②Is Fire a Gas? - The Fascinating States of Matter [Reading Science]

Is fire a gas, or is it some other form of existence?
Photo by YouTube Screenshot

[Asia Economy Reporter Kim Jonghwa] Substances on Earth usually exist in solid, liquid, or gas states, but there are also substances that exist in a 'Gel' state like toothpaste or mayonnaise, or in a 'Plasma' state like lightning or auroras, as explored in the [①Is Glass a Liquid? - The Fascinating States of Matter] segment.

Besides gels and plasma, there are substances whose states change depending on conditions or whose states are difficult to define. A prime example of such a substance is 'Fire.'

Looking at fire’s form, it is clearly neither solid nor liquid. Observing its flickering in the wind, one might think it is closest to a gas. However, unlike gases, fire has shape and volume. If it is not a gas, could it be plasma as some suggest?

To turn a gas into plasma, continuous heat of at least several thousand to tens of thousands of degrees Celsius is required. However, fire occurs without applying such extreme heat. For example, wood’s ignition point is only 400~470℃, so fire occurs once this temperature is exceeded. Therefore, it is logically less plausible to classify fire as plasma.

Fire does not even have a common chemical formula. Solids, liquids, and gases all have definite chemical formulas. This means fire is neither solid, liquid, nor gas. If it is not solid, liquid, gas, or plasma, then what exactly is fire?

If fire does not correspond to a state of matter, it is necessary to consider the possibility that it is something other than matter. First, consider ignition. Ignition is the moment fire occurs, which requires burning a substance. This process is called combustion. For combustion to occur, fuel (a combustible material), a temperature above the ignition point, and oxygen are necessary.

If any one of these conditions is missing, combustion does not occur, and fire does not arise. Simply put, fire is a phenomenon where a substance becomes hotter than its ignition point and burns together with oxygen in the air, emitting light and heat. Does something come to mind from the phrase 'emitting light and heat while burning'? What burns itself to emit heat and light is 'energy.' Fire can be considered energy rather than a state of matter.

Generally, the visible color of fire is bright orange or red. However, the color of fire varies depending on the reacting elements. When fire is lit in the air, the flame is red, while the flame of a gas stove is blue. This is because although the conditions for combustion are met, the color changes depending on whether these conditions are completely burned or not.

The flame of a gas stove is blue. Why is that?
Photo by YouTube screenshot

Fire in the air is red because it is incomplete combustion that does not fully burn the oxygen remaining in the air, whereas the flame of a gas stove is blue because it is complete combustion that fully burns the oxygen contained in the fuel itself. The color of fire also changes depending on the element being burned. Sodium produces yellow flames, potassium produces purple flames, and copper produces turquoise flames. Fireworks use this principle.

If the issue is not the element being burned, temperature changes the color. The higher the temperature, the bluer the flame; the lower the temperature, the redder it becomes. The colors of flames visible to the human eye change in the order of initial red → orange → yellow → white → pale blue.

There is also a claim that at even higher temperatures, the color changes in the order of 'purple → black,' but after purple, it is invisible to the eye. In other words, it is outside the visible spectrum and cannot be confirmed. Could the claim that the flame becomes black because it is dark and invisible be a misunderstanding?