History Weblecture for Unit 38
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Gas collection instruments at the Musée Arts et Métiers, Paris
© 2009 Christe Ann McMenomy
The idea that matter can be broken down into indivisible bits with specific characteristics goes all the way back to Democritus and Leucippus, early Greek philosophers who lived before Aristotle. Aristotle and his contemporaries could not observe atoms, and so sought other explanations for the properties of matter. But during the eighteenth century, the atomic theory became popular and was supported by Newton, Descartes, and Leibniz, although without any serious experimental evidence. The one who was able to bring together the discoveries of Lavoisier, Priestley, and Black with the atomist theories of Descartes and Leibniz was a modest, color-blind Quaker by the name of John Dalton.
Dalton was worked as a teacher of mathematics and natural philosophy in Manchester and York. He wrote books on meteorology and English grammar and the vision of color before venturing into chemistry, where his early researches concentrated on the study of gases and their volumes and pressures. As recording secretary of the Manchester Literary and Philosophical Society, Dalton was able to hear and present papers on different aspects of gas theory.
Dalton realized that if volumes of gases combined in whole number proportions to form different substances, then matter might break down into discrete particles that, like a child's set of building blocks, could be broken down and set back up in different combinations. He resurrected the idea that minute individual particles make up all matter. Each type of particle or atom had specific characteristics which depended on the atom's shape and size. Atoms of the same type were of the same element and behaved the same way. All atoms of gold, for example, have the same specific characteristics (weight, ability to combine with other elemental atoms). All atoms of oxygen share the same specific characteristics also, but these are different from the characteristics for atoms of gold or other elements. This theory has some important ramifications.
Dalton's law of partial pressures follows naturally from the idea that gases are made up of individual particles which act independently. If a sample volume has two gases, the total pressure on the container is the sum of the pressures exerted by each of the gases, just as though the other was not present:
PTOT = PGas1 + PGas2
Dalton's major contribution was the realization of the consequences of assuming atoms and multiple elements as the basis for all matter. Although published in the early 1800s, Dalton's original observations on how elements combine are so well attested that we now call them the three basic laws of chemical composition and apply them to all macro-atomic (bigger than the atom) or chemical reactions.
The conservation of mass is a basic principle of the universe. It means that (excepting whatever initial incident got us here) no mass is now being created or destroyed. In an ordinary chemical reaction, there is no difference between the mass of the reactants and the mass of the products. In a nuclear reaction, any mass loss is compensated by a release of energy equal to the mass loss times the speed of light squared (Einstein's famous E=mc2). This principle, of course, had already been discovered by Lavoisier, and indeed, had been stated centuries earlier by Francis Bacon (1600s) but without experimental proof. Dalton and Lavoisier were able to demonstrate that mass is always conserved.
The law of constant composition means that a compound is always formed out of the same components. Changing the proportions of the components changes the compound into something else. If we assume that there are the same number of atoms in equal volumes of two substances, but that the mass of the atoms are different for different elements, we can account for all the proportions of volume or mass in chemical reactions.
For example, both water and hydrogen peroxide are composed of hydrogen and oxygen. But water is in the proportion 2 volumes of hydrogen gas to 1 volume of oxygen gas, or 2 masses of hydrogen to 16 masses of oxygen, a 1:8 proportion by weight. Hydrogen peroxide is formed from 2 masses of hydrogen to 32 masses of oxygen: a 1:16 proportion by weight. Water is always 1:8, hydrogen peroxide is always 1:16. The exact proprotions are constant for a given compound, and identify the compound.
The law of multiple proportions means that elements combine in whole-number ratios (because the atoms are particles which are combining). This makes it much easier to determine the composition of complex molecules. It also makes sense only if whole particles like atoms are combining. We never see combinations that imply half or a third of an atom is part of a compound.
Use the slide show below to see the animation of Dalton's law of the conservation of mass, and examples of the laws of constant composition and multiple proprotions. Different options show at different times in the presentation.
Go to the archive copy of Dalton's New System of Chemical Philosopy. Use the slider at the bottom of the page to go to page 141 (Chapter II: On the Constitution of Bodies). Read this short introduction to the chapter (pp. 141-144)
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