Science Sunday: The Black After the Fire

This should be a relatively short one. The other day, this question popped into my mind: Why is it that when things burn, they turn black? Physically, what exactly happens when something is being burned that causes that blackness to be left behind, and what is that blackness? The answer that made the most sense to me at first thought was that the blackness that was left behind was probably some form of carbon. Firstly, materials of pure carbon tend to appear blackish (e.g. graphite), except when their atomic bonds are arranged in the tetrahedra that create diamonds.

Secondly, organic matter (the type of stuff that tends to burn anyway, and which comprises much of what is on the surface of the Earth aside from air and water) is choc full of carbon, which tends to be bound to a bunch of other elements (your hydrogens, nitrogens, oxygens, etc.). When whatever compound that carbon is a part of heats up, what probably happens is that energy input into that compound goes into breaking the bonds between carbon and other elements, and the other elements vaporize while carbon creates new bonds with itself (and other oxygen atoms creating CO and CO2; carbon is really quite good at bonding with other elements). The hydrogen that’s bound to the carbon links up with the oxygen that’s being liberated and creates water, etc. I only took introductory chemistry, so my knowledge of such things is limited.

Thus was my initial speculation. Following what I’d always grown up being told to do, I looked it up. Luckily, such questions are easily answered by Google.

What we think of as “burning” is really the rapid oxidation of organic molecules. When organic molecules are oxidized, they combine with oxygen in the air and produce carbon dioxide, water, and energy (in the form of heat, light, and sound).

Propane combustion reaction, as an example.

For things like wood, you have this combustion reaction. The energy that’s released comes out in the form of fire and what’s left behind is more or less a carbon skeleton (alongside other inorganic compounds and whatever material wasn’t oxidized). Bing bam boom, simple answer for a simple question.

Looking at this, another question comes to mind: Oxygen comprises 20.95% of dry air by volume. Even at this small fraction, we’re effectively large, fleshy bags of organic matter, walking through veritable fields of oxygen all the time. If oxidation happens when objects combine with oxygen, and oxidation is highly exothermic (heat producing), why is it that organic compounds like trees, houses, and hell, PEOPLE, aren’t just on fire all the time?


Turns out that organic combustion is temperature-dependent. There’s some lower threshold beneath which something will not burn (at standard atmospheric temperature and pressure) called the activation energy. Once a substance reaches this temperature, if it is in the presence of a sufficient amount of oxygen, it will then have the energy necessary to sprout flames without an external heat source. Thus, your skin is just fine and safe. Try not to walk through any fires, though. I can’t guarantee your safety there.

And that’s it! Fire!