The student will:
Previously, we have used the term “oxidation” in reference to the oxidation state of atoms. For instance, we specified that the oxidation state of an atom in the alkali family is . This occurs because these atoms always lose one electron to form an ion. In comparison, the oxidation state of an atom in the halogen family is . Again, this is because a halogen atom tends to gain an electron to form its ions. In this chapter, we will re-examine the concept of oxidation in more details.
The term phlogiston was coined by Georg Stahl in a paper published in 1718. Stahl was expanding on a concept originated by his teacher, Johann Joachim Becher. The phlogiston theory held that all flammable materials contain a substance named phlogiston in addition to the other components of the material. Phlogiston was described as being colorless, tasteless, and odorless. The only way to experience phlogiston was to observe it departing a burning material. The escaping phlogiston could be observed as the smoke and the orange and yellow light that accompanied combustion.
When pieces of wood, paper, or other flammable materials were burned, the residue of the flammable materials weighed less than the original, which seemed to support the theory that some substance was leaving the burning material. Once a substance was completely burned, another supporting observation was that the residue could not be burned further, which suggested the phlogiston was gone. The fact that a material soon stopped burning when it was burned in a closed container was also used to support the theory of phlogiston. The explanation was that the air had a maximum capacity for absorbing phlogiston, and once the air had absorbed the maximum, it could no longer support combustion. For the same reason, the same air from the container would not support the combustion of another material nor would it support life.
The theory of phlogiston was quite widely accepted. When nitrogen was discovered in 1772, it was referred to as phlogisticated air because it would not support combustion nor life, and when oxygen was discovered by Priestly in 1774, it was referred to as de-phlogisticated air because it could accept more phlogiston, that is, support combustion and life.
The first contradictory observation occurred with the combustion of certain metals such as magnesium. Experimenters determined that when magnesium burned, it gained mass even though it was supposed to be losing phlogiston. Some proponents of the phlogiston theory attempted to rationalize the observation with explanations such as metallic phlogiston had “negative mass” or that phlogiston had a buoyancy effect because it was lighter than air. These suggestions were not very convincing and indicated another theory was needed.
Lavoisier and Combustion
The phlogiston theory remained the dominant theory until Antoine Lavoisier (1743-1794) demonstrated that combustion was actually a combination of the flammable material with some component of the air. Lavoisier used the work of Joseph Priestley (the discoverer of oxygen) to demonstrate that if all the products of combustion (smoke, ash, etc.) were retained by carrying out the combustion in a closed container, then all combustible materials gained mass, not lose mass. Lavoisier also demonstrated that the increase in mass of the combustible material was exactly equal to the loss of mass of the air in the container.
Lavoisier suggested that combustion was the combination of the flammable material with some component of the air – namely oxygen. This theory of combustion was quickly accepted, replacing the phlogiston theory. You should not think that the acceptance and later rejection of the phlogiston theory as some unfortunate flaw in the scientific method. It is, rather, an example of the normal functioning of the scientific method.
- The phlogiston theory of combustion suggested that a substance called phlogiston was escaping from the combustible material into the air.
- Lavoisier's theory of combustion was that the combustible material was combining with some component of the air.
- The theory of combustion replaced the phlogiston theory.
Further Reading / Supplemental Links
This web site offers text material on the demise of the phlogiston theory and some history about some early chemists.
- What would be the experimental difference between phlogisticated air and de-phlogisticated air?