How do you do, dear Friends of Science.
I am glad that you are attending my lecture today.
Today I would like to talk about a subject that you all know: the candle, especially
the candle flame. Candles have been used for many hundreds of years to enlighten rooms.
With their warm light they provide a particularly contemplative mood. But let´s not be satisfied
in such sentimental findings, we want to explore the essence of a candle flame scientifically.
We start with a simple question: What do we need to produce a flame, to start a fire?
Well, it turns out three things are required : firstly, a flammable substance must be present.
Examples of combustible materials are: oil, grease, wax, natural gas; this flammable substance
must generally be present in the gaseous state. If the substance is a liquid or – as with
candle wax — a solid, it must be melted and vaporized. For this purpose, a candle wick
is required. It kind of focuses the heat of the flame; the wax melts in the vicinity of
the wick – we have a temperature of about 80 degrees Celsius of the molten wax. The
molten wax flows up in the porous wick – like ink in a chalk or blotting paper — the so
called capillary rise. Directly on the wick, it is very hot, so that the liquid wax evaporates.
The wax vapor is about 700 degrees Celsius. Actually we can make the wax vapor visible
by taking a glass tube and pull off some gas from the region near the wick. We see the
white wax vapor. This vapor can also be seen shortly after blowing out the candle. For
a flammable substance to burn, it must come into contact with air – more specifically,
a component of the air, namely oxygen. A candle has to “breathe”, just like an animal or a
human being. The air the candle breathes is drawn to the flame from below by convection.
As this process only works with gravity, a candle will not work very well in weightlessness.
The sucked air cools the sides of the candle, so that the liquid wax is usually in a sort
of shell which is made up of solid wax, and is not running down. Close to the wick the
wax vapor mixes with the air to produce a combustible mixture. This mixture must be
ignited to burn. This brings us to the third and final condition for a flame — the third
side of the fire triangle – an ignition source must be available, a spark, heat, matches
— at least initially. A match will easily ignite a combustible mixture. It´s possible
to ignite the wax vapor from a just blown out candle at some distance from the wick.
Application of heat can ignite the combustible mixture. By removing heat I can also delete
the flame again. When I approach a metal wire net to the flame, the metal is carrying away
heat very fast, so that the remaining temperature is not sufficient to reach the ignition point.
A flame cannot burn through a wire mesh. This effect can be seen in mining lamps. Now we
come to the actual chemical reaction that keeps the candle flame alive. Wax vapor – a
paraffin hydrocarbon -and oxygen (from the air) form carbon dioxide, water vapor, and
a lot of heat. With suitable equipment, both reaction products could be chemically identified.
Much heat is generated, which is obvious. The combustion of 1 g of wax gives rise to
about 9 kcal of heat (38 kJ). This 1 g of wax will burn in about 10 minutes. The candle
thus produces a power of 38 kJ/600 sec or 60 Watts. The above-mentioned reaction takes
place in the blue part of the flame – where wax vapor and air mix. The blue glow comes
from the fact that some reactants will be energetically excited and emit light. The
heat generated in the reaction, has several consequences – first, it ensures that more
wax melts and evaporates at the wick. Then, there is another important effect: At high
temperatures, part of the wax vapor is decomposed. Similarly to how a cake turns black in the
oven at too high a temperature, paraffin wax will decompose to carbon soot. This soot can
be found in the yellow part of the flame. Carbon soot is usually deep black – it is
nothing more than graphite, you know from the pencil. However, the carbon soot in the
candle flame has a temperature of about 800 ° C and as any material having this temperature,
the soot will emit visible light, it will glow. The yellow color of the flame is just
glowing soot. You can show the presence of soot for example by holding a china plate
in the yellow zone of the flame or by deleting part of the flame with a wire mesh.
At the edge of the yellow zone the luminous glowing soot gets in contact to air. There
it reacts in a further exothermic chemical reaction to form carbon dioxide – CO2 – and
more heat. Actually this is the hottest part of the candle. A similar thing happens with
the wick when it emerges from the flame. The burning wick is composed essentially of graphite,
which burns at the flame border – also with the typical yellow temperature radiation.
We may easily check experimentally some of the chemical necessities I just mentioned.
I said that a flame must breathe. If we don´t allow it to breathe, it will go out. The flame
breathes air from below – if I put a glass tube on the flame, then air is sucked in from
below. If I interrupt the intake stream, the candle will go out. In principle, any fuel
which can be brought into the gaseous state, is good as candle material. So it is not difficult
to make a candle from butter or oil. If I burn one teaspoon of butter in a candle flame,
100 kcal of heat will be produced. Well, a butter candle flame does not burn as uniform
as a wax candle. This is due to butter not exclusively being made up of fat, but also
some non-combustible constituents, for example water. The wick of the candle must not be
made necessarily of cotton yarn. Each porous , somehow heat resistant material, can be
used, as chalk or even blotting paper. If I want a candle flame to go out, I may provide
either too little or too much air to the flame. We have discussed the effect of too little
air. Too much air simply means to blow out the candle. This blowing out can be accomplished
using a drum. A drumbeat creates a blast of air – a vortex of air more specifically – which
may move from the drum to the candle. If I’m shooting well, I can blow it out. Well, try
this at home and see if you´re doing better than me. Thank you for your attention.