The law of multiple proportions states that when two elements form more than one compound between them, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers. This means if you fix the amount of one element, the amounts of the second element in different compounds will relate to each other as simple whole number multiples. For example, nitrogen and oxygen form several oxides. If you fix 14 grams of nitrogen, the masses of oxygen that combine with it in different oxides are 8, 16, 24, 32, and 40 grams, which correspond to the simple whole number ratio 1:2:3:4:5. This pattern supports the idea that atoms combine in fixed, discrete ratios, which was key evidence for Dalton's atomic theory
. Another example is carbon monoxide (CO) and carbon dioxide (CO₂). In CO, 12 grams of carbon combine with 16 grams of oxygen; in CO₂, 12 grams of carbon combine with 32 grams of oxygen. The ratio of oxygen masses (16:32) simplifies to 1:2, a small whole number ratio, illustrating the law
. In summary, the law of multiple proportions reveals that when two elements form multiple compounds, the masses of one element that combine with a fixed mass of the other are related by simple whole number ratios, reflecting the discrete nature of atoms combining in fixed proportions.
