The Chemistry of Fire

31st January 2018

Science

We’ve all heard of fire, and see it almost every day in matches, lighters, fireworks, gas hobs, and fire places. But this ostensibly simple reaction is actually a complex scientific event.

What is Fire Exactly?

Fire is an exothermic, self-perpetuating reaction that happens when a solid, liquid or gas-phase fuel undergoes rapid oxidation. This is known as combustion, and the reaction releases heat, light and further chemical reactions.

While allowing other reactions to take place, combustion is predominantly characterised as the combining of combustible materials with an oxidising agent.

When a fuel and an oxidising agent are heated passed a certain temperature, exothermic chemical reactions occur and are maintained by the additional heat energy that they continually generate.

Therefore, for a fire to be produced from combustion, these four elements are essential. This relationship can be conceptualised in the fire tetrahedron.

Fire Tetrahedron

Fire Tetrahedron

The fire tetrahedron is a representation of the elements necessary for a fire to occur. The four faces of the tetrahedron are fuel, an oxidising agent (usually oxygen), heat energy, and an uninhibited chain reaction.

Fuel

Fuel refers to a flammable or combustible material that is used to begin the process of combustion. While fuel can be a solid (wood), liquid (gasoline) or gas (propane), the materials will only burn when in the vapour phase. This means that in order for fire to begin, a combustible gas-phase must exist.

The gas-phase is achieved when a material is heated passed its flash point in order to exert a vapour pressure that can ignite in air and support combustion.

An example of a solid combustible material, and perhaps the most common form of fuel, is wood. This compound has high molecular weight molecules that include materials like lignins and naturally occurring carbohydrate cellulose.

For wood to reach the necessary gas-phase, these materials must undergo thermal decomposition by pyrolysis. This occurs when the wood is heated past its flash point, which causes the cellulose, and other materials, to decompose into small molecules that can then exist in gas-phase. When these gases reach their ignition temperature, burning begins.

Heat Energy

Heat energy is required to start the ignition of the fuel and to get it to the minimum temperature required for it become self-sustaining. This is referred to as the ignition temperature.

Heat energy is produced during combustion because the reaction is exothermic. Heat is released when chemical bonds are broken and formed during chemical reactions. Since these reactions are ongoing, combustion releases more heat than is needed to sustain a fire. This is what makes a fire self-perpetuating, and also what makes it hot.

Oxidising Agent

An oxidising agent is required to support burning by reacting with the fuel. Oxygen in the air is the most common agent used. Once the volatile gases released by the fuel have reached ignition temperature, the compound molecules break apart and recombine with oxygen to form water vapour, carbon dioxide, various combustion products, and more heat. This process is called oxidation, and can be recognised as burning and smoke.

Uninhibited Chain Reaction

The final face of the tetrahedron is the uninhibited chain reaction that is enabled by the reaction between fuel, heat and oxygen. An uninhibited chain reaction refers to the self-perpetuating capability of combustion.

Because of the continual reactions taking place between fuel and oxygen, which generate surplus amounts of heat energy, the flame will always be hot enough to keep the fuel at ignition temperature. Therefore, the fire will continue to burn as long as there is enough fuel and oxygen available. This process similarly finishes when these sources have been expended.

giphy

Conflagration: How Fire Spreads

The danger of these chemical reactions is the fact that they are self-sustaining. Fire can spread precisely because of the uninhibited chain reaction that occurs from combustion, and from the heat energy that keeps the fuel above ignition point.

The heat of the flames are able to heat surrounding fuel, whether that’s nearby wood or flammable liquids. If this nearby fuel is heated passed its flash point, volatile gases will be released as the fuel enters into its gas-phase. At this point, the flames are able to ignite the gas and spread. As long as there is fuel and available oxygen, fire is able to propagate.

When it comes to how fire travels, it all comes down to gravity. The hot gases in the fire are hotter and less dense than the surrounding air. Therefore, they move upward to where there is lower pressure. This is why fire travels uphill, and also why flames are pointed.

Flame Chemistry

There are a variety of colours associated with flames. These depend on the chemical composition of the fuel being burned, the reaction products being generated, and the heat at which it’s burning. For example, the colour blue in flames is due to the presence of carbon and hydrogen, but it also indicates that it is the hottest part of the flame. If there were copper compounds being burned, on the other hand, the flames would be green.

Colour variation in a flame is due to uneven temperature. A typical example of this is when a fire undergoes incomplete combustion. This happens when there is not enough oxygen to keep up with the burning of fuel, and it is commonly seen in camp fires. This because there is only 21% oxygen in our atmosphere, and while this is enough to cause oxidation it is not enough to keep up with the multiple chemical reactions that take place during combustion.

Being unable to react with oxygen, some of the fuel carbonises with itself to create soot. Soot gets extremely hot and begins to emit a visible white light. The soot particles that rise in the air begin to cool, causing their emission spectrum to shift to infrared. This is why the top of a fire is usually red while the bottom in more yellow-white. When there is complete combustion, meaning that there is a sufficient supply of oxygen, the flame will burn blue. The changes in flame colour caused by combustion rates is most easily seen in Bunsen burners, where you can manually control the amount of oxygen being fed to the flame.

Logs burning on a camp fire

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