Why do flour and starch explode?

During the Second World War, Hitler’s air force continued to bomb the British, and the bombs fell from the sky. The owner of a British flour mill secretly glad that the bomb did not hit his factory, but almost at the same time as the bomb fell, there was a big explosion in the workshop, and the roof flew into the sky. The power of the explosion exceeded the destruction of the bomb. At the same time, several other flour mills also exploded.

This peculiar explosion caused the factory to suffer heavy losses and was inexplicable because there were no bombs on the factory, and there were only flour and machinery in the workshop, and there were no explosives such as explosives.

So what is the cause of this strange explosion?

It turned out that due to the explosion of the bomb, the dust in the workshop picked up the flour contained in the air to a certain concentration, and exploded after the fire. The explosive is flour.

Someone has to ask, how can flour not be explosive?

We know that the pulverizer in the flour mill processes the wheat into very fine and fine flour. The pulverizer consumes electricity and works on the material being processed to crush the material. Among them, part of the work done by the pulverizer is converted into energy and stored on the surface of the granules after being pulverized. This part of energy is called "surface energy" in physicochemistry. Moreover, for a certain substance, the greater the degree of pulverization, that is, the smaller the particle, the larger the surface area, and the larger the surface energy. For example, a 1 kilogram of silica has a surface energy of 0.2 joules, which is small, and it is only equivalent to the work of lifting a 1 kilogram object up to 0.02 meters. However, if it is pulverized into fine dust like flour, its surface energy can reach 2.7 × 106 joules, which is equivalent to the work of lifting the same heavy object 2700 meters, the surface energy is increased by 10 million times. !

Because dust has such a high surface energy, it is easy to undergo physical or chemical changes to release its energy compared to bulk materials. The reason is that the water in the upper part has extra potential energy than the water in the lower part, so it has to flow to the lower part. Therefore, these fine dusts, which usually seem insignificant, are mixed with the air in a suitable condition. In the event of fire, it will rapidly undergo a fierce combustion reaction, releasing huge energy in an instant, such a fearful thing. - The flour explosion will follow.

Not only flour, but also flammable dust such as cocoa, cork, wood, light rubber, leather, plastic, and almost all organic compounds and various inorganic materials such as sulfur, iron, magnesium, cobalt, etc., if these dusts are When a certain concentration is reached in the air, as long as it encounters an open flame, even a fire of stars will cause an uproar - a violent explosion, and sometimes the explosion of these fine dust is no less than the destruction of the bomb. In 1846, a big bang occurred in the Harvey coal mine in the United Kingdom, when the famous scientist Faraday wrote in the report of the British Ministry of Internal Affairs to investigate the explosion: "The burning and explosion of methane mixture will rise in the tunnel. All the coal dust, and make it on fire." So, the starch workshop and the flour mill, if the dust density is relatively large, will cause an explosion.