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Unit 1: Matter and the Rise of Atomic Theory—The Art of the Meticulous

Section 7: The Properties of Chemical Compounds—Claude Louis Berthollet and Joseph Proust

Antoine Lavoisier's research inspired more work to identify chemicals and determine how and why they reacted with each other. Throughout the 18th century, chemists tended to use terms such as element, compound, and mixture loosely, but Lavoisier called for making these terms more precise. He also predicted that some substances that were thought to be indivisible might turn out to be compounds once scientists developed better methods of decomposing them.

Two French chemists, Claude Louis Berthollet and Joseph Proust, both of whom had worked with Antoine Lavoisier, debated extensively in the early 19th century over whether chemicals in compounds always combined in fixed proportions. Proust (1754–1826), a professor and research chemist, proposed his Law of Definite Proportions in 1797. In a series of experiments, he heated varying quantities of copper carbonate (CuCO3) (Figure 1-11) to decompose it into copper, carbon, and oxygen and then compared the ratios of the masses of these components. Based on the results, Proust contended that the composition of chemical compounds was fixed, with each component accounting for a specific fraction by weight.

Copper Carbonate Forming on a Copper Roof; Pure Copper Carbonate in a Vial

Figure 1-11. Copper Carbonate Forming on a Copper Roof; Pure Copper Carbonate in a Vial

© Left: Wikimedia Commons, Creative Commons License 3.0. Author: TitaniumCarbide, 12 November 2010. Right: Wikimedia Commons, Creative Commons License 3.0. Author: W. Oelen, 27 May 2005.

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Copper Carbonate Forming on a Copper Roof; Pure Copper Carbonate in a Vial

Figure 1-11. Copper Carbonate Forming on a Copper Roof; Pure Copper Carbonate in a Vial

The roof of the Château Frontenac in Quebec City, Canada, shows the blue-green color associated with copper carbonate forming on the surface of copper roofing plates. This is a natural process whereby the elements of nature, including acid rain, heat, and sunlight, cause the copper roof to react. The reaction that took place on the copper roof is the reverse of the reaction that Proust had done when he was converting the copper carbonate back to copper metal. The second image shows a close-up of purified copper carbonate powder.

Berthollet (1749–1822) was equally experienced. He had helped develop Lavoisier's chemical nomenclature and researched many chemistry applications, including bleaches, dyes, metalworking, and munitions. Some of his notable discoveries included the bleaching properties of chlorine and the chemical composition of ammonia. While traveling on a French military expedition to Egypt in 1798, Berthollet observed an unusual phenomenon at the Natron Lakes, a group of saltwater lakes in limestone beds northwest of Cairo. (Figure 1-12) There, he saw that natron, a Greek term for "soda ash" (sodium carbonate, Na2CO3, a chemical used for glassmaking and many other industrial applications), had formed along the shoreline, produced by reactions between saltwater and limestone (calcium carbonate):

CaCO3  +   2 NaCl   →   Na2CO3  +  CaCl2

Berthollet realized that this was the reverse of a reaction that he had seen in the laboratory, in which calcium chloride reacted with soda ash to produce calcium carbonate and salt:

Na2CO3   +  CaCl2   →   2 NaCl   +   CaCO3

Owens Lake: A Mostly Dry Lake with Salt Deposits

Figure 1-12. Owens Lake: A Mostly Dry Lake with Salt Deposits


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Owens Lake: A Mostly Dry Lake with Salt Deposits

Figure 1-12. Owens Lake: A Mostly Dry Lake with Salt Deposits

Similar to the Natron lakes in Egypt, Owens Lake in California was a site for soda processing due to the salts that surrounded it as well. The reddish color around the lake now comes from bright pink halophilic (salt-loving) bacteria that grow on these deposits.

Berthollet had discovered a reversible reaction, which chemists would later notate using a double arrow (equilbrium arrows) to indicate that the reaction could move in both directions. This indicated to Berthollet that conditions such as heat, pressure, and the amount of reagents present influenced the course and products of chemical reactions. After further study, he concluded that the composition of some compounds could vary within certain limits.

Proust and Berthollet debated this issue for nearly a decade, until John Dalton's atomic theory (discussed in Section 8) confirmed Proust's position. Dalton's work clarified the difference between mixtures, whose compositions can vary, and chemical compounds, which have constant compositions. Berthollet's reaction mixtures contained impurities, so in fact they were mixtures, not pure compounds. The debate between Berthollet and Proust shows the importance of accurate measurement: To determine the exact composition of a compound, chemists had to measure the relative masses of the elements of which it was composed.



A substance composed of two or more elements that are chemically combined in definite proportions. A compound's makeup never changes: For example, a molecule of carbon dioxide always contains one atom of carbon and two atoms of oxygen.


A substance that cannot be broken down into simpler substances through chemical processes.


A combination of two or more substances in which each ingredient retains its own chemical identity. Unlike chemical compounds, the makeup of mixtures can vary: For example, air samples from a polluted city and a forest will have different compositions.


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