Image credit: Piotr Siedlecki via Public Domain Pictures (source)

All of us are familiar with fire, and yet most of us would have a hard time describing exactly what fire actually is. When we look at a burning candle, a wood fire in the fireplace, or a gas stove in operation, we see a glowing flame and we feel the heat. On television or the internet, we see images of forest fires and buildings on fire. Because of experiences such as these, fire seems familiar and we have a pretty good idea of how it behaves. But what exactly is fire?

Describing the Nature of Fire

You could say that the nature of fire has been a burning question for millennia. ;-) Fire has been known since before civilization began, and therefore the quest to understand fire is as old as civilization itself. Several ancient civilizations considered fire to be one of the basic elements — in other words, a fundamental substance that cannot be broken down into components. The ancient Greeks considered the four basic elements to be earth, water, air, and fire. In this line of thinking, all other substances on earth are simply blends of the four basic elements. Several ancient civilizations across Asia held very similar ideas.

Even today, if you asked a hundred random people to describe what fire is, several of them might describe it as a substance. When you see a flame, you can clearly make out the edges — the apparent surface of the flame. If fire has a surface, then you might assume it to be a substance. If so, then everything beneath the surface of the flame must be the substance called fire. However, this is not correct. Fire involves a complex mixture of substances, interacting in specific ways — and therefore any complete definition of “fire” should mention these substances. However, fire is not a distinct substance — or even a substance at all. Furthermore, the substances within a fire can exist outside of a fire.

If fire is not a substance, then perhaps fire is a type of energy. This idea seems quite logical. We can clearly see and feel that fire gives off both light and heat. In some very compelling ways, fire seems comparable to electricity — which most of us know to be a form of energy. Both fire and electricity are powerful, somewhat mysterious, capable of doing great work for us, and yet dangerous if they get out of hand. For nearly two centuries before electricity was widely used to power machinery, the great machines of the Industrial Revolution were primarily driven by steam, which depended upon fire as the source of energy. So if you were to ask 100 random people to explain fire, quite a few of them would likely describe it as a type of energy. And yet fire is not actually a form of energy — although any complete definition of fire must explain the role of energy in fire.

So if fire is neither a substance nor a form of energy, then what could it possibly be? In fact, fire is a process, and only by describing it as a process can we fully make sense of it. In this process, energy that is stored in certain substances is released as heat and light, while the substances themselves are simultaneously converted to other substances. In other words, fire involves a dual conversion — a conversion of substances, and a conversion of energy. Let’s look at this more carefully.

Combustible Materials Plus Oxygen

In the parlance of firefighting, any combustible material — that is, any substance that is capable of burning — is considered to be a fuel. This includes many familiar materials such as wood, paper, plastics, and most fabrics. From the standpoint of a chemist, the phrase “capable of burning” means that the bonds between the atoms in the substance are not at the lowest possible energy state. If the bonds between these atoms should get shuffled around to result in a lower energy state, then the excess energy would be released. For many types of atoms, the lowest possible energy state is when that atom is connected to oxygen atoms. This is certainly true of carbon and hydrogen — by far the most common atoms in the “fuels” that firefighters commonly deal with. Therefore the presence of free oxygen is usually an essential requirement for a fire to take place.

The process of combining with oxygen is called oxidation, and therefore fire involves oxidation. But then again, a wet nail becoming rusty is also an example of oxidation, and we would certainly not say that the nail is on fire. Oxidation also constantly occurs within our own bodies, and in fact the process is essential to life. The reason that we require oxygen in our air is so our bodies can oxidize stored sugars and fats, thereby releasing energy whenever and wherever it is needed. And while we might use an expression such as “burning up calories” to refer to this oxidation process, we certainly don’t believe that there is actually a fire in our cells or in our blood stream. The upshot is that oxidation can occur at different rates — the rapid oxidation that occurs in a fire, the extremely slow oxidation that results in iron rusting (and the corrosion of other metals), and the medium-speed oxidation that occurs when our bodies metabolize sugars and fats.

In fact, fire and the metabolism of our food have several features in common, the most important being that the fuels consist primarily of materials dominated by carbon-to-carbon (C-C) and carbon-to-hydrogen (C-H) bonds — the kinds of bonds that characterize substances that come from living creatures. Even plastics can be traced back to living things, because they are usually made from petroleum products, which are the remnants of plants that lived long ago.

When a flammable material burns, the energy it gives off (heat and light) had previously been stored in its chemical bonds (also called atomic bonds — the bonds between the atoms). In almost all cases, this energy can be traced back to sunlight captured by plants through the process of photosynthesis. Plants capture the energy of sunlight by converting it to chemical energy — in other words, storing the energy in chemical bonds. To put it another way, plants use the energy of sunlight to convert molecules of carbon dioxide and water into molecules of sugar. Later on, when the molecules of sugar are converted back to carbon dioxide and water, the energy is released. Sugars can be converted to starches or to cellulose — the principal component of wood and paper — without losing the stored energy. Through more complicated processes, sugars can also be converted to fats, proteins, hydrocarbons, and other substances characterized by large numbers of C-C and C-H bonds. (Such materials are called organic compounds.) All of these substances retain the stored chemical energy that the original sugars once contained, and therefore all of them can burn under the right circumstances.

This leads to the rather interesting conclusion that fires exist because of plants. Just as virtually all life on earth depends on the food energy captured by photosynthetic plants, fire usually depends upon similar plant energy, stored in the form of wood, paper, textiles, plastics, and various man-made organic compounds synthesized from petroleum. When you see a fire — virtually any fire — the energy released as heat and light was originally captured from sunlight by plants.

Just as fire and our bodies make use of similar fuels, the end-products of both types of oxidation are rather similar. In our bodies, when our stored energy (the sugars and fats) become completely metabolized, then nothing is left except for carbon dioxide (CO2) and water (H2O). That is because sugars and fats contain only three types of atoms — carbon (C), hydrogen (H), and oxygen (O) — and if you provide enough additional oxygen, then the lowest possible energy state is to convert all of the carbon into carbon dioxide and all of the hydrogen into water. In almost all fires, the fuel also consists primarily of carbon, hydrogen, and oxygen atoms, and therefore complete combustion also results primarily in the production of carbon dioxide and water.

Fire as a Chain Reaction

If fire and the metabolism of our food are so similar, then what exactly distinguishes these two types of oxidation? One key difference is in what drives each process. The metabolism of our food is driven by the internal control mechanisms of our bodies, mediated by various enzymes. These enzymes select the time and place where each molecule of fuel is oxidized. Furthermore, the enzymes ensure that most of the available energy is captured and put to work driving essential biochemical reactions, rather than being released as heat. Among other important uses, such processes allow us to control our muscles — but all kinds of other processes going on in our bodies also require the use of energy. In contrast, a fire sustains itself through a chain reaction process. In other words, the fire itself produces conditions that allow the fire to continue — typically in a rapid, uncontrolled fashion. In this regard, a fire is analogous to the process that occurs in a nuclear reactor, which can also sustain itself through a chain reaction.

When we see a fire growing or spreading, it can seem rather obvious that a chain reaction is occurring. A fire can seem like a highly contagious disease, spreading by direct contact through a susceptible population of flammable objects. Materials that have not yet “caught fire” will burst into flame when touched by an existing flame or ember — as if the fire were an epidemic. What is far less obvious is that all fires are in fact chain reactions. Even a burning candle is a chain reaction, although the flame tends to remain a constant size and in a fixed location.

The chain reaction in a candle flame can be compared to a conveyor belt. There is a constant train of material running upward from the candlestick, through the wick, into the air and the flame, and up beyond the flame. The material that glows brightly in the flame has only a brief moment of brilliance, and then is pushed upward and away, replaced by other material that then gets its own moment of brilliance. Without the conveyor belt feeding the chain reaction, the flame would instantly disappear.

The chain reaction of a candle flame can be divided into several steps:

1) The heat of the flame melts some of the candle wax, forming a pool of liquid wax at the base of the exposed wick.

2) As the wax evaporates from the upper parts of the wick (in step 3 below), the liquid wax moves up through the wick to replace it. This wicking action is familiar to us in other contexts. For example, if we hold a paper towel so that a corner of it touches a puddle of water, then water will climb up through the fibers of the paper, wetting the towel.

3) The intense heat of the flame at the top of the wick causes the liquid wax to evaporate into the air, where it mixes and comes into contact with oxygen molecules.

4) The gaseous air/wax mixture, pulled upward by the vertical flow of air and heated by the flame, becomes so hot that it undergoes combustion. In other words, the molecules of wax are rapidly oxidized, releasing heat and light. If the combustion is complete, then only carbon dioxide and water molecules remain. Otherwise particles of soot (incompletely burned material) also remain. Soot is mostly amorphous carbon, containing large numbers of C-C bonds that were not oxidized.

5) As this glowing mix of gases is pulled upward — driven by the fact that the hot gases are much less dense than the cooler air surrounding the flame — the gases soon become too cool to glow, even though the upward flow of gases continues far above the visible candle flame.

This multi-step process is a chain reaction because the heat released in step #4 drives all of the other steps, producing a conveyor belt of gaseous fuel to burn, and then pushing the burned material out of the way. Likewise, the flame on a gas stove is also a chain reaction, although it has fewer steps, because the fuel is already a gas. Less obviously, a log fire in a fireplace is also a chain reaction, as the heat of the fire causes the wood to decompose into combustible gases that mix with oxygen in the air and then burn. (This heat-driven decomposition into gases is called pyrolysis.) The flames that dance above the logs reveal the locations where the combustion of gases is taking place. A blue flame (such as on a gas stove) usually indicates complete combustion, while a yellow flame indicates incomplete combustion. Incomplete combustion means that not all of the carbon is fully oxidized into carbon dioxide. Instead, some of it is converted into other carbon-based materials, such as particles of soot. The strong yellow color in a typical flame comes from the tiny bits of soot as they are heated to the point that they glow yellow.

The upshot is that fire — a process involving several steps — is more than a flame. The flame is certainly where the key part of the process occurs — the rapid combustion of the gaseous air/fuel mixture. In other words, the flame is the zone where the fuel is oxidized, releasing heat and light. But the area beneath the flame is also an essential part of the process, because this is where the fuel/air mixture is prepared and fed into the flame. And the vertical flow of gases and smoke above the flame also plays an important role, rapidly carrying away the used materials.

The Energy Hump

Of course, the subtle question behind all of this is how any combustible materials could exist in the first place. Going from a high-energy state to a low-energy state is as natural and automatic as water flowing down a hill. If the energy contained in organic molecules is so great, then why don’t these materials instantly oxidize when exposed to the oxygen in the air? Why don’t materials such as wood, paper, and plastic spontaneously decompose into carbon dioxide and water? Why do you have to touch them with a flame before they will start to burn?

The answer is that there is an energy “hump” in the process. Although the atomic bonds in the unburnt fuel contain a lot more energy than the bonds in the burned materials, you first have to break the existing bonds — and this requires energy. Therefore, to get over the hump, you have to put enough energy into the system to break the bonds, after which the process will “flow downhill” to the lower energy state. In this regard fire can be compared to a siphon. Imagine that you have two tubs of water sitting near each other, one several feet higher than the other. The water in the lower tub is at a lower energy state (with regards to gravity) than the water in the upper tub. However, the water will not automatically jump from the higher tub to the lower, because it would first have to climb up the sides of the higher tub. But if you connect the two tubs with a siphon — that is, a tube filled with water — then the water will flow from the higher tub up over the edge of the tub and then down to the lower tub. The equivalent siphon effect for fire is that combustible materials have to be heated to their combustion points in order to get over the hump and flow down to the lower energy state.

Thus fire needs heat to sustain the chain reaction — and the fire itself provides the heat. This explains why firefighters say that fire requires three ingredients — fuel, oxygen, and heat — and if you eliminate any one of the three, then you can extinguish the fire. Spraying water on a fire primarily attacks the heat component, mostly because evaporating water sucks a lot of heat out of whatever it is touching. (This is why the evaporation of perspiration cools your skin.) But you can also put out a fire by starving it of oxygen, or by removing the source of fuel.

Summary

So now we are finally ready to define “fire”. Fire is a chain reaction process in which substances containing stored energy in their chemical bonds are exposed to oxygen and high levels of heat, inducing any substances not already in a gaseous state to turn into gases, and then inducing this gaseous fuel to undergo rapid oxidation, releasing the stored energy as heat and light, which thereby continues the chain reaction.