What is fire?

During the mid-1800s, Michael Faraday captivated young audiences with his Christmas Lectures at the Royal Institution in London. One of his recurring topics of interest was fire.

Faraday's fascination lay specifically with candles, as they held fascinating insights into the mechanics of fire within their delicate flames. While you may have encountered explanations of fire in chemistry class, a mere chemical formula fails to capture the essence of what fire truly is, just as a recipe cannot fully convey the taste of chocolate chip cookies.

The vibrant array of colors displayed by a candle flame is one of its striking features. Objects emit light due to black body radiation, a concept we explored in our video about the color of the universe. The lower portion of the flame, being hotter, radiates a blue glow, while the middle region, relatively cooler, emits a yellowish-orange hue.

Within the confines of the flame, an intricate dance of hundreds of chemical reactions unfolds. The oxygen in the air, along with the carbon and hydrogen present in the candle, do not initiate these reactions on their own. They require external heat to kickstart the process. The solid fuel of the candle undergoes vaporization and disintegration into smaller fragments under the influence of heat. This transformative process is known as pyrolysis, an indispensable step for the existence of a flame.

Occasionally, you may observe a dark cone encircling the wick of a candle, devoid of any visible flame. This area signifies the presence of vaporized wax, which has yet to ignite.

The hydrocarbons in the air, along with the hot oxygen, vigorously collide, prompting the rearrangement of their constituent atoms. At times, electrons within these atoms become excited, emitting light upon returning to their normal states. This phenomenon elucidates the blue glow emanating from the bottom of the flame.

Not all the carbon within the candle undergoes complete conversion into carbon dioxide (CO2). As a result, residual carbon atoms converge, giving rise to minuscule particles of soot. These particles heat up and emit shades of orange and yellow, reminiscent of the glowing coals beneath a grill. The majority of a candle's luminosity emanates from this radiant soot.

Gradually, as the flame reaches its tip, all the soot is consumed, leaving behind only carbon dioxide and water, which disperse into the surrounding air. If you wish to explore the various components of a flame, a simple cold metal object can serve as your tool.

Higher up in the flame, you'll find water vapor, while the yellow section harbors soot. Adjacent to the wick, you may even discover unburned wax that can be recovered.

Flames possess an enthralling visual appeal, almost entrancing to behold. Oh, my apologies for digressing. Let's return to the topic at hand: the shape of flames.

Gravity exerts its force, pulling down cooler and denser air, while simultaneously causing hot air to ascend. This buoyancy effect bestows flames with their characteristic form. However, in zero gravity environments such as the space station, a flame's appearance would markedly differ.

The chemical and quantum reactions responsible for the radiant glow of a flame can only occur within specific regions. When a candle flame comes into contact with the surrounding air, its appearance may deceive you, as it resembles a solid cone. However, in reality, candle flames are actually hollow inside.

As long as there is a continuous supply of fuel and oxygen, a flame will persistently burn. But why does this happen? The intensity of a flame's heat does not stem solely from the molecular disintegration taking place. It is the formation of new molecules and the creation of fresh chemical bonds that generate heat. This heat, in turn, propels the chain reaction forward, vaporizing more fuel, causing molecules to collide more forcefully, and sustaining the fire's combustion.

Throughout our history, as a species, we have gathered around fires for countless years. The act of sitting together, pondering questions, and sharing stories in the glow of a flickering flame played a significant role in shaping our humanity.

Let your curiosity thrive and continue to explore the wonders that surround you.