The Law of Definite Proportions


Antoine Lavoisier was later killed during the French revolution, but another French chemist, Joseph Louis Proust, escaped to Spain. Proust's work in Spain was a continuation of Lavoisier's work. Instead of weighing the entire system, as Lavoisier did, Proust weighed each of the parts of the system. One of his first experiments involved the three elements copper, carbon, and oxygen. He discovered that he could combined them to form a compound called copper carbonate. To do this, he took five ounces of copper, four ounces of oxygen, and one ounce of carbon (Asimov 22). After the reaction was complete, Proust always had 10 ounces of copper carbonate.
This, of course, is an example of Lavoisier's conservation of matter. Proust, however, found out that if he didn't use the 5:4:1 proportion, then there would be leftover elements. He then went on to show that this was true for many other compounds as well. By 1799, Proust was certain that all compounds were built out of different elements in certain, definite proportions. His discovery is now known as the law of definite proportions.

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