Dr. Lauth´s Lab #4 - How a candle burns - Fire triangle, butter candle & candle drum

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.