Short answers, pulled from the story.
The first controlled chemical reaction discovered by humans was not a complex synthesis or a precise measurement, but the simple, terrifying act of starting a fire. Before the 1st of May 1536, humanity had no concept of the chemical reaction that would eventually power the modern world, yet the instinct to harness fire for warmth and cooking predates written history.
Combustion is a high-temperature exothermic redox process where a fuel reacts with an oxidant, usually atmospheric oxygen, to produce oxidized gaseous products. While the colloquial meaning of burning implies the presence of flames, combustion does not always result in fire because a flame is only visible when substances undergoing combustion vaporize.
In the microgravity environment of space, the thermal and flow transport dynamics behave quite differently than in normal gravity conditions, fundamentally altering the shape and behavior of a flame. A candle's flame takes the shape of a sphere in such an environment, as the influence of buoyancy on physical processes is considered small relative to other flow processes that would be present at normal gravity.
Combustion instabilities are typically violent pressure oscillations in a combustion chamber that can reach as high as 180dB, threatening the structural integrity of engines. In rockets, such as the F1 used in the Saturn V program, instabilities led to massive damage to the combustion chamber and surrounding components until the problem was solved by re-designing the fuel injector.
Complete combustion is greatly preferred because the formation of carbon monoxide produces less heat than formation of carbon dioxide, and carbon monoxide is a poisonous gas. When breathed, carbon monoxide takes the place of oxygen and combines with some of the hemoglobin in the blood, rendering it unable to transport oxygen.