law of definite composition
C2Academic, Technical, Scientific
Definition
Meaning
A fundamental principle of chemistry stating that a given chemical compound always contains its component elements in a fixed proportion by mass.
More broadly, the principle that a substance with a specific chemical composition has a consistent, invariant makeup regardless of its source or method of preparation.
Linguistics
Semantic Notes
Often referred to simply as Proust's Law, after Joseph Louis Proust. It is one of the fundamental laws of stoichiometry and is distinguished from the law of multiple proportions.
Dialectal Variation
British vs American Usage
Differences
No significant differences in usage. Spelling follows regional conventions within the sentence (e.g., 'composition' vs. 'compoſition' is historical only).
Connotations
Purely technical and scientific in both varieties.
Frequency
Used with identical frequency and context in scientific and educational settings in both the UK and US.
Vocabulary
Collocations
Grammar
Valency Patterns
The law of definite composition states that...According to the law of definite composition,...... is consistent with the law of definite composition.Vocabulary
Synonyms
Strong
Neutral
Weak
Vocabulary
Antonyms
Phrases
Idioms & Phrases
- “It's as fixed as the law of definite composition. (rare, metaphorical)”
Usage
Context Usage
Business
Not applicable; purely scientific term.
Academic
Central to introductory and advanced chemistry courses, textbooks, and research papers in stoichiometry.
Everyday
Virtually never used outside of educational or scientific discussion.
Technical
Foundational concept in chemical analysis, materials science, and forensic science.
Examples
By Part of Speech
adjective
British English
- The definite-composition principle is fundamental.
- A definite-composition analysis was performed.
American English
- The definite-composition principle is fundamental.
- A definite-composition analysis was performed.
Examples
By CEFR Level
- Water always has hydrogen and oxygen. This is a scientific law.
- The law of definite composition says that pure water always has the same elements in the same ratio.
Learning
Memory Aids
Mnemonic
Think: DEFINITE = DE-FINITE = The FINAL, fixed composition of a compound is definite and doesn't change.
Conceptual Metaphor
A RECIPE: Just as a cake recipe always requires the same fixed amounts of flour, sugar, and eggs, a compound has a fixed 'recipe' of elements.
Watch out
Common Pitfalls
Translation Traps (for Russian speakers)
- Avoid literal translation like 'law of determined composition'. The established Russian equivalent is 'закон постоянства состава' or 'закон определённых соотношений'.
- Do not confuse with 'закон кратных отношений' (law of multiple proportions).
Common Mistakes
- Confusing it with the 'law of conservation of mass' or 'law of multiple proportions'.
- Incorrectly stating it applies to mixtures (it applies only to pure chemical compounds).
- Using 'definitive composition' instead of 'definite composition'.
Practice
Quiz
Which of the following best illustrates the law of definite composition?
FAQ
Frequently Asked Questions
It is most famously associated with the French chemist Joseph Louis Proust, who formulated it based on analytical work between 1794 and 1804.
No. It applies specifically to pure chemical compounds. Alloys (like brass) and solutions (like saltwater) are mixtures and can have variable composition.
The law of definite composition states that one specific compound has a fixed composition. The law of multiple proportions states that when two elements form more than one compound together, the masses of one element that combine with a fixed mass of the other are in a ratio of small whole numbers.
Yes, non-stoichiometric compounds (berthollides), such as some metal oxides and sulphides, show slight variations in composition. However, for the vast majority of molecular compounds and ionic compounds, the law holds true.