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There are several videos of the reaction, where some amount of burning thermite explodes on a contact with ice.
An "original" video: http://www.youtube.com/watch?v=IuPjlYxUWc8
A Mythbusters confirmation: http://www.youtube.com/watch?v=BnHR4cMXiyM

There are several speculative explanations of the phenomenon, summarised in the end of the latter video. And none of them sounds to me satisfactory enough.

Is there any detailed investigation and/or explanation of the effect?

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Unfortunately, there doesn't seem to be a definite scientific study done on the cause. There are some logical explanations, as phycker pointed out, but unfortunately, it seems that mankind hasn't done every experiment there is to do, yet. :] –  voithos May 24 '11 at 18:05
""it seems that mankind hasn't done every experiment there is to do, yet."" Yes, of course. But please think that Thermit is a commercial thing, what Goldschmidt AG did or not did, You never know. Because Thermit reaction has some military application, such experiments maybe were made, but they will not write about in scientific journals. –  Georg May 24 '11 at 20:00

7 Answers 7

This is probably a question for the proposed Chemistry Stack Exchange. Here is one explanation given by Jamie Hyneman of MythBusters. The MythBusters video showed: Thermite place on top of ice → Ignited → Exothermic reaction begins → Big Explosion. Once the thermite has been ignited, a highly exothermic reaction follows it. This exothermic reaction releases a huge amount of energy so fast that the ice becomes superheated and turns into vapor. This vapor condenses around the fine thermite particles forming a steam-thermite aerosol.The thermite mixture continues to react further, resulting in a steam explosion throwing particles around.

Yet another explanation is that the high temperature produced during the thermite reaction results in the following reaction: $$3Fe(s) + 4H_{2}O(g) → Fe_{3}O_{4}(s) + 4H_{2}(g)$$

The hydrogen released will explode exothermically in air: $$H_2 + \frac{1}{2}O_2 → H_{2}O$$

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Personally, I favor the latter explanation. –  Mitchell May 24 '11 at 18:07

The latter is the better choice, whist the videos of the explosion are very red/orange. This hue is caused by the rapid burning of the thermite which is aerosolized by the hydrogen and oxygen explosion. You'll note that the same results do not occur if you drop thermite into water (Thermite is sometimes used for underwater welding.)

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I'm not so sure the latter is the better choice. In the presence of oxygen doesn't Hydrogen burn with a blue-ish hue? Getting its oxygen from both the surrounding air and any extra oxygen atoms/molecules that haven't bound themselves to the Thermite. The explosions I've seen are all heavily red/orange, indicating it's the Thermite burning, not the hydrogen.

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It's actually a combination of a couple of events occurring fairly rapidly due to the extreme heat of the Thermite reaction. Heat from the Thermite reaction melts the ice into liquid water, which can now combine with the reacting Thermite. Burning metals like those in Thermite (Iron, Aluminum) react faster with water than with oxygen, so more energy is released. This evidences itself as an explosion, similar to the explosion caused by dropping metallic potassium into water.

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You can't see hydrogen combusting in the explosion because of the amount of solids expelled by the explosion. All you see is iron oxide mist. Also, hydrogen doesn't combust merely in the presence of oxygen, it just burns as oxygen is available. Even though it burns rapidly, it's still not combustion. You need an optimum fuel-air mixture. And i don't believe that is the case. A steam explosion also seems unlikely, so we'll never really know for sure until someone drops burning thermite into water where oxygen isn't readily available and see what happens. The most likely answer is that the iron oxide will react violently with the water at the last stage before the explosion. You can actually see the transition between the two different reactions in the intensity of the reaction.

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Aluminium burns like crazy when burned in steam, so my guess it that the steam created by the heat on the ice causes the aluminium to react rather violently hence a major explosion. I'm guessing this doesn't happen in water because the reaction requires steam AND oxygen which is present in air, but when using thermite under water, though you are creating heaps of steam, there is no oxygen present. I remember doing an experiment in high school where our teacher got an aluminium pencil sharpener glowing hot and then stuck it in a stream of steam coming from a beaker - the sharpener burned so rapidly that it shot off and bust a hole right through the ceiling! Just my guess!

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I think Terry and Bernhard have part of the puzzle. The reaction that Terry is talking about is:

$$2\mathrm{Al} + 3\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{Al}_2\mathrm{O}_3 + 3\mathrm{H}_2$$

"Aluminium powder heated in steam produces hydrogen and aluminium oxide. The reaction is relatively slow because of the existing strong aluminium oxide layer on the metal, and the build-up of even more oxide during the reaction."

Source: http://www.chemguide.co.uk/inorganic/period3/elementsreact.html

I would think that if the reaction stated by Bernhard and this one are the ones that take place, the same explosion should occur when thermite is burned over water.

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There are a couple of videos burning thermite over water in youtube, and they don't have an explosion. More interestingly, there's some videos of burning thermite over frozen ponds/lakes and they don't show explosions either. Can anybody think about the variables that change between: -Conventional ice block with thermite over it (original ice block explosion video and Mythbusters one) -frozen lake with thermite over it. The first thing that comes to my mind is the temperature differential between air and ice. And perhaps different ice purity? –  Esteban Aug 14 '13 at 22:53

protected by Qmechanic Aug 13 '13 at 18:42

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