Phlogiston theory, once a leading theory in early chemistry, proposed that combustible materials contain a fire-like element called “phlogiston” that gets released during combustion. The essence of this theory was that all flammable materials contain this substance, and burning them releases phlogiston into the air.
- Phlogiston Theory is an obsolete scientific theory from the late 17th and early 18th centuries, proposing that a fire-like element, phlogiston, is released during combustion.
- The theory is important because it represented a crucial step in the development of chemical science, providing an early framework for understanding combustion and oxidation.
- Evidence Against Phlogiston Theory:
- Joseph Priestley’s discovery of oxygen contradicted the existence of phlogiston.
- Antoine Lavoisier’s experiments proved that combustion was a process of oxidation, involving the combination of a substance with oxygen, not the release of phlogiston.
Origin of the Word “Phlogiston”
The term “phlogiston” comes from the Greek word “phlogistos,” meaning “to set on fire.” This name directly relates to the process of combustion that the theory aimed to explain.
How the Phlogiston Theory Works
According to this theory, all combustible materials contain phlogiston, which releases into air during burning. The visible flame and heat are direct manifestations of phlogiston’s release. The theory implies that once all the phlogiston leaves a material, combustion ceases, leaving behind a ‘dephlogisticated’ substance (ash, in the case of wood).
This theory explains not just why things burn, but also why metals gain mass when calcined (now understood as oxidation), assuming that phlogiston has a negative mass. The release of phlogiston into the air sustains life and fire, and when air became saturated with phlogiston, it no longer supports either.
- Phlogistated: A term used to describe substances that supposedly contain phlogiston and release it through burning.
- Phlogistated Air: Air containing phlogiston. The theory proposed that burning releases phlogiston into air.
- Deflogistated: Referring to air or substances that had absorbed phlogiston, supposedly making them unable to support combustion or life.
- Deflogistated Air: Gas lacking phlogiston. According to the theory, this was air prior to combustion.
History of Phlogiston Theory
- Empedocles and Aristotle (430-322 BC): The Greek philosophers laid the foundation for the theory by proposing that all matter consisted of four elements, including fire, which was closely related to the concept of phlogiston.
- Johann Joachim Becher (1667): Introduced the concept of “terra pinguis,” a fatty earth, which was later adapted into phlogiston theory.
- Georg Ernst Stahl (1703): Renamed “terra pinguis” to “phlogiston” and formulated the phlogiston theory, building on Becher’s ideas.
- Johann Heinrich Pott: Stahl’s student refined and spread the theory in a way that ordinary people understood.
- Christian Juncker: Expanded on Stahl’s ideas and demonstrated that combustion requires air.
Challenging and Replacement of Phlogiston Theory
The two key scientists in overthrowing phlogiston theory were Joseph Priestley and Antoine-Laurent Lavoisier. Giobert introduced Lavoisier’s work to Italy.
- Joseph Priestley’s Discoveries: While Priestley accepted the phlogiston theory, his experiments with “dephlogisticated air” (oxygen) showed that air played a role in combustion and respiration.
- Antoine-Laurent Lavoisier: Lavoisier disproved the theory through meticulous experiments. He demonstrated that combustion involved oxygen from the air combining with the burning material, refuting the notion of phlogiston release.
- Amedeo Giobert (1792): An Italian chemist who refuted the theory and supported Lavoisier’s theory of water composition.
Joseph Priestley’s Experiments
Joseph Priestley, an English chemist, conducted several groundbreaking experiments in the 18th century that challenged the phlogiston theory.
- Discovery of Oxygen (1774): Priestley discovered a gas, which he initially called “dephlogisticated air,” later known as oxygen. He produced this gas by heating mercuric oxide and observed that it enhanced combustion and supported life better than ordinary air. This discovery was critical because, according to the phlogiston theory, air should become phlogistated (saturated with phlogiston) and thus less supportive of combustion and life after a substance burned in it.
- Experiments with Mice and Plants: Priestley conducted experiments where he placed mice in a closed space and observed that they eventually died when the air became ‘phlogistated’. However, if he placed a plant in the same space, the air became ‘dephlogisticated’ over time, allowing the mouse to survive longer. These experiments suggested a kind of air purification process, which was in direct contradiction to the phlogiston theory.
Antoine-Laurent Lavoisier’s Experiments
- Combustion and Oxygen: Lavoisier replicated Priestley’s experiments and went further by demonstrating that the gas responsible for combustion (oxygen) was a distinct chemical element. He showed that when substances burn, they combine with oxygen, and the total mass remains constant, contradicting the idea of a material losing phlogiston during burning.
- Quantitative Experiments: Lavoisier conducted meticulous quantitative experiments, carefully weighing reactants and products before and after chemical reactions. He found that the weight gained during combustion equaled the weight of the oxygen consumed. This was inconsistent with the phlogiston theory, which could not account for the gain in mass.
- Water Formation Experiment: In a landmark experiment, Lavoisier synthesized water from hydrogen and oxygen, demonstrating that water was not an element (as previously thought) but a compound formed from these gases. This further undermined the phlogiston theory, which could not explain such synthesis.
Phlogiston Theory – Further Study
Here are several suggested activities for expanding your understanding of phlogiston theory and its relevance to chemistry:
- Context: Briefly describe the state of scientific knowledge and methodology during the time when the phlogiston theory was prevalent. This helps in understanding why such a theory was initially plausible and widely accepted.
- Role in the Development of Chemical Nomenclature: Lavoisier not only refuted phlogiston theory but also introduced a systematic chemical nomenclature. How do you think the rejection of phlogiston theory contributed to the advancement of chemical language and classification?
- Concept of Chemical Elements and Compounds: Elaborate on how the downfall of phlogiston theory helped in the evolution of the concept of elements and compounds, which is fundamental to modern chemistry.
- Discussion of Scientific Methodology: Use the transition from phlogiston to oxygen theory as a case study in the scientific method. Highlight how hypothesis testing, experimentation, and willingness to discard old theories in light of new evidence are crucial in scientific progress.
- Impact on Other Scientific Fields: Briefly explore how the debunking of phlogiston theory influenced other areas of science, such as physics and biology, particularly in the understanding of gases and biological processes.
- Biographical Sketches of Key Figures: Learn about the lives and careers of Priestley, Lavoisier, and other scientists involved. This humanizes the story of phlogiston theory and makes the history more engaging.
- Legacy and Philosophical Implications: Discuss the philosophical and historical implications of the phlogiston theory. How did its rise and fall influence the philosophy of science and the public’s perception of scientific authority?
- Comparisons to Modern Theories: Draw parallels between the phlogiston theory and modern theories in chemistry. These comparisons illustrate the continuous evolution and refinement of scientific ideas.
- Best, Nicholas W. (2015). “Lavoisier’s “Reflections on phlogiston” I: against phlogiston theory”. Foundations of Chemistry. 17 (2): 137–151. doi:10.1007/s10698-015-9220-5
- Boyle, R. A (1673). Discovery of the Perviousness of Glass to Ponderable Parts of Flame. London: Essays of Effluvium.
- Conant, James Bryant (ed.) (1950). The Overthrow of Phlogiston Theory: The Chemical Revolution of 1775–1789. Cambridge: Harvard University Press.
- Priestley, Joseph (1774-1777). Experiments and Observations on Different Kinds of Air. 3 vols. London W. Bowyer and J. Nichols.
- White, John Henry (1973). The History of Phlogiston Theory. New York: AMS Press Inc. ISBN 978-0404069308.