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In the seventeen hundreds phlogiston was thought to be an unobservable
substance which was the counterbalance of calx when things burned. If a
substance was dephlogisticated, then it had lost its flammable
phlogiston and was now in the more essential form, its calx. It was
Lavoisier, the Father of Modern Chemistry, who refuted the phlogiston
theory of combustion. Lavoisier in 1789 published a book, Traité
élémentaire de chemie, in which he purported his Oxygen Theory to
refute the ruling idea of the existence of a substance called phlogiston
which had been used for almost a century up to that time
to explain how substances burn. Two other scientists in the time
of Lavoisier had isolated oxygen chemically, Carl Scheele in 1772 and
Joseph Priestley in 1774. Scheele had called the substance 'fire
air.' Priestley called the substance he had isolated by directing
sunlight through a lens onto mercurius calcinatus (mercuric oxide, a red
powder) so as to heat it, "dephlogisticated air."
The equation for Priestley's experiment would be as follows:
2HgO(s) + heat ---> 2Hg(l)
+ O2
Lavoisier
developed an accurate scale which could measure quantities of mass in
the range of the nearest 10-4 g (to 0.0005g). He
then performed several experiments with combustion. He found that there
was a conservation of mass ultimately, that the mass of the reactants
and the mass of the products remained unchanged by the combustion.
This became part of the key to his newly found understanding that air
was comprised of oxygen, nitrogen, and carbon dioxide. In a meeting
with Priestley he was told of Priestley's discovery of dephlogisticated
air in the experiment Priestley had performed. This had inspired
Lavoisier to rethink air not as losing a given substance in the process
of combustion, the purported substance called phlogiston, as much as
being comprised of one or more substances which had reactive abilities.
Indeed, Lavoisier coined the term 'oxygen' to describe the substance
which was most active in the combustion events he had witnessed.
Lavoisier was astute enough as an empirical scientist to recognize that
the oxidation of metals and the respiration of living things, as well as
combustion, all were examples of the same type of reaction which he had
been able to analyze through the primary tool of accurate measurements
in the weight of reactants and products in his combustion experiments.
Along with his theory of oxygen, Lavoisier established the nature of an
element for the annals of chemistry whereby an element was defined as a
substance which could not be broken down further. He named 33 such
elements, and these were described in his book, Traité élémentaire
de chemie.
Lavoisier's
great breakthrough in refuting the theory of phlogiston came when he
recognized that a substance, the liquid mercury, could change state upon
being heated in a closed container to solid mercuric oxide, reducing the
volume of air in so doing; then it could be changed back with further
heating from that so-called "calx" of mercury to make
the explanation of phlogiston now untenable. Phlogiston could be
released upon the first heating, but it could not be put back when the 'calx' was
re-heated to give the original liquid mercury plus O2 since
according to the existing theory of the nature of phlogiston such a
replenishment should have been deemed impossible.
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