Antoine Lavoisier:

Antoine Lavoisier (1743-1794) is often called the father of modern chemistry. He is possibly the greatest scientist France ever produced. The son of a wealthy Parisian lawyer, Lavoisier completed a law degree to please his parents, but his real interest was in science. In 1768 he was elected to the prestigious French Academy of Sciences in which he was a rising star, becoming director in 1785.

Peter Atkins, the noted UK chemist, credits Lavoisier as "The Father of Modern Chemistry" for three reasons.

First, Lavoisier introduced a new language of chemistry, which swept away the old terminology based on the natural origins of things, such as flowers and trees - terminologies that said nothing about the chemical composition of the material.

Second, Lavoisier emphasized the fundamental distinction between elements and compounds and established the basic rules of chemical combination. One of his most famous experiments was a public demonstration that water is made up of the two elements, hydrogen and oxygen.

Third, Lavoisier introduced precise measurement into chemistry and so turned it into an exact physical science.

Lavoisier is best remembered for overturning the theory of phlogiston.

Phlogiston was a hypothetical substance, postulated in the 17th century to explain combustion. The theory held that combustible substances contain phlogiston and combustion is essentially the process of losing phlogiston.

The British chemist Joseph Priestley (1733-1804) discovered that air is composed of several gases, one of which is essential to animal life, which Priestley called "dephlogisticated air". He generated it by heating mercuric oxide and collecting the gas that was given off. Priestley showed Lavoisier how to make dephlogisticated air.

Lavoisier re-named dephlogisticated air as oxygen. The emanation of oxygen from mercuric oxide suggested to Lavoisier that chemical decomposition could be quantified. He ran the experiment in both directions. First he burned mercury in oxygen and measured the amount of oxygen that combines with the mercury to make mercuric oxide.

Next he took the mercuric oxide and heated it to expel the oxygen, leaving mercury behind. When he measured the oxygen generated, it was exactly the amount that had been taken up before.

The overall process was revealed as the combination and uncoupling of fixed quantities of mercury and oxygen. Combustion of mercury was revealed as chemical combination with oxygen and therefore, phlogiston was no more. 

Oxygen theory of combustion

During late 1772 Lavoisier turned his attention to the phenomenon of combustion, the topic on which he was to make his most significant contribution to science. He reported the results of his first experiments on combustion in a note to the Academy on 20 October, in which he reported that when phosphorus burned, it combined with a large quantity of air to produce acid spirit of phosphorus, and that the phosphorus increased in weight on burning. In a second sealed note deposited with the Academy a few weeks later (1 November) Lavoisier extended his observations and conclusions to the burning of sulfur and went on to add that "what is observed in the combustion of sulfur and phosphorus may well take place in the case of all substances that gain in weight by combustion and calcination: and I am persuaded that the increase in weight of metallic calces is due to the same cause."

Joseph Black's "fixed air"

During 1773 Lavoisier determined to review thoroughly the literature on air, particularly "fixed air," and to repeat many of the experiments of other workers in the field. He published an account of this review in 1774 in a book entitled Physical and Chemical Essays. In the course of this review he made his first full study of the work of Joseph Black, the Scottish chemist who had carried out a series of classic quantitative experiments on the mild and caustic alkalies. Black had shown that the difference between a mild alkali, for example, chalk (CaCO3), and the caustic form, for example, quicklime (CaO), lay in the fact that the former contained "fixed air," not common air fixed in the chalk, but a distinct chemical species, now understood to be carbon dioxide (CO2), which was a constituent of the atmosphere. Lavoisier recognized that Black's fixed air was identical with the air evolved when metal calces were reduced with the charcoal and even suggested that the air which combined with metals on calcination and increased the weight might be Black's fixed air, that is, CO2.

Joseph Priestley

Joseph Priestley, an English chemist known for isolating oxygen, which he termed "dephlogisticated air."

In the spring of 1774 Lavoisier carried out experiments on the calcination of tin and lead in sealed vessels which conclusively confirmed that the increase in weight of metals in combustion was due to combination with air. But the question remained about whether it was combination with common atmospheric air or with only a part of atmospheric air. In October the English chemist Joseph Priestley visited Paris, where he met Lavoisier and told him of the air which he had produced by heating the red calx of mercury with a burning glass and that released air had supported combustion with extreme vigor. Priestley at this time was unsure of the nature of this gas, but he felt that it was an especially pure form of common air.

Lavoisier carried out his own researches on this peculiar substance. The result was his famous memoir On the Nature of the Principle Which Combines with Metals during Their Calcination and Increases Their Weight, read to the Academy on 26 April 1775. In the original memoir Lavoisier showed that the mercury calx was a true metallic calx in that it could be reduced with charcoal, giving off Black's fixed air in the process. When reduced without charcoal, it gave off an air which supported respiration and combustion in an enhanced way. He concluded that this was just a pure form of common air, and that it was the air itself "undivided, without alteration, without decomposition" which combined with metals on calcination.
After returning from Paris, Priestley took up once again his investigation of the air from mercury calx. His results now showed that this air was not just an especially pure form of common air but was "five or six times better than common air, for the purpose of respiration, inflammation, and ... every other use of common air." He called the air dephlogisticated air, as he thought it was common air deprived of its phlogiston. Since it was therefore in a state to absorb a much greater quantity of phlogiston given off by burning bodies and respiring animals, the greatly enhanced combustion of substances and the greater ease of breathing in this air were explained.

Pioneer of stoichiometry

Lavoisier's researches included some of the first truly quantitative chemical experiments. He carefully weighed the reactants and products of a chemical reaction in a sealed glass vessel so that no gases could escape, which was a crucial step in the advancement of chemistry. In 1774, he showed that, although matter can change its state in a chemical reaction, the total mass of matter is the same at the end as at the beginning of every chemical change. Thus, for instance, if a piece of wood is burned to ashes, the total mass remains unchanged if gaseous reactants and products are included. Lavoisier's experiments supported the law of conservation of mass. In France it is taught as Lavoisier's Law and is paraphrased from a statement in his "Nothing is lost, nothing is created, everything is transformed. Mikhail Lomonosov (1711–1765) had previously expressed similar ideas in 1748 and proved them in experiments; others whose ideas pre-date the work of Lavoisier include Jean Rey (1583–1645), Joseph Black (1728–1799), and Henry Cavendish (1731–1810).