Why the flame of gas stoves is blue and what it means when it turns yellow-red

Anyone who has one gas cooker at home knows that, once turned on, i stove one is formed immediately blue/light blue flamevery different from the red/orange one generated by a candle or a wood-burning fireplace. We might think that the color of the flame depends on its fuel, which in the case of stoves it is typically methane gas or sometimes, in areas not served by distribution systems, LPG in cylinders. In reality, even these gases in the wrong conditions can form more reddish flames: malfunctioning stoves lead to the formation of residues of carbonaceous particles commonly called soot, which when incandescent emit a strong yellow-red colour. There blue flame it simply indicates the complete combustion of hydrocarbon (methane or LPG), without residuesobtainable only pre-mixing the fuel with the right amount of air: a principle made famous by the German chemist Robert Wilhelm Bunseninventor of the so-called “Bunsen burner”, an instrument present in many laboratories. Be careful though: “complete combustion” does not mean that the flame does not release polluting compounds, such as nitrogen oxides (NO_x). This is why it is always important ventilate the rooms during cooking.

Bunsen and the perfect gas/air mixture

The blue flame is an indicator of complete combustion of all the gas we use, without leaving residues: it means that combustion is taking place in the most optimal way, thanks to a perfect balance of the reagents (gas, fuel and oxygen, oxidising agent). With the advent of modern chemistry, scientists understood that matter was composed of molecules and atoms, and that reactions among different compounds followed precise mathematical rules: as in a recipe, the “ingredients” must be mixed using the right quantities to obtain the best result. In chemistry, this ratio between ingredients is called a “stoichiometric ratio”.

In our stoves a chemical combustion reaction takes place and the right stoichiometric ratio for a burning gas (fuel) is obtained by providing a adequate volume of air and therefore the right amount of oxygen molecules (oxidizing).

The chemist Robert Wilhelm Bunsen experimented in 1857 several modifications to existing burners, widespread for decades in chemical laboratories in large European cities, where gases such as methane they were already used for lighting buildings and streets. By making some openings in a metal cylinder, that can be opened or closed at will with a special “collar”, Bunsen managed to adjust and finely mix methane and air before reaching the flamethus obtaining perfectly adjustable combustion and reaching higher temperatures.

Even today, the achievement of the right air/gas ratio is indicated by the flame changing from an orange/yellow colour, due to the incandescent soot particles, to a blue flametypically divided into an area of ​​more intense color (where the warmest area is located, approximately 1500° C at the highest point) and a larger, almost invisible area.

Because the flame is really blue and not another color

It all depends on the temperature reached and the electromagnetic radiation that some particles emit during the combustion reaction. Each material, when heated, emits a radiation in the infrared (what we perceive as heatapproaching the flame), but above a certain temperature, this radiation it also falls in the visible field. For example, the yellow/red light it is due to the “black body radiation” of soot particles. This is what happens to too oven resistanceswhich become “glowing red” when in operation, or ai filaments of an old light bulb which at higher temperatures (3000 °C) shine one intense, yellow light.

In the absence of soot, however, we can observe one much less intense and bluish lightdue in this case to intermediate molecules in combustion reactions: it is about radicalsunstable species such as OH* and CH*. The electrons of these molecules they can gain energy due to high temperatures, and fall back to the ground state they give up energy in excess in the form of electromagnetic radiation that our eye sees as “light”. These radicals emit radiation between 300 and 500 nmwavelengths that appear to our eyes between purple and blue-blue.

The spread of gas cookers

The complete combustion of the gas eliminates carbon residues, i.e. sootwhich causes objects near flames such as those of a candle to blacken: this also allowed greatly reduce unburned gases (which remain after combustion), because a gas mixed well with oxygen burns completelyproducing only CO₂ and humidity.

The spread of gas cookers (and boilers).built using this principle, has allowed us to move from the use of fireplaces and wood ovens to safer and cleaner cooking and heating methods in our homes, freeing them from soot. In Italy almost the 69% of families owns one gas cookerdespite the ever-increasing diffusion of electric cookers and especially induction hobs.

In gas stoves the methane is pre-mixed with air before leaving the burner, e.g under normal conditions the flame will always be blue: duct blockages or other problems can lead to formation of redder flamesindicating the need for maintenance.

The classic is an exception yellow flame that we see when foam or salt water comes out of the pan on fire: in that case, the momentary color of the flames depends on the presence of the sodium in water and in particular the distribution of its electrons, a characteristic exploited in the chemical analysis called “flame-tested” to identify salts or metal powders.

The gas problems

As anticipated, however, complete combustion does not free us from harmful emissionswhich can easily increase indoor pollution. During combustion, in fact, the heat favors the formation of nitrogen oxides (NO_x), in particular, nitrogen dioxide (NO₂) that can cause irritation to the airways, especially in children, and favor the onset of asthma; moreover, combustion reduces the amount of oxygen present in the air while enriching that of CO₂although the quantities of gas burned for a meal are small.

A CLASP studywhich involves 7 European countries including Italyhighlights how the NO_x are present at concentrations up to 3 times higher in homes with gas stoves, especially in the kitchen area. The study is not particularly extensive (there are only 40 homes monitored in Italy, of which only 2 with electric/induction hobs) but the conclusions are in line with other international studies: also for this reason, as well as to reduce CO emissions₂, New York State (USA) has banned the installation of gas systems in newly built buildings.

What measures can we take, in addition to changing appliances, an certainly expensive alternative? Surely use hoods and ventilation systems that they bring unwanted cooking products outside remains the number one precaution. Another trick is to ventilate the rooms well: “isolate” the kitchen, closing the doors inwards and opening the windows when possible to promote air circulationcan help further reduce NO concentrations_x in our homes, as long as the outside air is relatively clean.