The identity and relative amounts of substances making up a material or object.Chemical composition is the identity and relative amounts of substances that make up a material or object. It is the fundamental 'recipe' of matter, distinguishing a pure element from a compound, an alloy, or a complex mixture. Knowing the composition of something — whether a distant star, a speck of dust, or a piece of metal — is the first step to understanding its origin, its properties, and its behavior.
The concept of chemical composition solidified in the late 18th and early 19th centuries with the work of Antoine Lavoisier, who established the law of conservation of mass, and John Dalton, who proposed the atomic theory and the law of definite proportions, showing that compounds always contain the same elements in the same ratio.
Composition is determined by identifying the elements or molecules present and measuring their proportions. This can be done through techniques like spectroscopy, which analyzes the light a substance emits or absorbs, or mass spectrometry, which sorts atoms and molecules by their mass. The relative amounts are often expressed as percentages by mass, by atom count, or as ratios, and these numbers dictate everything from a material's melting point to its chemical reactivity.
It is used everywhere from metallurgy, where the exact mix of elements in an alloy determines its strength and memory properties, to astrobiology, where the elemental composition of a planet's atmosphere or surface can hint at the presence of life.
The smell of a substance is rarely due to a single molecule; even a simple odor like that of a rose is a complex mixture of dozens of different chemical compounds, meaning its 'composition' is what your nose actually detects.
