Combustion is an observable kind of reaction, familiar as burning and rusting, in which a material combines with something from the air and gains mass in the process. That last clause is the one worth pausing on. Burning and rusting look like losses. A candle shortens. A log becomes ash. A nail thins as its surface flakes off. For most of the history of chemistry, people trusted the appearance and told a story of something escaping: a substance they called phlogiston that was supposed to leave a material as it burned.
The story was a reasonable one, given the evidence available. It was also wrong. When Antoine Lavoisier began weighing metals before and after they rusted, he found that the metals gained weight. Not lost, gained. Whatever was happening in combustion was not a loss of some internal stuff; it was the addition of something from outside. That something was a portion of the air, and it was what we now call oxygen.
This is not a small historical detail. The reason combustion matters in the foundations of chemistry is that the careful weighing of burning and rusting materials is what forced the principle of conservation of mass into place. Before Lavoisier's scales, one could believe that mass was roughly conserved and let the anomalies go. After his scales, the anomalies were no longer anomalies; they were accounted for, and the accounting closed. The mass that appeared to leave the candle went into the air as gases and warmth. The mass that appeared to arrive on the rusting nail came from the air itself. Weigh the whole system, and the totals hold.
At our level, combustion is not a story about what happens in some unreachable layer. It is a story about what a scale reads. A scale reads the mass of the metal before rusting; a scale reads the greater mass of the metal-plus-rust after. A scale reads the candle; a scale, if the air is captured, reads the gases that come off it. The observable facts alone, carefully measured, are enough. Combustion is the case where chemistry first learned the discipline of trusting the scale over the story.
Question
When a metal rusts, it gains mass. Where is the extra mass coming from?
Reveal Merrill's answer →Hide answer
From the air. Specifically, from the oxygen in the air, which combines slowly with the metal to form rust. For a long time people told the story in reverse, claiming a stuff called phlogiston was leaving the metal as it rusted or burned. But then the metal would have to lose mass, and it doesn't. It gains. Lavoisier put both stories on the scale, and only one of them could explain what the scale said. The extra mass on the rusting iron is the air it has quietly been absorbing.