How Hot is Fire?

Table of Contents

According to scientists, early humans started using fire approximately one million years ago. The discovery of fire has been considered to be one of the first great innovations of humankind. Scientists also hypothesize that the use of fire played a role in the evolution of humans. This is known as the cooking hypothesis.

Fire is a part of many other processes apart from cooking. For example, fire is also used for the clearing of land for agriculture, for making heat and light, for the incineration of waste, as a weapon, or for destruction.

Yet, what exactly is fire? how hot is it and what are its properties?

The discovery of fire
The discovery of fire by early man – the use of flints to start fire. Credit: Margaret A. McIntyre/Wikimedia Commons

What is Fire?

Components of the fire triangle or the combustion triangle – oxygen, fuel and heat. Credit:Gustavb/Wikimedia Commons

Fire is the effect observed because of the process of combustion . Combustion is a special type of chemical reaction in which oxygen reacts with some sort of fuel. In this reaction, oxygen and fuel are known as reactants of the chemical reaction. A combustion reaction results in the generation of heat and light in the form of a flame.

 

The rate at which the reactants react in a combustion reaction is usually very high. Thus, more energy is generated in this reaction than that which can escape into the surroundings. This results in an increase in the temperature of the reactants, speeding up the reaction even more. In order for combustion to occur, the fuel must be heated to a specific temperature known as the ignition temperature .

 

Depending on the amount of oxygen available, a combustion reaction may be relatively slow or fast. If the rate of combustion is high enough to form a flame, then it is called burning. The fire will occur as long as there is a supply of the three components – fuel, oxygen, and heat. This is also called the fire triangle .

The products  of a combustion reaction that involves hydrocarbon-based fuels – such as petroleum, gasoline, and coal – are carbon dioxide and water along with energy. Thus, the entire chemical reaction in the process of combustion may be written as follows:

 

Fuel + Oxygen + Energy → Carbon Dioxide + Water + More Energy

 

The energy released results in the generation of heat and light in the form of a flame. A flame or a fire has a definite composition and a complex structure. Flames may exist in various forms. They may be formed at low temperatures as well as at extremely high temperatures. The energy released excites particles such as charged atoms, molecules, and electrons, generating light.

What happens when we light a match?

Fire
The lighting of a matchstick – an example of a combustion reaction. Credit: Jith JR/Wikimedia Commons

When a match is struck, the friction at the head of the matchstick heats the head to ignition temperature. Thus, the chemicals present at the head can now react at that temperature and generate more heat. This heat escapes into the air and burns with a flame. However, if the wind blows at the head or if the head is moist, friction is not sufficient to raise the temperature of the matchstick, so the flame is not formed.

Once a flame is formed, its heat raises the temperature of the nearby layer of the matchstick and of the oxygen in the air around it. Thus, the wood of the matchstick and oxygen in the surrounding air react in a combustion reaction. Now, the flame of the matchstick eventually goes out. This is because equilibrium is reached in the combustion reaction.

Why is fire hot?

Flames formed because of combustion (burning). Credit: Vyacheslav Argenberg/Wikimedia Commons

Now that we have seen that fire is the product of a combustion reaction, we will understand why fire is hot. Fire is hot because there is more energy generated than that which is needed for the combustion reaction.

 

Where does this additional energy come from? This energy or heat comes from the energy stored in the fuel. As seen above, the fuel reacts with oxygen to give rise to carbon dioxide and water. This reaction requires the breaking and making of chemical bonds in the reactants and the products. Breaking the chemical bonds between oxygen molecules requires energy. However, the formation of the chemical bonds in carbon dioxide and water releases much more energy. Thus, the net energy released is greater than the energy required in this chemical reaction.

How hot is fire?

Although we know that fire is hot, exactly how hot is it? There is no definite temperature point for fires because the amount of energy released differs greatly depending on several factors. The chemical composition of the fuel, as well as the amount of oxygen available for the combustion reaction, determine the temperature of the fire. Furthermore, the portion of the flame used to measure the temperature must be considered.

A fire in which wood is used as fuel may have a temperature greater than 1100° C (2012° F)—however, different types of wood burn at different temperatures. For example, pine wood is known to produce twice as much heat as fir wood or willow wood. Dry wood used as fuel also burns hotter than green wood.

What about the temperature of a flame generated in a combustion reaction using a fuel that is not wood? Propane , for example, burns at 1980° C (3596° F), and acetylene in oxygen burns at 3100° C (5612° F), which is hotter than any wood.

Can the temperature of a fire be seen?

flame color temperature
Flame Color Temperature/ Image Credit https://www.cityfire.co.uk

We can roughly estimate the temperature of a fire by its color. A deep red fire is about 600–800° C (1112–1800° F). An orange-yellow fire is about 1100° C (2012° F). A white flame is hotter than both, having temperatures of 1300–1500° C (2400–2700° F). A dazzling white flame is the hottest flame of all, with a range of 1400–1650° C (2600–3000° F). Thus, the next time we spot a fire, we can roughly guess how hot it is!

Glossary

Combustion: A rapid chemical reaction involving oxygen and producing heat and light.

 

Ignition Temperature: The minimum temperature needed to start or cause combustion.

 

Products: Substances present at the end of a chemical reaction.

 

Reactants: Substances present at the beginning of a chemical reaction

 

Fire Triangle: A simple model showing the necessary components for most fires

 

Equilibrium: A state wherein the reactants and products remain unchanged

 

Propane: a flammable gas made of carbon and hydrogen

 

Acetylene: a colourless pungent-smelling gas made of carbon and hydrogen

Anne Marie Helmenstine, P. D. (2020, November 21). Why fire is hot (and how hot it is). ThoughtCo. Retrieved February 9, 2023, from https://www.thoughtco.com/why-is-fire-hot-607320

 

Encyclopædia Britannica, inc. (2023, January 27). Combustion. Encyclopædia Britannica. Retrieved February 9, 2023, from https://www.britannica.com/science/combustion

 

Hirst, K. K. (2019, May 4). When did people begin using fire? Retrieved February 9, 2023, from https://www.thoughtco.com/the-discovery-of-fire-169517

 

What is fire? Science Learning Hub. (n.d.). Retrieved February 9, 2023, from https://www.sciencelearn.org.nz/resources/747-what-is-fire

Copyright @smorescience. All rights reserved. Do not copy, cite, publish, or distribute this content without permission.


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