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Name: Terri
Status: student
Grade: 12
Country: Australia
Date: Spring 2013


Question:
If an ice cube has air bubbles in it, will it be easier to fracture? If an ice cube has particles of salt or sugar in it, will it be harder to break?



Replies:
Terri,

This is a very complex question because there are many factors involved in ice fracture and even under lab conditions, it is difficult to isolate any one factor - so the data on this can be difficult to interpret.

What we know in general, is that ice fails or breaks along grain boundaries. Imagine a cube of ice as being made up of smaller blocks of ice (about 1mm in diameter). These grains separate from each other when force is applied - producing a fracture or a crack. These cracks tend to propagate (since cracks become focal points of applied energy). When the cracks get big enough or there are enough of them, the ice fails.

The problem is the fracture formation can occur in many different ways. Grains can slip past each other (one sliding upward, say, while the other slips downwards), they can fuse in one direction while leaving a gap in the other direction (creating holes or fissures), they can melt - thus producing the complication of wet versus dry fissures, etc.

So it is very hard to say that there is any one absolute effect of bubbles. If there are very few bubbles, then the bubbles can actually stop the propagation of a crack. If there are very many bubbles, then the bubbles can be weak points where grains can flow into and induce cracking elsewhere. The size of the bubbles also matter. A large enough bubble would be like breaking a hollow container made of ice walls. Small enough bubbles (smaller than the grain size) might be considered cracks or fissures or they might be negligible.

Doing experiments on actual cubes of ice is also tricky. Since the growth of the ice cube (its history) will control grain sizes, bubble distribution, etc. So only the most highly controlled ice growth (difficult to do) will yield meaningful results - but then this would be limited to the very specific case and not really a "real world" ice.

Sorry, this reads like a no answer, but hopefully, I've shown you how complex the situation is.

Greg (Roberto Gregorius) Canisius College


Hi Terri,

An ice cube with entrapped air bubbles will not be as strong as one with no entrapped air, simply because there is less ice in the one containing the air bubbles. The more air it contains, the less ice there is and the easier it is to break.

As for salt or sugar crystals embedded in the ice, I simply don't know how you could achieve this. Both salt and sugar are very soluble in water, and would certainly dissolve quickly in the water you are trying to freeze, before it was able to freeze into an ice cube, and thus would not exist as discrete salt or sugar crystals embedded in the ice cube.

Regards, Bob Wilson


There are many complexities involved if you want them to be (for example, 'ice' has a large number of different structures, depending on conditions)... and when you add salt or sugar, the ability of the water to freeze changes too (freezing point depression). All that aside, the simplest black-or-white answer is that ice with lots of air bubbles is weaker (think of an icy sponge versus solid ice), and ice with lots of dissolved solute in it is weaker (think Popsicles versus solid ice)... but there are many counter-examples and experimental limitations that could get in the way.

Hope this helps, Burr Zimmerman


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