Why are candles burned in glass

Now is the time of year when we can think about the soot: The wood or coal burns in the fireplace stove, candles spread cosiness with their cozy light. Many now ask the question: Why does soot form on the windows of a fireplace when the flame is bright and the heat is high? Why do candles soot? Soot is made up of carbon. It should just burn away under the reaction conditions.

Image 1: Soot on a chimney pane
(Photo: flower)


We can investigate this with a simple experiment.

Experiment: Soot formation in a candle flame
We light a candle or a tea light and let it burn a little. Then we hold a glass in the flame for a short time. Danger! Cheap glass can crack.
Result: The glass first steams up with water, which then dries away. Soot then forms.
Note: If you use refractory glass (e.g. quartz glass, Duran ), you can even burn off the soot again with a dimmed Bunsen burner flame.

This is how we used to make our own goggles for viewing a solar eclipse.

Many people assume that burning carbonaceous materials such as coal, wood and paper, as well as hydrocarbons (e.g. diesel oil or candle wax) only produces water and carbon dioxide. Ultimately, that's also true - such as B. with hydrocarbons:

However it is from fuels to H.2O and CO2 a long chemical path that is anything but direct. For example, in the course of this reaction chain, carbon has to be formed somehow.

In this context, it should be remembered that the common fuel materials such as wood and paper as well as coal are ultimately derivatives of hydrocarbons, so that we can conduct the discussion on the basis of the more manageable reaction processes of this group of substances.

At first, gases and radicals such as B.

The H atoms can also be taken over directly by oxygen molecules, which are known to be radicals themselves. Of course, the oxygen can also react directly with the hydrocarbons - with the formation of CO2 is being prepared. In any case, hydroxyl radicals HO are formed or hydroperoxide radicals HOO and of course H2O molecules.

There are an unbelievable number of other possible reactions, in which ions are also involved. Ultimately, all H atoms can split off.

The consequence of this `` stripping '' of the hydrocarbon molecules is that there must be free carbon atoms. These fly around very quickly in the hot gas phase and look for a bonding partner. This can also be other carbon atoms.

When the C atoms hit cooler surfaces, they lose energy and assemble to form carbon atom aggregates. One speaks of `` Quenching '' the flame. In the process, the graphite lattice, which is particularly favorable in terms of energy, is formed - this is the soot.

In the same way, soot development is a blakenden candle flame to explain. The only difference is that no cold surface is necessary to quench the hot flame gases. The cold ambient air takes on this role here.

What we have discussed also applies to diesel engines that are considered environmentally friendly. It is well known that soot wells up from their exhaust - one of the main sources of the notorious fine dust.

With precise technical control (for example with regard to temperature, pressure or the presence of certain foreign substances), various modifications of the carbon can be obtained when hydrocarbons are burned. Instead of a carbon black coating made of graphite, it is even possible to produce pure diamond coatings - or even fullerenes and nanotubes.


Something else has to be said about the color of the flame
It is noticeable that the flame of burning wood appears yellow. This is always the case when soot is heated - this is also an indication that there is free carbon in the flames. The more oxygen there is, the less carbon can hold. Schoolchildren know this from the Bunsen burner: By increasing the air supply, the flame is `` dimmed ''.

The fact that the ash glows purple is due to the potassium compounds such as potash, which are released when wood is burned.

Photo 2: furnace flame
(Photo: flower)

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