What triggers an explosion?

Viewers of action movies or fans of the TV show Mythbusters have seen countless explosions. Fireball, detonation at a very high speed, involving explosives. Everyone can picture this destructive event. But do you really understand what is underneath this phenomenon? How an explosion occurs?

Definition of an explosion

Back to the basics, to what do we refer to when we talk about an explosion?

An explosion is a massive energy release with a quick increase of volume and are often at an important temperature and emit gases. Faster the reaction, higher the pression. Then the matter will retrieve the atmospheric pressure causing a blow and a loud sound.

If the blow is slower than the sound, we call it a deflagration. If the blow is faster than the sound, we call it a detonation, generating a shock wave by breaking the sound barrier [1].

Causes of an explosion

Chemical reactions occur when reagents interact. The products of such reactions can have a different state (solid, liquid, gas) and/or less density which increase the volume, often with heat production. They are the most common explosives with one of the first being gunpowder.

An explosion can also result from a mechanical process. It happens when an element (often a liquid) in a container increases its volume thus the internal pressure. Typically when the said-element is heated, changing its state by fusion or evaporation. It is sometimes combined with chemical reactions.

The last kind of explosion is nuclear, the power of the atom. We all have in mind the destructive power of a nuclear bomb. A whole post would be necessary to explain everything. Meanwhile, I’ll give you a video by Arvin Ash that explains it: [2].

Only a human origin?

When talking about explosions we often think about human-made action. Some natural events can cause explosions too.

First, volcanoes. Explosive eruptions occur when a viscous magma (rich in silicium) block the chimney preventing the gas from evacuation. Pressure rises to the point when the explosion happens, releasing suddenly a massive amount of gas and magma in the air. [3]

Then, we have a natural yet considerably more powerful explosive event, supernovae. A supernova is a star’s death. There are two categories: thermonuclear supernova when a white dwarf (high density star) attract matter from a star in its neighbourhood, and core collapsing supernova when the star’s core is made of iron (the most stable element in the periodic table) disturbing its upper layers stability which will be blown away by the iron turned into neutrons. [4]

Application in technology

Explosives serve their purpose in civil engineering, placed at tactical locations, weak spots, on building infrastructures to tear them down.

Another piece of technology that most of you use almost every day depend on explosions to work. I am talking about engines, like car engines. Here again, there will be a deeper explanation on another post.

Explosion is often associated with destruction and needs to be dealt with carefully. However, understanding these processes gives us the opportunity the prevent them. Like always, scientific knowledge is key to evolve in the world.

References

[1] All You Need Is Science, Breaking the sound barrier

[2] Arvin Ash, Hydrogen Bomb: How it Works in detail. Atomic vs thermonuclear bomb, https://youtu.be/fYuVzbIu_8o

[3] Skinner, Brian J. (2004). Dynamic Earth: An Introduction to Physical Geology. John Wiley & Sons. Inc. Hoboken, NJ. ISBN9780471152286

[4] Bethe, H. A. (1990). “Supernovae”. Physics Today. 43 (9): 736–739. doi:10.1063/1.881256