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Chemistry Index

For reference: The development of the Phlogiston Theory

Chemical Element Identification Timeline
Robert Boyle
Hennig Brand
Georg Brandt
Henry Cavendish
Axel Frederik Cronstedt
John Dalton
Antoine Lavoisier
Albertus Magnus
Joseph Priestley
Carl Wilhelm Scheele

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The end of the 18th century

By the mid-18th century laboratory apparatus had become more sophisticated. One example was the collection of gases. At first gases were collected in bladders that were not so 'air-tight' and made it impossible to obtain measurements such as volume and weight. Researches began to collect gases over water using a pneumatic trough, a bell jar and a specially shaped retort flask. Soon Mercury replaced the water so that soluble gases could be collected. Gases could now be accurately measured and tested. Many famous scientists turned their attention to the study of gases.

Carl Scheele, Henry Cavendish and Joseph Priestley were respected scientists who studied gases. All three of them adhered to the phlogiston theory. Cavendish, who was the first to identify the gas that we know as hydrogen, believed that he had isolated phlogiston. He named his new gas "phlogisticated air". Joseph Priestley produced a gas he called "dephlogisticated air" (that we know as oxygen) because it supported combustion.

 

 

Experiment 3: Lavoisier experiments with water

Boiling water was forced into an iron pipe surrounded by a furnace.

Apparatus5

Observations

The steam produced moved through the apparatus and was collected in a container where it cooled and condensed into water.

The experiment also produced a gas.

Results

  • The condensed water weighed less than the water at the start of the experiment.

  • The gas collected was highly flammable (Lavoisier recognized his gas as Cavendish's "phlogisticated air").

  • When the iron pipe was dried and weighed it had increased in weight.

  • The weight of the "phlogisticated air" + the increase in weight of the iron pipe = the decrease in weight of the water

Lavoisier ignited the "phlogisticated air" (produced by the reaction) in oxygen and measured the weight of the water formed. It was equal to the weight of the water lost during the experiment. He was also able to confirm the

2 parts "phlogisticated air" : 1 part oxygen

that Cavendish had found.

Lavoisier called "phlogisticated air" hydrogen (Greek meaning 'water forming')

Conclusion

Lavoisier concluded the formation of rust (iron calx) was due to something from the water mixing with the iron bar.

He knew that oxygen ignited in hydrogen formed water. His experiment suggested that the water could be split into hydrogen (collected as a gas) and oxygen (that mixes with the iron causing rust).

 

Lavoisier had shown the Law of Conservation of Matter. He said "Nature is a closed system. In any transformation no amount of matter is never lost and none is gained". His 1775 Easter Memoire, his 1783 publication Réflexions sur le phlogiqtique and his 1789 Traité élémentaire de chimie were direct attacks on the phlogiston theory.

 

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Antoine Lavoisier and the demise of the phlogiston theory

 

It was Antoine Lavoisier, working in Paris, who used accurate measurements and deductive reasoning to raise serious doubts about the phlogiston theory. He completed the series of experiments shown below.

Experiment 1: Heating Mercury in atmospheric air

Appartatus01

A diagram of the apparatus used by Lavoisier
at the start of the experiment.

Lavoisier used 4 ozs (113.4 grammes) of mercury. There were exactly 50 cubic inches (819.35 cm3) of air in the bell jar.

The furnace was ignited and the apparatus was heated. There was no change observed during the first 24h but after 48h Lavoisier observed red spots on the surface of the mercury.

Apparatus2

After a few days of continual heating the surface of the mercury was completely covered in red.

Apparatus3

A diagram of the apparatus at the end of the experiment.

Once the apparatus had cooled down, Lavoisier took the new measurements.

He discovered that 8 cubic inches of gas weighing exactly 3.5 grains (0.23 grammes) had "disappeared" from the bell jar.

The mercury + calx in the retort flask has gained exactly 3.5 grains.

When he tested the gas left in the bell jar he found that it had lost its "active" ingredient. It no longer supported combustion or respiration.

Lavoisier concluded that the Mercury had taken this "active gas" from the air.

 

Experiment 2: It was Joseph Priestley's observations on heating mercury calx that gave Lavoisier the information he needed to move on. According to Priestley

equation1

Lavoisier carefully removed all of the calx from the surface of the mercury from Experiment 1. He weighed the calx to make sure that he had the full 3.5 grains.

The mercury calx was placed in a retort flask over the furnace. The apparatus was assembled as in the first experiment but the liquid in the bell jar was brought up to a higher level.

Apparatus3

During this experiment Lavoisier's observations were the same as those of Priestley. A grey mist appeared in the retort flask and drops of mercury could be seen running down on the inside.

Apparatus5

When all of the red calx had disappeared he let the apparatus cool down before taking any measurements.

The volume of the gas in the bell jar had increased by 8 cubic inches and and the weight by 3.5 grains. Tests on the gas in the bell jar showed that it was "active" - it supported combustion and respiration.

Finally Lavoisier weighed the mercury in the retort flask. It had lost exactly 3.5 grains in weight.

Because the "active gas" produced an acidic gas after a candle had burnt in it (tested with litmus paper), Lavoisier named the "active gas" oxygen (from the Greek meaning acid producer).

 

For reference: The development of the Phlogiston Theory

 

 

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