What is the law of definite proportions?
The law of definite proportions (also called the law of constant composition) states that a given chemical compound always contains the same elements in the same fixed ratio by mass, regardless of its source, size, or how it was made. Water, for example, is always about 11.2% hydrogen and 88.8% oxygen by mass.
The answer
The law of definite proportions, also known as the law of constant composition, states that any pure chemical compound is always made of the same elements combined in the same fixed proportion by mass. It does not matter whether the sample is large or small, where it came from, or how it was produced—the mass ratio of its elements is constant.
Water (H₂O) is the classic demonstration. Every pure water sample contains hydrogen and oxygen in a mass ratio of about 1 : 8. Because 2 hydrogen atoms (≈2 g/mol total) combine with 1 oxygen atom (≈16 g/mol), water is always roughly 11.2% hydrogen and 88.8% oxygen by mass. A raindrop, a glacier, and lab-synthesized water all give the same percentages. This was one of the key observations that supported John Dalton's atomic theory in the early 1800s.
Why the ratio is fixed
The reason the proportion is constant is atomic. A compound forms when atoms combine in whole-number ratios to make a specific unit (a molecule or formula unit). Because each atom of an element has a characteristic mass, combining them in a fixed count automatically fixes the mass ratio. Water's formula is always H₂O—never H₃O or H₁.₅O in a pure sample—so the mass composition cannot drift.
A quick worked example: to find the composition of carbon dioxide (CO₂), take one carbon atom (12 g/mol) and two oxygen atoms (2 × 16 = 32 g/mol) for a total of 44 g/mol. Carbon is 12/44 ≈ 27.3% and oxygen is 32/44 ≈ 72.7% by mass—in every sample of pure CO₂ on Earth.
Who discovered it, and the related distinctions
The law is credited to the French chemist Joseph Louis Proust, who established it around 1797–1799 through careful mass measurements, defending it in a famous debate against Claude Berthollet (who wrongly believed composition could vary continuously). It is essentially the same statement as "constant composition"—the two names are interchangeable.
Students often confuse this with the law of multiple proportions (Dalton). The distinction is important:
- Definite proportions: for one compound, the mass ratio of its elements is always the same (water is always 1:8 H:O).
- Multiple proportions: when the same two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in small whole-number ratios (e.g., CO vs CO₂: the oxygen masses per gram of carbon are in a 1:2 ratio).
One caveat worth knowing: the law applies to pure stoichiometric compounds. A small class of substances called non-stoichiometric (berthollide) compounds, such as certain metal oxides and sulfides, can deviate slightly because of crystal defects—which is exactly what Berthollet was partly seeing. For the compounds you meet in general chemistry, though, the law of definite proportions holds precisely.
Frequently asked
Who discovered the law of definite proportions?
The French chemist Joseph Louis Proust established the law around 1797–1799 through precise mass measurements of compounds. He defended it in a well-known scientific debate against Claude Berthollet, who mistakenly argued that a compound's composition could vary continuously.
What is the difference between the law of definite and multiple proportions?
Definite proportions says one compound always has the same element-to-element mass ratio (water is always 1:8 H:O). Multiple proportions says when two elements form several compounds, the masses of one element combining with a fixed mass of the other form small whole-number ratios, as in CO versus CO₂.
How is the law of definite proportions demonstrated with water?
Pure water always contains hydrogen and oxygen in a mass ratio of about 1:8, or roughly 11.2% hydrogen and 88.8% oxygen by mass. Because 2 hydrogen atoms (≈2 g) bond with 1 oxygen atom (≈16 g), every sample—raindrop, glacier, or lab-made—gives the same percentages.
Is the law of definite proportions the same as constant composition?
Yes. "Law of definite proportions" and "law of constant composition" are two names for the same principle: a given pure compound always contains the same elements combined in the same fixed proportion by mass, regardless of its source or amount.