Chemistry

What causes Spontaneous Combustion



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The mother of all spontaneous combustions is a dramatic conflagration known as the thermite reaction, and the basis for this is KMNO4. That's the chemical notation for potassium permanganate. The other primary component is glycerin. When you mix glycerin- and it has to be very pure, because glycerin has a tendency to attach with moisture in the air- but if you mix glycerin with crystals of pure potassium permanganate, and wait for a minute or so, it will combust spontaneously. The flame is incredible, nearly a thousand degrees. In an earlier time, it was used to weld train rails together by dropping potassium permanganate crystals down between the rails, and then pouring in some glycerin. In a minute or so, it was show time- exothermically speaking.

The potassium permanganate and glycerin reaction, by itself, is impressive enough, but it's only the first half of the thermite reaction. The other two components of the thermite version of spontaneous combustion are fine aluminum metal powder and common iron oxide. I use the term, "common," because iron oxide is nothing but rust. When I was a boy, doing my thermite reactions back in Boston, I'd get my iron oxide just by scraping down deep into the rust on an anchor chain. My dad worked in the shipyard. I got my aluminum powder from a pile where they were cutting and grinding big sheets of aluminum into various shapes. The supply was inexhaustible. I used a very fine kitchen strainer, and then a flour sifter, to screen out all the larger chunks and get down to a fine powder. My mom had no idea what I was up to, until the day I burned a hole through the bottom of her best frying pan.

Now the thing about the two secondary metals in the thermite reaction is that they won't react until the temperature gets up near a thousand degrees. In chemistry, that's called a high activation energy. But once they start reacting, the two metals will burn together and get the flame up to nearly three thousand degrees. Every chemical reaction has it's own level of activation energy. The aluminum and rust combination needs almost a thousand degrees. That gets supplied by the reaction of the potassium permanganate and glycerin. But then, this primary reaction has a required activation energy too. It turns out to be somewhere around seventy degrees, Fahrenheit, if the glycerin is fairly pure- low water content- and the potassium permanganate grains are small. So in a low temperature environment, the mixture can be assembled, and it'll stay stable until it gets warm. The reaction will refuse to take place at all if it's too cold.

The reaction has it's own chemical oxygen, too, so it can't be smothered or extinguished. Because of this useful feature, thermite is commonly used for underwater welding. There was an informal and clandestine demonstration of the thermite reaction a few years ago, back east at MIT, when some unnamed student used small thermite reactions to weld a few of the Boston trolley cars to the tracks.

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