Monday, September 7, 2015



Phlogiston – An Early Chemical Theory

Phlogiston is an early chemical theory, a hypothetical principle of fire, of which all combustible substance has been in part composed. The phlogiston theory is an out-dated scientific theory which hypothesized a fire like element known as phlogiston which comprises of combustible bodies and are released at the time of combustion.

The name has been derived from the Ancient Greek word phlogiston – burning up and was first stated in 1667 by Johann Joachim Becher. The theory makes an attempt in explaining the burning processes like combustion and rusting that are now together known as oxidation. The phenomenon of burning was caused by the liberation of phlogiston with the de-phlogisticated substance that is left as ash or residue.

In 1669, Becher put forth his views that substances comprised of three types of earth which according to him were vitrifiable, the mercurial and the combustible. He presumed that when a substance tends to burn, combustible earth was liberated. Hence, wood was a combination of phlogiston and wood ashes. Georg Ernst Stahlapplied the name, phlogiston from Greek meaning `burned’ to the hypothetical substance, around the beginning of the 18th century.

Metals Composed of Calx & Phlogiston

Stahl was of the belief that the corrosion of metals in the air, for instance the rusting of the iron was a form of combustion and when changed to its calx, or metallic ash, its oxide, in modern terms, phlogiston tend to get lost. Hence metals were composed of calx and phlogiston.

The role of air is merely to carry away the liberated phlogiston. The main objection to the theory is that the ash of organic substance tends to weigh less than the original while the calx seems to be heavier than the metal. This was of less significance to Stahl whoconsidered phlogiston as an immaterial principle instead of an actuals substance.

Phlogiston was considered as a true substance as chemistry advances and great attempt was made in accounting for the weight changes that were noticed. Some presumed that it was pure phlogiston, when they discovered that hydrogen was very light in weight and very flammable.

Theory Condemned by Antoine Lavoisier

The phlogiston theory was condemned by Antoine Lavoisier between 1770 and 1790 who researched on the gain or loss of weight when tin, lead, phosphorus and sulphur went through reactions of oxidation or reduction – de-oxidation and portrayed that the newly discovered element – oxygen was involved.

Though a number of chemists like the notable Joseph Priestly, one of the discoverers of oxygen, made attempts to retain some types of the phlogiston theory and by 1800, almost every chemist accepted the accuracy of Lavoisier’s oxygen theory.

It was believed that when an object would burn, it released its phlogiston – an element without taste, mass, odour or colour, leaving behind a powdery substance, calx. Objects burnt in air were considered to be rich in phlogiston and the fact that a fire burned out when the oxygen was removed was a proof that oxygen could only absorb a limited amount of the substance.

 The theory also leads to the notion that the human need to breathe, had a main function which was to eradicatephlogiston from the body. The complete concept was superseded by Lavoisier’s discovery that combustion could only take place with the help of oxygen.

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