Well now let us discuss about the glowing fire which glows even brighter when we blow. Isn’t it strange? Every one of us knows that we take in oxygen and give out carbon dioxide. We have been learning this since our childhood, may be since 1st or 2nd standard in our school. We take in oxygen for oxidation of food to release energy in the form of ATP (Adenine Triphosphate). Oxygen is required by each and every cell of our body for cellular respiration.
Oxygen reach to each of the cell in the form of oxy-hemoglobin complex and carbon dioxide is released from there in the form of carboxy-haemoglobin complex. Use of oxygen is not limited just for our body but also for ignition (burning). Without oxygen, fire is nearly impossible. Oxygen is good supporter of burning but not itself a combustible gas. On the other hand, carbon dioxide is neither combustible nor a supporter of combustion. It is a good fire extinguisher instead.
Whenever carbon dioxide is exposed to fire, it forms a blanket around the fire stopping the oxygen to reach to the fire to make it burn continuously. This is added up by the weight of carbon dioxide being more than that of oxygen and other atmospheric gases. It can hence extinguish the fire relentlessly. Now, the confusion can reach to the peak. We exhale carbon dioxide which is a good fire extinguisher. Despite this fact, the fire glows brighter when we blow. I do not recommend you to go and try it in case of a burning candle. It will surely go off. We all have at least once tried it in case of fire on logs or woods.
Let me now explain you the actual reason behind it. The pressure difference created by the blowing carbon dioxide near the fire plays the role. The blowing carbon dioxide having greater velocity than the atmospheric air causes to lower the pressure of air near the fire, given by Bernoulli’s theorem (pressure is inversely proportional to the velocity). This pressure difference causes nearby atmospheric air to rush towards the fire. As I have mentioned that oxygen is very good supporter of combustion, with such large amount of oxygen being rushed towards fire, it glows with significant brightness. Another question may arise in your mind, why we see using a hollow, narrow pipe to blow the air in fire? It is favored by Bernoulli’s theorem and equation of continuity followed by principle of conservation of mass.
When air enters through a narrow pipe, the area of cross-section decreases so that the velocity of air increases, given by the equation of continuity (area of cross-section is inversely proportional to the velocity). It can be observed better in case of liquid. As the velocity increases, the pressure decreases, given by the Bernoulli’s theorem. As there is smaller cross-section in a narrow pipe, there is greater velocity leading lower pressure. To balance the difference in pressure, there is greater chance of rushing of oxygen towards fire to make it glow brighter. This is the reason behind using hollow, narrow pipe for blowing the air in fire.
Then why doesn’t this work in case of candle? Actually, the burning candle does not have enough surface area of ignition to favor this phenomenon. When carbon dioxide blows towards candle, immediate blanket is formed which blocks the passage for oxygen within a moment to turn off the fire. However, in case of fire on wood or logs, the fire has greater surface area which cannot be easily covered by the amount of carbon dioxide we exhale from our tiny lungs. But of course, if we blow carbon dioxide in extent enough to form a blanket around the fire, then any sort of normal fire we use in our daily life can be extinguished. I personally recommend you to experimentally try this and conclude on your own for deeper and clearer idea about what I have just explained.
Discovered by Madhusudan Duwadi
