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Fire is not really a solid, a liquid, a gas, or even a textbook plasma — it is a rapid chemical reaction (combustion) whose products happen to be hot, glowing gases. An everyday candle or wood flame is mostly a hot gas mixture (carbon dioxide, water vapour, soot particles) with only a tiny fraction of its atoms ionised. Hotter flames, like an oxyacetylene torch, are ionised enough to genuinely qualify as plasma — the “fourth state of matter”.
Every object that has ever existed on this planet can be classified into three basic states of matter: solid, liquid or gas. That’s what we were told to believe in our science classes, right? But… what about fire? Imagine holding fire in your hands like a brick, storing it in some kind of vessel or even better, trying to fill a balloon with a raging inferno! Doesn’t seem possible, right? To all those people who scratched their heads during those boring chemistry classes, here’s a piece of trivia: fire doesn’t really belong to any of the aforementioned groups. In fact, the closest state of matter that it can be compared to is plasma. Plasma?
What on God’s green earth is that?
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How Is Fire Like A Gas?
Before jumping to the fun part, one must rule out the possibilities of fire being a gas or a solid (Surely, it’s not a liquid, right?) So… why can’t fire be a gas? In its true nature, fire does share some properties with other gases. Like gas, it does not have a definite shape or volume, unless it is enclosed in an appropriate container. The visible conflagration that one witnesses is simply gas that is still reacting and providing illumination. However, fire doesn’t expand evenly like other gases in a given closed container.
Furthermore, fire is incapable of forming structures like filaments, beams and double layers under the influence of a magnetic field. Thus, fire cannot serve as an electromagnet when exposed to a magnetic field, which is not the case with solids, liquids or gases. So basically, fire’s atomic structure is acting like Switzerland did during the World War! Not taking sides…. nice and complicated.
Also, the laws of physics dictate that one cannot extract more energy out of a given substance without investing more energy into it. This fact eliminates the slightest possibility of fire being a solid, liquid, or gas. All types of fire gradually die out and cannot continue to exist in their natural state forever, unlike the above. For example, a funeral pyre would eventually die out if it was not constantly furnished with oxygen and flammable material.
Fire And Plasma? Plasma!
Let’s rewind and take a look at some of the earlier theories developed by humans to give tangible meaning to their outlandish findings. Before Sir William Crookes identified what he called "radiant matter" in cathode-ray experiments in 1879 (the term plasma for this fourth state of matter was actually coined by the American physicist Irving Langmuir in 1928), mankind only believed in three states of matter. Long before that, the ancient Greeks classed fire as one of the four classical elements, alongside earth, water and air.
Plasma is a hot, electrically conducting gas in which some atoms have been stripped of one or more of their electrons. The resulting soup of positively charged ions and negatively charged free electrons (the nuclei, including their protons and neutrons, stay intact) lets plasma respond strongly to electric and magnetic fields and behave as a single collective system rather than as a bunch of independent atoms. It pretty much behaves like a high school punk who decides to abandon his set of friends, only to succumb to a life of solitude. Aside from the hypothetical existence of “Dark Matter”, plasma is the most abundantly and scientifically accepted form of ordinary matter found throughout the universe!
Plasma resembles a gas more than any other state of matter, but it behaves very differently from a gas. This is because the free electrons are not in constant physical contact with one another due to a lack of affinity towards each other. This means that plasma can flow like a liquid or a fluid, comprised of specific areas that are like groups of atoms sticking together. This property of plasma differentiates it from all other gases.
So what should you actually say if a teacher asks you which state of matter fire is? The most honest scientific answer is this: fire is a chemical reaction (combustion) whose visible product is a hot gas, with a very small fraction of its atoms ionised. Ordinary fires — a candle, a campfire, a gas stove — are weakly ionised, so they sit closer to a hot gas than to a true plasma. Only very hot flames such as an oxyacetylene torch, an arc-welding flame or the plasma inside a lightning bolt clearly cross into the plasma state. So when popular sources say "fire is plasma", take it with a pinch of physics: it’s a useful shorthand, but the more accurate description is that fire contains a small amount of plasma alongside lots of hot gas.
