How does carbon monoxide and carbon dioxide demonstrate the law of multiple proportions?

The law of multiple proportions, announced by the English chemist John Dalton in 1803, states that when two elements combine to form more than one compound, the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers.1

Carbon monoxide (CO) and carbon dioxide (CO₂) serve as a classic demonstration of this law. Dalton observed that these two compounds are formed by the same elements—carbon and oxygen—combining in different proportions by weight. Specifically, carbon dioxide contains twice as much oxygen by weight as carbon monoxide for a fixed amount of carbon.2

This relationship results in a simple, integer ratio of 2:1 regarding the oxygen content between the two compounds. This consistency provided powerful experimental evidence for Dalton’s theory that matter is composed of discrete, indivisible atoms, as it suggested that elements react in definite, predictable proportions based on their constituent atomic masses.12