Anyone who has a gas stove at home knows that, once the stove is turned on, a small blue/light blue flame immediately forms, very different from the red/orange flame 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 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. The blue flame simply indicates the complete combustion of the hydrocarbon (whether methane or LPG), without residues, which can only be achieved by pre-mixing the fuel with the right amount of air: a principle made famous by the German chemist Robert Wilhelm Bunsen, inventor 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 (NOx). For this reason it is always important to ventilate the rooms during cooking.
Bunsen and the perfect gas/air mixture
The blue flame is an indicator of the 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 the reactions between 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”.
A chemical combustion reaction takes place in our stoves and the right stoichiometric ratio for a burning gas (fuel) is obtained by providing an adequate volume of air and therefore the right quantity of oxygen molecules (oxidizing agent).
In 1857, the chemist Robert Wilhelm Bunsen experimented with various modifications to existing burners, which had been widespread for decades in the chemical laboratories of large European cities, where gases such as methane were already used for lighting buildings and streets. By making some openings in a metal cylinder, which could be opened or closed at will with a special “collar”, Bunsen managed to finely regulate and mix methane and air before reaching the flame, thus obtaining perfectly adjustable combustion and reaching higher temperatures.
Even today, the achievement of the right air/gas ratio is indicated by the passage of the flame from the orange/yellow colour, due to the incandescent soot particles, to a blue flame, typically divided into an area of more intense color (where the hottest area is located, at around 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. As every material heats up, it emits infrared radiation (what we perceive as heat as we approach the flame), but above a certain temperature, this radiation also falls into the visible range. For example, yellow/red light is due to “blackbody radiation” from soot particles. This is what also happens to the heating elements of an oven, which become “glowing red” when in operation, or to the filaments of an old light bulb which at higher temperatures (3000 °C) shine with an intense, yellow light.
In the absence of soot, however, we can observe a much less intense and bluish light, due in this case to the intermediate molecules in the combustion reactions: these are radicals, unstable species such as OH* and CH*. The electrons of these molecules can acquire energy due to high temperatures, and by falling back to the fundamental state they release excess energy in the form of electromagnetic radiation that our eye sees as “light”. These radicals emit radiation between 300 and 500 nm, wavelengths that to our eyes appear between violet and blue-blue.
The spread of gas cookers
The complete combustion of the gas eliminates carbon residues, i.e. soot, which blackens objects near flames such as those of a candle: this has also made it possible to greatly reduce unburned gases (which remain after combustion), because a gas well mixed with oxygen burns completely, producing only CO2 and humidity.
The spread of gas cookers (and boilers), built using this principle, has made it possible 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 69% of families own a gas cooker, despite the ever-increasing diffusion of electric cookers and especially induction hobs.
In gas stoves, methane is pre-mixed with air before leaving the burner, and under normal conditions the flame will always be blue: blockages in the ducts or other problems can lead to the formation of redder flames, indicating the need for maintenance.
An exception is the classic 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 sodium in the water and in particular the distribution of its electrons, a characteristic exploited in the chemical analysis called “flame test” to identify salts or metal powders.
The gas problems
As anticipated, however, complete combustion does not free us from harmful emissions, which can easily increase indoor pollution. During combustion, in fact, the heat favors the formation of nitrogen oxides (NOx), in particular, nitrogen dioxide (NO2) which can cause irritation to the airways, especially in children, and promote the onset of asthma; Furthermore, 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 study by CLASP, which involves 7 European countries including Italy, highlights how NOx 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 emissions2the State of New York (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? Certainly using hoods and ventilation systems that bring unwanted cooking products outside remains the number one precaution. Another trick is to ventilate the rooms well: “isolating” the kitchen, closing the doors inwards and opening the windows when possible to encourage air circulation, can help to further reduce NO concentrationsx in our homes, as long as the outside air is relatively clean.
