New 杏吧原创 has teamed up with Discovery Channel鈥檚 MythBusters to attempt to solve a mystery. Thermite and ice can make an explosive combination, so don鈥檛 try this experiment at home 鈥� watch it safely on the web at . We want to find out why the explosion happens. Thermite is a mainstay of pyrotechnics, comprising a mixture of metal and metal oxide powders that burns at extremely high temperatures in a tightly focused area. Thermite is not, by itself, explosive, but if you ignite a bucket of thermite on top of blocks of ice, there is an enormous bang once it has burnt through the bucket. MythBusters presenters Jamie Hyneman and Adam Savage are seeking a convincing scientific explanation of this violent reaction. Will The Last Word readers solve the mystery?
鈥� The most common type of thermite reaction, which uses aluminium powder and ferric oxide, produces molten iron at 2900 掳C, twice the melting point of the pure element. This means that the iron will not solidify quickly even when in contact with ice.
Hot iron reacts with steam to produce hydrogen and iron oxide. Molten iron will ignite this hydrogen, so a puddle of iron hovering on a blanket of steam above an ice block will be blasted into droplets by a hydrogen explosion. The resulting iron aerosol will ignite in air to produce bright yellow sparks, just like the ones seen from fireworks filled with iron filings.
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The iron-steam reaction was also used by in 1784 as a proof that water was composed of hydrogen and oxygen. In his experiment, he used a red-hot gun barrel.
John Rowland Derby, UK
鈥� For more than 20 years, I performed a series of demonstration lectures based on fast reactions alongside the late Michael Burnett, my colleague at Queen鈥檚 University Belfast. We carried out dozens of thermite reactions, and were aware of reports of explosions. Thankfully we never experienced one, but then our procedure was different to that used by the MythBusters team and others suffering an explosion. We placed the thermite mixture in an earthenware pot suspended over a large bucket of ice. The ice in this case was used purely to contain and cool the resulting reaction residue and not to induce an explosion.
Our good fortune didn鈥檛 stop us speculating on the cause of such explosions. We considered two possible explanations. First, the MythBusters one of the dissociation of water into hydrogen and oxygen, followed by explosive recombination. Second, steam being trapped within the solidifying metal, generating high pressure and ultimately an explosion. On balance we favoured the latter.
Our experience suggested that about 2 litres of a 2-to-1 mixture of hydrogen and oxygen is required to produce a substantial explosion. Using our first theory, this would not only require up to a couple of grams of water to dissociate and recombine, but also for the explosive recombination to be delayed until such an amount had dissociated and accumulated. So we thought it more likely that a similar amount of water could be trapped inside the metal lattice, which would provide the pressure required for a large explosion.
鈥淎bout 2 litres of a 2-to-1 mixture of hydrogen and oxygen is required for an explosion鈥�
Brian Walker Belfast, UK
惭测迟丑叠耻蝉迟别谤蝉鈥� Jamie Hyneman writes:
鈥� The responses are impressive but I am left with some questions.
What about the quantities involved? The explosion we saw in the experiment was not, in my opinion, the result of the conflagration of a few litres of gases or aerosolised metals, which my guess is all the thermite had time to create in that instant while splashing around on the ice鈥檚 surface. There was enough blast energy to break off large chunks of ice and hurl them a great distance, and the visible diameter of the explosion was about 15 metres.
Was this a result of these things happening in a fairly small volume and creating a small explosion which aerosolised the unburned thermite which in turn exploded on a larger scale? Or perhaps it was just steam that aerosolised the powders and did the same? One test would be straightforward to do: aerosolise some thermite in the presence of a source of ignition and see if the result is similar. My sense is that it may be, as we have seen vigorous explosions from many aerosolised powders. I will make a point of doing this in the near future and will let New 杏吧原创 readers know the results.