Why every drop of water is chemically identical to the next

Science · Published 28 October 2026 Swipe more like this →

In short: If you take a sample of pure water from the ocean and another from a mountain stream, they are chemically indistinguishable. This isn't just a coincidence of nature; it is governed by a fundamental law that dictates how matter assembles itself at the atomic level.

The Law of Constant Composition, also known as the Law of Definite Proportions, states that any given chemical compound consists of the same elements combined in exactly the same proportions by mass. This means that regardless of where a substance originates, its elemental makeup remains fixed. For example, water is always composed of hydrogen and oxygen in a 2:1 atomic ratio, forming H2O molecules.

This principle was first established through the research of French chemist Joseph-Louis Proust. It serves as a cornerstone for defining compounds and allows chemists to use formulas—like CH4 for methane—to represent specific ratios of atoms. By understanding these fixed proportions, scientists can convert chemical formulas into mass percentages, a vital tool in analytical chemistry for identifying unknown substances.

However, the law has notable limits. While it holds strictly for small molecular compounds like water, it is less precise for large or complex structures known as non-stoichiometric compounds. In these cases, the ratio of elements can vary slightly between samples. A primary example is iron selenide (FeSe), which often forms in a ratio close to Fe0.9Se.

These slight deviations from constant composition are not just chemical quirks; they can change the physical properties of a material entirely. In the case of iron selenide, this non-stoichiometry is what gives rise to its unique magnetic properties, allowing it to function as a low-temperature superconductor.

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