The Discarded Phlogiston Theory in Early Chemistry History

Relating Phlogiston, Dephlogistated Air, and Calyx

A chemist looking at a tube of liquid in a lab

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Mankind may have learned how to make fire many thousands of years ago, but we didn't understand how it worked until much more recently. Many theories were proposed to try to explain why some materials burned, while others didn't, why fire gave off heat and light, and why burned material wasn't the same as the starting substance.

Phlogiston theory was an early chemical theory to explain the process of oxidation, which is the reaction that occurs during combustion and rusting. The word "phlogiston" is an Ancient Greek term for "burning up", which in turn derives from the Greek "phlox", which means flame. Phlogiston theory was first proposed by the alchemist Johann Joachim (J.J.) Becher in 1667. The theory was stated more formally by Georg Ernst Stahl in 1773.

Importance of Phlogiston Theory

Although the theory has since been discarded, it's important because it shows the transition between alchemists believing in the traditional elements of earth, air, fire, and water, and true chemists, who conducted experimentation that led to the identification of true chemical elements and their reactions.

How Phlogiston Was Supposed to Work

Basically, the way the theory worked was that all combustible matter contained a substance called phlogiston. When this matter was burned, the phlogiston was released. Phlogiston had no odor, taste, color or mass. After the phlogiston was freed, the remaining matter was considered to be deflogistated, which made sense to the alchemists, because you couldn't burn them any more. The ash and residue left over from combustion was called the calx of the substance. The calx provided a clue to the error of phlogiston theory, because it weighed less than the original matter. If there was a substance called phlogiston, where had it gone?

One explanation was the phlogiston might have negative mass. Louis-Bernard Guyton de Morveau proposed it was simply that phlogiston was lighter than air. Yet, according to Archimede's principle, even being lighter than air couldn't account for the mass change.

In the 18th century, chemists did not believe there was an element called phlogiston. Joseph Priestly believed flammability might be related to hydrogen. While phlogiston theory didn't offer all the answers, it remained the principle theory of combustion until the 1780s, when Antoine-Laurent Lavoisier demonstrated mass was not truly lost during combustion. Lavoisier linked oxidation to oxygen, conducting numerous experiments which showed the element was always present. In the face of overwhelming empirical data, phlogiston theory was eventually replaced with true chemistry. By 1800, most scientists accepted oxygen's role in combustion.

Phlogisticated Air, Oxygen, and Nitrogen

Today, we know that oxygen supports oxidation, which is why air helps to feed a fire. If you try to light a fire in a space lacking oxygen, you'll have a rough time. The alchemists and early chemists noticed that fire burned in air, yet not in certain other gases. In a sealed contained, eventually a flame would burn out. However, their explanation wasn't quite right. The proposed phlogisticated air was a gas in phlogiston theory that was saturated with phlogiston. Because it was already saturated, phlogisticated air did not allow the release of phlogiston during combustion. What gas were they using that didn't support fire? Phlogisticated air was later identified as the element nitrogen, which is the primary element in air, and no, it won't support oxidation.

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Helmenstine, Anne Marie, Ph.D. "The Discarded Phlogiston Theory in Early Chemistry History." ThoughtCo, Aug. 28, 2020, Helmenstine, Anne Marie, Ph.D. (2020, August 28). The Discarded Phlogiston Theory in Early Chemistry History. Retrieved from Helmenstine, Anne Marie, Ph.D. "The Discarded Phlogiston Theory in Early Chemistry History." ThoughtCo. (accessed May 30, 2023).