Chemical composition
The chemical composition (or simply composition) is a concept in chemistry that has different (but similar) meanings if referred to a single pure substance or a mixture.
Composition of a substance
The chemical composition of a pure substance it corresponds to the relative amounts of the elements that constitute the substance itself. It can be express by the empirical formula.
For example the formula for water is H2O: this means that each molecule it is constituted by 2 atoms of hydrogen (H) and 1 atom on oxygen (O).
Composition of a mixture
The chemical composition of a mixture can be defined as the distribution of the single substances that constitute the mixture (called "components"). In other words, it is defined giving the concentration of each component.
Because there are different ways to define the concentration of a component, as a consequence there are also different ways to define the composition of a mixture. For example it can be expressed as molar fraction, volume fraction, mass fraction, molality, molarity or normality.pt:Composição química
