Making chula burn
We blow carbon dioxide from our mouth for making fire in a chula. Carbon dioxide is a fire extinguisher. Then how is it possible to make the chula burn?
SIJU P. RAJ
Kollam, Kerala
When we breathe out air it still contains a substantial amount of oxygen (about 16 per cent). The exchange of oxygen and carbon dioxide between the blood and air in the lungs is by diffusion and it cannot be completed to deplete 100 per cent of oxygen from the air (and also carbon dioxide from the blood). Further, when we blow the breath by our mouth, this mixes with the air that contains slightly more oxygen and a lot of nitrogen.
The reason for blowing to make fire in a chula is to remove the ash from the charcoal or firewood so that fresh hot combustible material comes in contact with the air containing oxygen.
Blowing also helps to direct the flame and heat in a desired direction to assist combustion. More than the carbon dioxide it is the abundant nitrogen that should come in the way. But there is enough oxygen to sustain combustion and blowing helps to make fire. In a fire extinguisher using carbon dioxide, a lot of it is blown to completely mask the fire from oxygen in the air. We cannot do it by our breath since breath is not powerful enough and also it contains almost as much oxygen as the air.
An interesting related problem is the candle flame where due to natural buoyancy the hot lighter gases move up and the fresh air is replenished from below. This cannot take place in a zero gravity atmosphere in a satellite in outer space and a candle lit even in an oxygen filled chamber gets extinguished. This is because carbon dioxide and other gases produced by combustion create a blanket around a candle flame and sooner or later diffusion alone cannot allow oxygen to penetrate up to the flame.
Also, note that on the earth a candle or a flame can be blown out even by an oxygen jet if its velocity is sufficiently high. You can repeatedly and vigorously blow on a lit incense stick to extinguish it. This is not by withholding oxygen. But to extinguish a cigarette in the same fashion you may need an elephant!
MOHAN D. DESHPANDE
M.S. Ramaiah School of Advanced Studies
Bangalore
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