PHYSICAL CHEMISTRY IN BRIEF

CHAP. 1: BASIC TERMS [CONTENTS] 36

1.6 Pure substance and mixture

We speak about a pure substance (chemical individual) when only one kind of molecules is

present in a system. When a system contains more kinds of molecules, we speak about a

mixture. The substances of which a mixture is composed are itscomponents. According

to the number of components we distinguish binary mixtures consisting of only two compo-

nents, ternary mixtures consisting of three components, quarternary mixtures consisting of four

components, etc. In addition to thermodynamic quantities used to describe pure substances

(temperature, pressure, volume), the description of a mixtures also requires knowledge of the

composition of all its phases, which may be expressed using one of the quantities listed below.

1.6.1 Mole fraction of theithcomponentxi

Definition

xi=

ni

n

, (1.5)

whereniis the amount of substance of componenti,nis the total amount of substance of all

components

n=

∑k

j=1

nj, (1.6)

andkis the number of components in the mixture.
U Main unit:dimensionless quantity.
It follows from the definition (1.5) that the sum of mole fractions equals one

∑k

i=1

xi= 1. (1.7)

Note:Instead of mole fractions, the expression mole percent is often used, meaning 100-

times the mole fractions.