Lavoisier and the chemical revolution

In the 1770s chemists already knew how to make many gases, but they explained them with a theory that turned out to be backwards. Antoine-Laurent Lavoisier fixed it with an instrument that any merchant would recognize: a very good balance.

This article explains the phlogiston theory he overturned, the experiments that revealed the role of oxygen, the law of conservation of mass, the new language of chemistry he helped create and the end of his life during the French Revolution.

A wealthy man with a laboratory

Lavoisier was born in Paris on 26 August 1743. He trained in law but spent his time on science, and in 1768 he joined the Academy of Sciences. The same year he bought a share in the Ferme générale, a private company that collected taxes for the king. It made him rich and paid for one of the best-equipped laboratories in Europe.

His wife, Marie-Anne Paulze Lavoisier, was a full partner in the work. She learned chemistry, translated English papers into French, recorded experiments and drew the precise engravings of apparatus for his books.

The problem with phlogiston

The leading theory of the time said that anything that burns contains phlogiston, a substance that escapes into the air during burning. A burned metal (a “calx”) was the metal minus its phlogiston.

But when metals are heated in air to form a calx, they become heavier. If something left the metal, why did it gain weight? Some chemists suggested phlogiston had negative weight. Lavoisier suspected instead that something from the air was being added.

Weighing a fire

In 1772 Lavoisier burned phosphorus and sulfur and found that they too gained weight, while absorbing air. He then heated metals such as tin in sealed glass vessels. The total weight of the sealed vessel did not change; only when he opened it did air rush in.

In October 1774 Joseph Priestley visited Paris and described a new gas he had made by heating red mercury calx (mercury oxide). Candles burned brilliantly in it. Carl Wilhelm Scheele in Sweden had prepared the same gas a few years earlier. Lavoisier repeated the work and understood what it meant.

The twelve-day experiment

Lavoisier heated mercury in a retort connected to a measured volume of air for twelve days. A red calx formed on the mercury, and the volume of air dropped by about a sixth. The remaining air put out flames and could not support animals. He then heated the red calx strongly: it gave back mercury and a volume of gas close to what had been lost — the same gas Priestley had made.

Air, he concluded, is a mixture. One part combines with metals and supports burning and breathing; the other part does not. He named the active part oxygène in 1777, from the Greek for “acid former”, because he believed (wrongly) that all acids contain oxygen. The inactive part he called azote — today’s nitrogen.

Water is not an element

Henry Cavendish had noticed that burning “inflammable air” (hydrogen) produced water. In 1783 Lavoisier and Pierre-Simon Laplace repeated the experiment and drew the conclusion Cavendish had not: water is a compound of hydrogen and oxygen. Lavoisier also decomposed steam over red-hot iron to recover the hydrogen. One of Aristotle’s four elements was gone.

Conservation of mass

Lavoisier’s method rested on a principle he stated clearly: in every operation, the quantity of matter is the same before and after. It is often summarized as “nothing is lost, nothing is created, everything is transformed”. Today we explain it with atoms: a reaction only rearranges them, which is why chemical equations must be balanced.

See how equations are balanced in What is a Chemical Reaction.

A new language for chemistry

Old chemical names were a jumble: “oil of vitriol”, “butter of antimony”, “flowers of zinc”. In 1787 Lavoisier, with Louis-Bernard Guyton de Morveau, Claude-Louis Berthollet and Antoine Fourcroy, published the Method of Chemical Nomenclature. Names would now describe composition: sulfuric acid, copper sulfate, zinc oxide. The system is the basis of the names chemists still use.

In 1789 he published the Traité élémentaire de chimie (Elementary Treatise of Chemistry). It defined an element as a substance that cannot be broken down further by chemical analysis, and listed 33 “simple substances”. Most are true elements, such as oxygen, hydrogen, sulfur and the metals; a few, like lime, later turned out to be compounds, and the list also included light and “caloric”, a supposed fluid of heat.

Breathing is slow burning

With Laplace, Lavoisier built an ice calorimeter in 1782–1783 and compared the heat given off by a guinea pig with the carbon dioxide it breathed out. The numbers matched those of burning charcoal. Respiration, he concluded, is a slow combustion that uses oxygen to release heat — the start of the chemistry of metabolism.

The guillotine

Lavoisier also served the state: he improved France’s production of gunpowder, worked on the metric system and wrote on agriculture and public finance. But during the Revolution, the tax farmers were hated as symbols of the old regime. In 1793 they were arrested, and on 8 May 1794 Lavoisier was tried and guillotined in Paris, aged 50.

The mathematician Joseph-Louis Lagrange is reported to have said: “It took them only an instant to cut off that head, and a hundred years may not produce another like it.” Within two years the French government acknowledged the injustice.

Timeline

• 1743 — Birth. Antoine-Laurent Lavoisier is born in Paris.
• 1772 — Burning gains weight. Phosphorus and sulfur absorb air when they burn.
• 1774 — Priestley’s visit. The gas from mercury calx comes to Lavoisier’s attention.
• 1777 — Oxygen named. Combustion explained as combination with oxygen.
• 1783 — Water decomposed. Water shown to be hydrogen plus oxygen.
• 1787 — Nomenclature. A systematic language for chemical substances.
• 1789 — The Traité. Elements, conservation of mass and the new chemistry in one book.
• 1794 — Execution. Lavoisier is guillotined on 8 May.