January 07, 2022 7 min read
We all know that if you gather up a bunch of dry twigs, grass, and leaves and put a lit match to them, they’ll burn. Add some more sticks and bigger bits of wood and you’ve got fire ready for the barbecue. But how does fire actually work?
Fire is the result of applying enough heat to a fuel source, when you’ve got a whole lot of oxygen around. As the atoms in the fuel heat up, they begin to vibrate until they break free of the bonds holding them together and are released as volatile gases. These gases react with oxygen in the surrounding atmosphere. This chemical reaction causes a lot of heat, so much heat, in fact, that it can keep driving the reaction, as long as there’s enough fuel and oxygen still present, the reaction will become self-sustaining. The actual flames of the fire are the release of some of the heat energy as light.
Courtesy: Science 4 Fun
Fuel + Oxygen + Energy → Carbon Dioxide + Water + More Energy
Why Is Fire Hot?
In a nutshell: Fire is hot because the energy stored in fuel is released suddenly. The energy required to start the chemical reaction is much less than the energy released.
How Hot Is Fire?
There is no single temperature for fire because the amount of thermal energy that is released depends on several factors, including the chemical composition of the fuel, the availability of oxygen, and the portion of the flame being measured. A wood fire may exceed 1100° Celsius (2012° Fahrenheit), but different types of wood burn at different temperatures. For example, pine produces more than twice as much heat as fir or willow and dry wood burns hotter than green wood. Propane in air burns at a comparable temperature (1980° Celsius), yet much hotter in oxygen (2820° Celsius).
Other fuels such as acetylene in oxygen (3100° Celsius) burn hotter than any wood.
The color of a fire is a rough gauge of how hot it is. Deep red fire is about 600-800° Celsius (1112-1800° Fahrenheit), orange-yellow is around 1100° Celsius (2012° Fahrenheit), and a white flame is hotter still, ranging from 1300-1500 Celsius (2400-2700° Fahrenheit). A blue flame is the hottest one of all, ranging from 1400-1650° Celsius (2600-3000° Fahrenheit). The blue gas flame of a Bunsen burner is much hotter than the yellow flame from a wax candle!
Hottest Part of a Flame
The hottest part of a flame is the point of maximum combustion, which is the blue portion of a flame (if the flame burns that hot). However, most students performing science experiments are told to use the top of the flame. Why? Because heat rises, so the top of the flame's cone is a good collection point for the energy. Also, the cone of the flame has a fairly consistent temperature. Another way to gauge the region of most heat is to look for the brightest portion of a flame.
Fun Fact: Hottest and Coolest Flames
The hottest flame ever produced was at 4990° Celsius. This fire was formed using dicyanoacetylene as fuel and ozone as the oxidizer. Cool fire may also be made. For example, a flame around 120° Celsius may be formed using a regulated air-fuel mixture. However, since a cool flame is barely over the boiling point of water, this type of fire is difficult to maintain and goes out readily.
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