Simple Science
Why do candles go out when you blow
on them?
© Jerry Jacobs
At a glance
Fire requires 3 factors
to burn: Oxygen, Heat
and Fuel
Removing one of these
factors will put out the
fire
Blowing on a candle
flame causes air to
move across the wick
removing the heat
from the flame
The wax can no longer
reach a sufficiently
high temperature to
burn and the candle is
extinguished
It wouldn’t be a birthday party without the traditional blowing out of birthday
candles on top of the cake. The science behind extinguishing a candle is in
fact far more complicated than the act of extinguishing the candle. The
process of burning is actually the substance reacting rapidly with air. In order
for a fuel, in this case the wax, to react with oxygen it requires energy. By
bringing a flame to the wax you give the wax enough energy to react quickly
with the oxygen, producing fire. As we know fire produces energy, the energy
produced by the fire allows more wax to melt and burn forming a chain
reaction which allows the candle to continue to burn. Different materials
require different amounts of energy to ignite. When you build a fire, you put
a layer of paper underneath a layer of wood, on top of which you put your
coal. The coal requires the most energy to ignite followed by the wood
followed by the paper. Each layer produces enough energy when it burns to
set the layer above on fire. This means that you can burn coal without having
to have an incredibly hot flame in your hand.
Oxygen
Heat
Fuel
To put out a fire you either need to remove the fuel, oxygen or heat. When
you blow on a candle the movement of the air across the wick removes the
heat from the flame. This prevents the wax from reaching the temperature
it needs to be at to burn and so puts out the fire. Another more brave way
to put out a candle is to squash the wick. This works since you prevent the
wax from traveling up the wick to the flame. This removes the fuel from the
fire. A final way to put out a candle is to cover it with a glass. The wick will
continue burning, using up the oxygen, until there is no longer any oxygen in
the glass for it burn.
Centre for Lifelong Learning
Alex Brown, 2012
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