- How does a candle burn?
Where does the wax go?
Candle wax is just paraffin, a hydrocarbon. As the name
implies, a hydrocarbon contains just hydrogen (H) and carbon
(C). When candles burn, most of the wax reacts with oxygen
(O) in the air to form water vapour (H2O)
and carbon
dioxide
(CO2). Both water vapour and carbon dioxide are
gaseous and
invisible. Thus when a candle burns, the wax goes into the
air.
Candlelight (Photo: Courtesy of H.Y. Chiu)
When a candle heats up, some of the wax melts. The wick
carries the melted wax up to a point where it vapourizes and starts
combustion, i.e. the wax reacts with the oxygen in the air to form
water vapour and carbon dioxide. Heat is released during the
combustion, and this keeps the melting and vapourizing going until all
the wax runs out.
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- Can a candle burn without a
wick?
When a candle burns, the melted wax is carried by capillary action up
the wick. The same mechanism works in plants, where
water is carried up from the root to other parts of the
plant.
A puddle of melted wax that is left on the table will not
burn. So will a block of wax. This is because there
is not enough oxygen in contact with the wax. Only vapourized
wax has sufficient amount of oxygen in the surroundings to keep the combustion
going.
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- Why is candlelight yellow and
gas-grill light blue?
It all depends on how much oxygen is
around. Lots of oxygen makes blue flames, while limited
oxygen produces yellow flames.
The amount of oxygen that is available
for candlelight is still not sufficient to give complete combustion,
i.e. for all the wax to become water vapour and carbon
dioxide. Under the heat, some of the wax (paraffin) breaks
down into tiny particles of carbon, which is called soot.
These particles are heated up under the high temperature and glow with
a bright yellow light. This makes candlelight
yellow. On their way up the flame, most of the particles find
enough oxygen to burn themselves out.
Some of the carbon particles, i.e. soot,
do remain, however. You can catch them by putting a knife or
spoon in the flame for a few seconds. The blade will collect
a black coating of carbon – the same material you find on the inside
of a chimney. Soot is a source of pollution, and is commonly
found in poorly maintained vehicles and from coal-burning homes and
industries.
All in all, there is too much fuel and too little oxygen going on in
candlelight. This makes the combustion far from an efficient
process.
A gas flame (Photo: Courtesy of H.Y. Chiu)
In contrast, a gas-grill flame is more efficient. It uses gaseous
fuel, i.e. no vapourizing is required. The burning is almost
complete and the flame is much hotter than candlelight and can reach
several hundred degrees Celsius. The fuel molecules emit blue
and green light when hot. The human eye is more sensitive to
blue light, hence we perceive a blue flame.
For even more efficient combustion, people use pure oxygen.
After all, air is only about 20% oxygen. Using pure oxygen
and acetylene gas, a welder’s torch reaches over 3 000 degrees Celsius.
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