmental conditions of that autumn must've been just right.

What local conditions are necessary to form clear ice on a lake?

Cool, still air and cold permafrost under the lake bed A sudden, freezing cold front of air and a warm lake bed Still air just below freezing and a warm lake bed

2 solutions

Blake Farrow Staff
Apr 3, 2018

What's going on here?

Finding clear ice on a frozen lake indicates that the lake must have frozen slowly. Ice crystals usually begin to form (or nucleate ) near impurities or edges, and spread outwards, adding nearby water molecules to the ice's crystalline lattice. The crystal lattice is tight and there isn't room for impurities like air, dust, or dissolved minerals to fit in the crystal. If the ice crystals form slowly enough, these types of impurities have time to diffuse away from the crystal as it grows. The larger the ice crystal can slowly grow without freezing around an impurity, the fewer scattering ice surfaces there are that can lead to cloudiness.

How to avoid cloudy ice

Slow-freezing ice rejects impurities by giving them time to diffuse away. The very last part of the ice to freeze is where the trapped air and impurities have been cornered. Most ice cubes are particularly cloudy in the middle because the water is freezing from all six cold sides. In a lake with cool air and a relatively warm lake bed, the freezing occurs from the top only and never freezes all the way to the bottom -- the impurities and air are still beneath in the liquid water near the lake bed.

Clear ice is also pure

The ability of slow freezing to "purify" water of impurities was noticed in the late 19th century when pollution was first identified as a problem. Even if a lake begins full of impurities and pollutants, the top layer of clear ice after a few cool days of slow freezing is nearly perfectly pure.

That is reasonable phenomenological description of ice (and any other regular liquid to solid transition) except for explaining the correct choice as against the others. Honestly I think the first choice maybe better since it is a front that travels across the lake so the ice will spread slowly- the second could lead to ice spreading across the surface at the speed once it nucleates and driving impurities down in the lake not up above the lake.

Mark Smith - 3 years, 1 month ago

The important distinction between the first two answers is that for clear ice to form, the freezing must occur slowly. This won't happen if there is a freezing cold front that is well below zero. The speed at which even the slowest cold front might move across a lake is far more rapid than crystallization, and so likely wouldn't lead to directional freezing in a lateral direction.

Your last sentence is indeed correct -- still air just below freezing sitting above the lake would indeed drive impurities down into the lake. This is the goal of slow, directional freezing, and the reason why lake ice can appear clear despite the source water being full of impurities: The the impurities and air bubbles are pushed down and remain beneath in the liquid water near the slightly warmer lake bed.

Blake Farrow Staff - 3 years, 1 month ago

I learned about making clear ice cubes in third grade science. We learned that the temperature must be just below freezing and the water must be a little bit warm. Next time I could have even tried to make clear ice cubes. I would rate this problem a 2because it was really clear to me.

Lucia Tiberio - 3 years, 1 month ago

The answer could have been: none of the above; being a Canadian lake, the water in it was exceptionally pure because Canadians are angelic people who do not pollute their environment.

S I - 3 years, 1 month ago

A cube (ice cube) has 6 sides...

Gregor Shapiro - 3 years, 1 month ago

Thank you kindly, Gregor. fixed!

Blake Farrow Staff - 3 years, 1 month ago

So I suppose I understand the answer as you described it, but how, with the information we were given, were we supposed to come up with the answer?

I chose "A sudden, freezing cold front of air and a warm lake bed". The option "still air just below freezing etc." sounds like the conditions in a freezer which the information giver acknowledges doesn't result in clear ice" so I rejected it. The option "cool, still air and cold permafrost" doesn't seem exclusive to late autumn as cool air can happen at any time of the year and permafrost is permanent, so I rejected this too. The last option seemed the most plausible then, not only by elimination, but through a general understanding of the seasons and temperatures. A sudden freezing cold front makes sense as it's late autumn and thus temperatures are starting to drop and also the warm lake bed makes sense as bodies of water change temperature more slowly than the air around it.

So yeah, I'm not denying that I am wrong, I'm just confused as to how anyone is able to get the answer without understanding the deeper chemistry and physics behind ice formation.

Christopher Lok - 3 years, 1 month ago

How can the "correct" answer be "a sudden freezing cold front of air and a warm lake bed" if it takes slow freezing to reject the impurities and air bubbles? Surely slow freezing requires continuous air just below freezing, not a sudden cold front?

Joan Cunningham - 3 years ago

Hi Joan-

The correct answer for this problem is indeed "Still air just below freezing and a warm lake bed". Is it not showing that for you?

Blake Farrow Staff - 3 years ago

Laura Gao
Apr 15, 2018

Ice that is not clear always has impurities like air bubbles trapped in it. Clear ice is pure water. Having clear ice means that the water had frozen slowly, because there would've been enough time to get the impurities out of the water. Still air just below freezing and a warm lake bed allows the ice to freeze slowly. If there had been a sudden freezing cold front of air (like in the 1st answer choice), then the water would have frozen very quickly, and trapped all the air inside the ice. If there was permafrost under the lake bed (like in the 3rd answer choice), then the water would have frozen a long time ago, and it would remain frozen for probably a long time due to the permafrost.

Fancy Ice Redux

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