The law of multiple proportions is one of the fundamental laws of stoichiometry and was first discovered by the English chemist John Dalton in 1803. The law states that when chemical elements combine, they do so in a ratio of small whole numbers. For example, carbon and oxygen react to form carbon monoxide (CO) or carbon dioxide (CO2), but not CO1.3. Further, it states that if two elements form more than one compound between them, the ratios of the masses of the second element to a mass of the first element will also be in small whole numbers.
The Three Laws
The total mass of all products of a chemical reaction is equal to the total mass of all reactants of that reaction. These statements are summaries of many observations, which required a tremendous amount of experimentation to achieve and even more creative thinking to systematize as we have written them here. By making these assumptions, we can proceed directly with the experiments which led to the development of the atomic-molecular theory. It is also called hard law theory. Dalton believed in this theory.
Goals: The statements above, though correct, are not more vague than they might first appear. For example, exactly what do we mean when we say that all materials are made from elements? Why is it that the elements cannot be decomposed? What does it mean to combine elements into a compound? We want to understand more about the nature of elements and compounds so we can describe the processes by which elements combine to form compounds, by which compounds are decomposed into elements, and by which compounds are converted from one to another during chemical reactions.
One possibility for answering these questions is to assume that a compound is formed when indestructible elements are simply mixed together, as for example, if we imagine stirring together a mixture of sugar and sand. Neither the sand nor the sugar is decomposed in the process. And the mixture can be decomposed back into the original components. In this case, though, the resultant mixture exhibits the properties of both components: for example, the mixture would taste sweet, owing to the sugar component, but gritty, characteristic of the sand component.
In contrast, the compound we call iron rust bears little resemblance to elemental iron: iron rust does not exhibit elemental iron's color, density, hardness, magnetism, etc. Since the properties of the elements are not maintained by the compound, then the compound must not be a simple mixture of the elements.
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