Cobra Effect · Chemistry
Weighing everything changed chemistry
How weighing everything before and after overturned an old idea about burning.
7 cards, read aloud in 3:20, with a test and sources.
In the eighteenth century, chemists explained burning with a substance called phlogiston.
They thought anything that could burn contained phlogiston, and released it into the air as it burned. Wood lost weight as it burned, which seemed to fit. A candle in a closed jar went out, they said, because the air had filled up with phlogiston. But some metals got heavier when heated in air, which was much harder to explain.
Antoine Lavoisier weighed everything, before and after.
Lavoisier was a French chemist who believed careful measurement could settle arguments. He heated tin inside a sealed glass vessel, so that nothing could get in or out. Part of the tin turned into a powdery crust, called a calx. Weighed as a whole, the sealed vessel was no heavier and no lighter than before.
The metal had gained exactly the weight that the air had lost.
When Lavoisier weighed the contents of the vessel separately, the numbers told the story. The tin and its calx were heavier than the tin had been, and the air inside had lost the same amount. Something from the air had combined with the metal. Heating a metal in air did not drive something out of it. It took something in.
In 1774 Joseph Priestley made a gas that made flames burn brighter.
Priestley, an English minister and chemist, used a lens to focus sunlight onto a red powder made from mercury. A gas came off, in which a candle burned brilliantly and a mouse lived well. Priestley still believed in phlogiston, and called it dephlogisticated air. Lavoisier repeated the work, and saw what the gas really was.
Lavoisier named the gas oxygen, and the phlogiston idea began to collapse.
He showed that ordinary air is a mixture, and that only part of it supports burning and breathing. That part was the gas he named oxygen. Burning, rusting and breathing all combine things with oxygen. There was no need for phlogiston at all.
Marie-Anne Paulze Lavoisier was his partner in the laboratory.
She married Antoine in 1771, and worked alongside him in nearly all his experiments. She translated scientific works from English, which he could not read. She made the detailed drawings of apparatus for his great book on chemistry, published in 1789. Her careful drawings show exactly how his experiments were done.
So when an explanation seems to fit, ask whether the numbers agree.
Phlogiston explained a great deal, until Lavoisier weighed everything carefully. In a sealed vessel, the total mass stayed the same, whatever changed inside. The idea that matter is neither created nor destroyed in a reaction became a foundation of chemistry. Lavoisier was executed in 1794, during the French Revolution, for his part in collecting taxes. His idea outlived him.
Sources
- The trials of Lavoisier, Science History Institute. Lavoisier’s work, his partnership with Marie-Anne, and his death in the Revolution.
- Lavoisier on the calcination of tin, Classic Chemistry, Le Moyne College. Lavoisier’s own account of heating tin in a sealed vessel.
- Marie-Anne Paulze Lavoisier: the invisible assistant, Chemistry World. The translator, illustrator and experimenter behind Lavoisier’s work.
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