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Antoine Lavoisier: Experiments
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Given all this uncertainty as to the composition of fixed air and its role in combustion, Lavoisier designed some new experiments. He set up some candle experiments which reinforced his results from 1773 that the volume of fixed air produced in combustion was equal to the volume of pure air consumed. Since the weights were so close, he thought that the two gases differed by some weightless substance such as matter of fire or phlogiston. In a few memoirs he even entertained the idea that phlogiston was matter of fire. However, another experiment in which he reacted charcoal and mercury calx and obtained fixed air, contradicted the idea that fixed air was merely a combination of common air and phlogiston or matter of fire. The results of this experiment implied (correctly) that fixed air was the combination of common air with charcoal.
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Lavoisier's researches included some of the first truly quantitative chemical experiments ever performed. He carefully weighed the reactants and products involved in a chemical reaction, a crucial step in the advancement of chemistry. He showed that, although matter can change its state in a chemical reaction, the quantity of matter is the same at the end as at the beginning of every chemical change. He burned phosphorus and sulfur in air, and proved that the products weighed more than the original. Nevertheless, the weight gained was lost from the air. These experiments provided evidence for the law of the conservation of matter or, in other words, the law of conservation of mass.
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Lavoisier was an excellent discoverer because he was quick to see the significance of new findings. He readily confirmed and extended the experimental discoveries of others and formed mental models to organize all of these ideas. He was one of the few chemists at the time to fully appreciate the importance of careful measurements of reactants and products. In order to make such careful measurements he invented a balance which was good to about .0005 grams. He proved the Law of Conservation of Mass, showing that the mass of the reactants had to equal the mass of the products.
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By 1772 Lavoisier had begun experiments that were to change the face of chemistry. He was helped by a large burning lens that belonged to the French Academy of Sciences. Arthur Donovan, author of a recent biography of Lavoisier, pictures his delight in having access to the lens as being 'exactly like that of 20th-century physicists who run experiments on gigantic particle accelerators - it was a machine capable of blasting apart substances previously considered immutable'. Lavoisier deposited his findings for safety in a sealed note with the secretary of the academy on 1 November that year. Within two months he was able to state that phosphorus and sulphur gained weight on burning, due to their combination with air.
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For example, in one experiment, Antoine put fruit into a sealed container, measured its mass, and then left it in a warm place for a few days. The fruit rotted and changed into a putrid mess. Gas was released from the decomposing fruit and droplets of water formed on the glass, but nothing escaped from the container. Lots of changes had taken place, but the mass of the sealed container and the rotten fruit was equal to the mass measured at the beginning of the experiment.
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Lavoisier noticed that the series of experiments he’d been working on strongly resembled the experiments he had done with birds. Respiration was believed to be similar to calcination for various qualitative reasons. However, Lavoisier suggested the correct correlation between combustion and respiration as early as November of 1773 when he wrote a memoir on respiration for the Academy of Science. At this point he had already started on the path that would lead him away from the phlogiston theory.
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