What is Fire?

 by Yuanzheng (Bruce) Mao

Figure 1 A flame. Image from: Word Stock Images

Fire, as one of the most useful tools for humans, has been employed by us for more than hundreds of thousands of years. [1] Without fire, life would have been disastrous: no cooked food when you are in hunger; no warmth when you are in winter; no power for many industries…. Before the advent of modern chemistry, fire was a mystery. The ancient Greeks thought there were 4 elements: earth, air, water, and of course, fire. Nowadays, we are so familiar with this hot stuff that most people pay little attention to it. What on earth is fire?

 

Fire, by the definition from Wikipedia, is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction products. [2] This means that fire is not an object, but an event (To be clear, in this article, a “fire” refers to an event, while a “flame” refers to the glowing, fluid, hot thing, although in real life I think they are usually interchangeable.). [3] It is an event where there are so many chemical reactions going on, releasing an enormous amount of energy, heating everything up. What’s noticeable about fire is the hot, bright, smooth, fluid-like flame. In fact, the substances involved in a flame are just some normal particles (e.g., water, carbon dioxide, unburnt fuels, ions, etc.). [2] [4] However, the really fascinating part about the flame is the fact that is glows, emitting light. So, here raises the question: Why does a flame glow?

Here is the explanation: Electrons in all matters move around and emit energy in the form of electromagnetic waves (thermal radiation). These electrons first gained energy, then release it, in the form of “light”. But the only problem with this “light” is that we are usually not able to see it due to our natural limits. For example, animals emit infrared “light” for exactly same reason as a flame glows, but we can’t see it with naked eyes. A special camera must be used. Luckily, the wavelength of radiation from a flame is just within our visual spectrum. Thus, we can see flames glowing. [5] [6]

References

[1] Wikipedia (2022), Control of fire by early humans [online] Last accessed 3 May 2022: https://en.wikipedia.org/wiki/Control_of_fire_by_early_humans

[2] Wikipedia (2022), Fire [online] Last accessed 3 May 2022: https://en.wikipedia.org/wiki/Fire

[3] Leeaundra Keany (2011), 20 Things You Didn’t Know About Fire [online] Last accessed 3 May 2022: https://www.discovermagazine.com/the-sciences/20-things-you-didnt-know-about-fire

[4] Veritasium (2012), What’s In A Candle Flame? [online] Last accessed 3 May 2022: https://www.youtube.com/watch?v=a7_8Gc_Llr8

[5] But Why? (2020), Why Do Hot Objects Glow? – Black Body Radiation [online] Last accessed 3 May 2022: https://www.youtube.com/watch?v=Mj2QOpQkSfI

[6] Wikipedia (2022), Thermal radiation [online] Last accessed 3 May 2022: https://en.wikipedia.org/wiki/Thermal_radiation