Combustion and oxygen

Combustion is a substance combining rapidly with oxygen, releasing heat and light; rusting is the same reaction run slow; breathing is a slow combustion inside the body. All three had looked like different mysteries. The unifying insight is that a specific component of ordinary air — about a fifth of it, the part Lavoisier named oxygène — is the reactive partner in every case, and that whatever the reaction rearranges, the total quantity of matter stays exactly the same.

Before this, chemistry had no reliable bookkeeping. The dominant idea was phlogiston: a fire-substance supposedly released when things burned. It explained a lot and was wrong, and while it stood, chemistry could not add up. Lavoisier's demand that every reactant and product be weighed, and his finding that burning is combination with oxygen rather than loss of phlogiston, turned chemistry from a collection of recipes into a quantitative science with a conservation law at its base. His 1789 textbook also gave chemistry a rational naming system and a list of about thirty-three "simple substances" that could not be broken down — the first modern table of elements.

A conservation-of-mass foundation that made chemical equations possible, and directly the next node: once elements combine in definite, weighable amounts, you can ask in what fixed proportions — which is Dalton's atomic theory. Understanding combustion also underlies every heat engine, every furnace, and eventually the analysis of what fuels release and what they cost the air.