Berthollet, Claude-Louis (1748-1822) was an Italian-born French chemist who first proved that chemical reactions and affinities are dependent upon physical factors, such as mass and temperature. With French chemist Antoine Laurent Lavoisier, he established a system of chemical nomenclature that reflected a newer understanding of the field and still serves as the basis of modern terminology in chemistry.

Berthollet was born to French parents in the Savoy region of what was then Italy, and began his studies in medicine at the University of Turin, qualifying as a physician in 1768. Four years later he moved to Paris, where he studied chemistry and earned an M.D. degree while continuing to study medicine. After being appointed personal physician to Madame de Montesson, wife of the Duke of Orléans, he was given a research laboratory in the Palais Royale; there he performed experiments introduced by the older chemist Lavoisier. In 1778, he earned a second doctor of medicine at the University of Paris, which did not recoginize his Italian degree. He became a member of the Royal Academy of Sciences in 1780. Berthollet soon joined a group of young scientists in discussions held by Lavoisier in Paris's Arsenal.

Initially Berthollet accepted the then-prevalent phlogiston theory of combustion. The phlogiston theory stated that when combustible materials were burned they released a substance called phlogiston, but Lavoisier's own more modem theory won Berthollet over and Lavoisier ultimately disproved the existence of phlogiston. Later, Berthollet himself disputed one of Lavoisier's theories and proved that not all acids contained oxygen. Berthollet also discovered that chlorine was an excellent bleaching agent.

By the late 1700's, the vocabulary of chemistry was a morass of inaccuracies and inconsistencies, and Berthollet became one of Lavoisier's primary collaborators in creating a new system of nomenclature, much of which still serves today. Published in 1787 as Méthode de Nomenclature Chemique, their system replaced outmoded names for elements with more precise terminology and clearly identified the makeup of compounds. “Liver of sulfur,” for example, became potassium sulfide.

In 1798, while at Lake Natron in Egypt, investigating natural resources for Napoleon, Berthollet noticed a high concentration of sodium carbonate (soda) in the land surrounding the lake. From this he correctly deduced that there had been a chemical reaction between the salt (sodium chloride), caused by surface evaporation, and the calcium carbonate from the area's limestone hills to create the sodium carbonate. But according to the affinities doctrine, the resultant compound should have been calcium carbonate. This led Berthollet to suspect that the quantity of a reactant, and other physical variables, affected chemical affinities. With further testing in Paris, he proved this to be true, his discovery being the precursor of what is now known as the law of mass action and paving the way for a deeper understanding of such affinities. These findings also led to Berthollet's most famous work, Essai de Statique Chimique (1803), a two-volume treatise that first put forth the concept of chemical balance and established rules, known as Berthollet's laws, for anticipating chemical reactions and affinities.

Berthollet also engaged in a long-running dispute with chemist Joseph Louis Proust, who had proposed that the elements making up any sample of a pure compound were always found in definite, or constant, proportions. Berthollet's experiments led him to believe otherwise and he argued vociferously against Proust for many years. He was ultimately proved wrong and the law of definite proportions was established, but these debates with both Lavoisier and Proust prompted further important discoveries in chemistry.

Berthollet was involved in important work under four different regimes. Napoleon named him director of the mint. He became a senator, a grand officer of the Legion of Honour, and a count in 1804, and served on several commissions, ranging from monetary reform to art restoration.

Berthollet discovered the composition of prussic (hydrocyanic) acid, sulfurated hydrogen (hydrogen sulfide), and ammonia; developed a method for extracting iron ore for the production of steel; and created a preparation of potassium chlorate that resulted in the first color fireworks. He also founded the École Polytechnique with colleagues Gaspard Monge and Louis B. Guyton de Morveau. With Marquis de Laplace, he established the Société d'Arcueil, a discussion group that included some of the most eminent scientists of the day.