What causes the colour of a flame?
The colour of a flame depends on three factors.
A flame is produced when something burns, that is, when it combines with oxygen in the air.
The process is called combustion.
The colour of the flame depend on
- What is burning.
- The temperature of the flame.
- The amount of air (oxygen) that is supplied.
What is burning
A match burns with a yellow flame, but a piece of magnesium metal burns with a brilliant white light.
The temperature of the flame
As the temperature increases, the colours go from dark red to bright red, then to orange, then to yellow, and finally to white-blue.
Thus, different parts of the flame will have different colors depending on the temperature.
The hotter parts, closer to the burning fuels, will be whitish-blue. The parts that are further away will be cooler and orange-reddish.
Orange corresponds to a temperature of about 1000 °C.
The amount of air
If enough oxygen is available, the substance will burn completely (complete combustion).
If there is not enough oxygen, to burn everything (incomplete combustion), the flame temperature will be lower.
Also, unburnt material such as soot (carbon) particles will glow orange in the flame.
In a candle flame, the air comes up from the bottom.
There is complete combustion. The flame is hot and has a blue colour.
Further up in the flame, there is less oxygen, because it has already been used up, and the temperature is lower.
Unburned carbon particles (soot) will glow orange in the flame.
You can see the same effect in the burning match.