Its not the substances themselves that burn but the

To move an airplane or a model rocket through the air, we must use a propulsion system to generate thrust. Different types of aircraft use different types of propulsion devices, but all aircraft rely on some type of engine to generate power. Rocket engines, internal combustion, or piston engines, and jet engines all depend on the burning of fuel to produce power. Burning a fuel is called combustion, a chemical process that we study in middle or high school.

Because combustion is so important for aircraft and rocket propulsion, we will review the fundamentals. Combustion is a chemical process in which a substance reacts rapidly with oxygen and gives off heat. The original substance is called the fuel, and the source of oxygen is called the oxidizer. The fuel can be a solid, liquid, or gas, although for airplane propulsion the fuel is usually a liquid. The oxidizer, likewise, could be a solid, liquid, or gas, but is usually a gas (air) for airplanes. For model rockets, a solid fuel and oxidizer is used.

During combustion, new chemical substances are created from the fuel and the oxidizer. These substances are called exhaust. Most of the exhaust comes from chemical combinations of the fuel and oxygen. When a hydrogen-carbon-based fuel (like gasoline) burns, the exhaust includes water (hydrogen + oxygen) and carbon dioxide (carbon + oxygen). But the exhaust can also include chemical combinations from the oxidizer alone. If the gasoline is burned in air, which contains 21% oxygen and 78% nitrogen, the exhaust can also include nitrous oxides (NOX, nitrogen + oxygen). The temperature of the exhaust is high because of the heat that is transferred to the exhaust during combustion. Because of the high temperatures, exhaust usually occurs as a gas, but there can be liquid or solid exhaust products as well. Soot, for example, is a form of solid exhaust that occurs in some combustion processes.

During the combustion process, as the fuel and oxidizer are turned into exhaust products, heat is generated. Interestingly, some source of heat is also necessary to start combustion. Gasoline and air are both present in your automobile fuel tank; but combustion does not occur because there is no source of heat. Since heat is both required to start combustion and is itself a product of combustion, we can see why combustion takes place very rapidly. Also, once combustion gets started, we don't have to provide the heat source because the heat of combustion will keep things going. We don't have to keep lighting a campfire, it just keep burning.

To summarize, for combustion to occur three things must be present: a fuel to be burned, a source of oxygen, and a source of heat. As a result of combustion, exhausts are created and heat is released. You can control or stop the combustion process by controlling the amount of the fuel available, the amount of oxygen available, or the source of heat.

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The ancient Greeks recognised that fire doesn’t have the same physical properties as earth, air or water, but do we know much more these days? Let’s look at the thorny question – what exactly is fire?

History doesn’t record what humans thought when they first saw fire, but one imagines it was something like ‘what on earth is that?’ And that’s a good question, because over a million years later, we still can’t pigeonhole fire into any of the fundamental states of matter that describe other substances so well.

What we know for sure is that fire is the visible effect of combustion – an exothermic chain reaction requiring the fire triangle: oxygen, heat and some type of fuel. Combustion resulting in fire can only occur between gases (liquid or solid fuels must first be vaporised for there to be a flame) and that gives us our first clue as to how to categorise fire – it’s a gas, right?

Gas, liquid or solid?

Well, a flame certainly has many of the properties of a gas: it has no fixed shape or volume, expanding and contracting as oxygen is added or taken away. But one of the defining characteristics of a gas is its ability to expand to fill any container it’s put in. Fire doesn’t do this, which instantly disqualifies it from being a gas.

Let’s think about liquids. Liquids don’t have to conform to a particular shape and they don’t expand to fill the container they are placed in – so it’s looking good for fire. But they do have a fixed volume, whereas fire doesn’t, so that’s another state of matter that doesn’t quite fit.

You might think solid seems like an unlikely answer to the conundrum, and you’d be right. Solids hold their shape, but if you were to try to pick up a flame, your hand would pass straight through it. And then probably hurt quite a bit.

There’s also another reason why fire doesn’t fall into any of the above: once the fuel or oxygen is exhausted, fire ceases to exist. But that isn’t the case for solids, liquids or gases. And that gives us an important clue as to where to look next.

Possibly plasma

Often called the fourth state of matter, a plasma is formed when gaseous atoms or molecules are ionised to such an extent that positively charged nuclei and the negatively charged electrons roam free of each other. To do this takes energy; take that energy away and the plasma ceases to be, becoming just gas again. Ring any bells?

Sadly though, plasma is not a perfect fit either. Like a gas, it expands to fill the volume in which it’s contained, whereas a flame does not. But what we can conclude (for now) is that, of the fundamental states of matter, fire is most like a plasma.

In fact, some very hot flames do contain plasma – when the energy inside them is sufficient to ionise enough of the air molecules. A classic example is the flame generated by burning acetylene in oxygen, which reaches an eye-watering 3100˚C. The paltry 1500˚C flame produced by a wax candle, on the other hand, is too low to be considered a textbook plasma.

Modern matter

While we still haven’t properly answered our early human’s question, ‘what is fire?’, we have discovered that the four natural states of matter are not sufficient to properly describe everything in the universe. This is something that physicists are acutely aware of, having come up with numerous ‘modern states of matter’ to explain the plethora of phases that can be observed in the lab.

Burning to find out more? Watch this fire TikTok.

Ian Farrell

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