Chemistry

What is a flame?

Flames from a bonfireFlames from a bonfireWe are all familiar with flames, the flickering tongues of coloured light that ripple upwards from bonfires and burning candles. Flames are the visible portion of hot gases that are produced when something burns. Only the hottest portion of the gases emits light, and it is this portion that we see as coloured flames. The cooler portion of the gases remains invisible.


Lighting a candleLighting a candle

Burning

To get something to burn (the scientific term is “combust”), oxygen, fuel and a source of heat are needed. The heat comes from striking a match, or focused light, or something else that is already burning. A candle is lit by lighting its wick, the cord sticking out of the top, with a match or a lighter. The rest of the candle is made of wax. The wax contains atoms—the basic building blocks of matter—of the elements carbon and hydrogen. They are the fuel for the combustion process. 



Blue flames from the burning of gas on a cooking ringBlue flames from the burning of gas on a cooking ring

Glowing gas

The heat causes the atoms in the candle wax to turn into gas. This is called pyrolysis. The atoms of carbon and hydrogen leave the surface of the wax through the wick, and immediately start to react with oxygen in the air. This chemical reaction is known as oxidation, and releases heat—which we can feel if we place our hand near the flame. As the atoms go through this process of oxidation, they are shaken violently and also radiate light. This is a process known as chemiluminescence. A pure flame from a fuel made of carbon and hydrogen atoms (hydrocarbons) glows blue.

 
 

The different colours in a candle flameThe different colours in a candle flame

Flame colours

Flames from a candle or a bonfire are a mixture of colours, including red, orange and yellow as well. How are these other colours produced? When a candle burns, some of the carbon is left behind after oxidation. These collect together to form tiny particles we call soot (smoke is the name we give to clouds of soot particles mixed in with gases). In the intense heat of the flame, these minute black bodies start to glow. This is called incandescence, and it gives a flame its range of colours.

The colours depend on the temperature: beginning with red, then with higher and higher temperatures, turning to orange, yellow and then white. A blue-coloured flame only emerges where there is little or no soot. Blue can often be seen near the base of the candle flame, where there is usually little soot.

On Earth, gravity determines how the flame burns. The hot gases in the flame are much hotter than the surrounding air, so they rise upwards. This is why flames are "pointed" at the top. If you were to light a fire in a place where gravity was very low—on a spaceship, for example—the flame would form a sphere.

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