What really happens in a candle flame

material Candle with the thickest possible wick, matches, wick material, wax cookies, spoon, glass tube (12 cm, d = 8 mm), microscope slide, aluminum wire mesh 4 × 12 cm, magnifying glass, possibly burner

The "candle" is a classic didactic teaching piece [1], [2] and enables Socratic-genetic learning in the sense of Martin Wagenschein. The teaching piece can be carried out at the primary level, but also at the secondary level. The 10 criteria for initiating an experiment must be carefully observed. The number of people taking part must not be too high. No more than 15 participants are recommended. Tables that are arranged in a circle or rectangle so that everyone sits opposite each other are suitable as seating arrangements. The room is slightly darkened. The photos are not a substitute for real observation, but they show details and can sharpen the power of observation. “The candle” is also often treated in connection with the subject of fire or air. Fire poems or air poems are suitable for reading aloud.

“We now want to experience a round of experiments together and also discuss things with one another. You express assumptions and attempts to explain. The following discussion rules apply:

  • Listen to others and speak up if you want to say something!
  • Respond to what others have said!
  • Try to make assumptions!
  • Suggest experiments that prove these guesses!
If there are suggestions for an experiment, the experiment is then carried out by everyone and the result is checked. You will now receive a number of work orders that should be processed under certain questions. The entire series of experiments is documented in writing. "

Task 1 Draw a candle flame!

Before the students draw a candle flame, they are made aware of possible features that they should pay particular attention to. They also hold the candle horizontally and watch whether the flame changes its position. After drawing the flame, suggested solutions are drawn on the board and discussed.


Exercise 2 What is burning on the candle? The wick or the wax?

By experimenting with wicks (without a candle), the students realize that the wick material burns relatively quickly without wax and is no longer usable. There is no real flame.

A wax cookie cannot be lighted with a match. Even if you melt the cookie with a candle flame in a spoon, it won't burn. Perhaps someone comes up with the idea that the liquid wax in the spoon has to be heated up until wax vapors arise that can be lit with a match. The wax in the spoon then continues to burn even without heating with a candle flame, as the flame heats the spoon so much that the high temperature for evaporating the wax is maintained. Note: This flame must never be extinguished with water, as this creates a flash of flame! The wax is left to burn out.

Another experiment to prove that the wax fumes burn with the candle is already known to every child as the “candle trick”: a candle is blown out and observed. A thin, white column of hot wax vapor rises from the wick. If you ignite the wax column further up with a match, the flame “jumps” downwards and re-ignites the candle. In this context, the concept of the states of aggregation can also be worked out. All transitions can be reproduced experimentally:

Exercise 3 Where is the wax vapor in the candle?

With the help of the slide, the glass tube and the wire mesh, the learners try to find out in which zone of the candle flame the wax vapor is located. If you hold the device close to the wick in the flame, you get a white smoke that is black above:

All three tools prove that the wax vapor is in the inner, darker zone of the flame and the soot is in the lighter zone. Another indication that the white smoke is wax vapor could already be proven with the help of the “candle trick”.

If you hold the wire mesh in the lower zone of the flame, a
white smoke, further up you get black smoke.

Exercise 4 Why is the candle lit? Who is responsible for that?
The slide became sooty when it was held into the upper, shining part of the flame. This leads to the assumption that the soot is responsible for the glow. The students now consider an experiment that proves this assumption. Various possibilities are conceivable: If you hold a charred match in a candle flame, it starts to glow. If you rub the ashes of two matches between two slides and sprinkle them into the non-glowing flame of a burner, a brief glow is also visible.

A charred match begins to glow in the candle flame.

Exercise 5 Does the candle flame have the same temperature everywhere?

Here, too, there are several options that can be developed:
a) When the heads of matches are ignited in the different zones of the flame, the duration of ignition varies.
b) If you hold a burned match in the flame for a short time, statements about the temperature can be made about the speed and the type of charring on the edge of the match.
As a result, we note: Directly above the wick, in the inner, darker zone, the temperatures are lower than in the upper or outer area of ​​the flame.

Exercise 6 What is the purpose of the wick?
When you light a candle, you can watch the cold wick (if it is long enough) with a magnifying glass how the wax in the wick liquefies. The wick transports liquid wax from the candle into the flame.
Task 7 The learners formulate what exactly happens in the candle flame
There is no such thing as a “model solution”; it is much more important that the students formulate the path to knowledge in their own words and present it in detail in their exercise book. The educational moment lies in the process of experimentation, discussion and reflection itself.

The candle flame is a self-sustaining system that is not easily transparent: In the inner, darker and cooler zone of the flame, liquid wax is evaporated into wax vapor. At the transition to the lighter zone, the wax vapor burns to soot. The soot begins to glow in the glowing zone because it is stimulated by the heat. By burning the wax vapor, new liquid wax is drawn out of the wick. The heat of the flame causes the wax in the wick to liquefy. Martin Wagenschein put it this way [4]:
"The candle itself:" Warmth rises "(but why?). It creates the flow, but it also needs it so that more and more air can get to it, because the air "uses itself up" as in any fire. - What a functional little machine such a candle is! Once ignited, the operation of this small gas factory continues and regulates itself. The gas tongue above, while burning, gives warmth; the same heat melts the wax. In the wick the liquid wax rises “by itself” (really, by itself?), And above the combustion heat generates the air flow that the flame needs to live again. It has two tasks and solves them at the same time: melt the fuel, evaporate it, prepare it, and draw in the combustion air. In addition, the third, which is the most important to us: it shines its warm light. It lures us into the optics. "
One question has not yet been answered: What happens to the soot when it starts to glow? In further experiments it would be possible to prove that carbon dioxide is formed as a combustion product of the soot above the flame. If the gases are sucked through a gas washing bottle with lime water, the turbidity of this gas can be demonstrated. The demonstrations on oxidation and mass conservation refer again to the candle experiments. Some approaches go even further and use the candle to develop the natural carbon cycle [3]. However, the approach here is limited to simple understanding with as little equipment as possible.
[1] Ueli Aeschlimann: Why is the candle flame lit? Writings of the Swiss Wagenschein Society, 2001
[2] M. Faraday: Natural history of a candle, Verlag B. Franzbecker 1979
[3] Eberhard Theophel: Kerze nach Faraday, In: Berg / Schulze: Lehrkunst - Lehrbuch der Didaktik, Luchterhand 1995, pp. 283 ff
[4] Martin Wagenschein: See and understand natural phenomena, Klett-Verlag 1980, p. 116