PHYSICAL CHEMISTRY IN BRIEF

CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 202

• When there is a minimum on the curve of boiling temperatures in an isobaric diagram
  for a binary system, we speak about an azeotrope with aboiling point minimum.
  It exists in systems with positive deviations from Raoult’s law, and consequently it is
  calledpositive azeotrope.
  On they–xdiagram [see Figure7.8], the system with a positive azeotrope is represented
  by the thick line. For the derivative of theyi dependence onxiin the vicinity of the
  positive azeotrope we can write

0 <

(

∂yi

∂xi

)

< 1. (7.31)

• In systems with negative deviations from Raoult’s law, an azeotrope with a boiling
  point maximum, or anegative azeotrope,may occur [see Figure7.7].
  For the derivative of theyidependence onxiin the vicinity of the negative azeotrope
  we write (
  ∂yi
  ∂xi

)

> 1. (7.32)

7.6.7 Effect of the non-volatile substance content on the boiling

pressure and temperature

If we add a small amount of a non-volatile substance 2 (ps 2 ≈0) to a pure liquid substance 1
at constant temperature, the initial boiling pressure, which is equal to the saturated vapour
pressureps 1 , always drops. After adding a non-volatile substance, the boiling pressurepbecomes
to a good approximation
p=x 1 ps 1. (7.33)

• The decrease in the boiling pressure at constant temperatureis given by the
  relation
  ∆p=ps 1 −p=x 2 ps 1

.

=

m 2 M 1

M 2 m 1

ps 1 , (7.34)

wherepis th e boiling pressure of a liquid solution of compositionx 2 ;m 1 andm 2 are

the masses of the components.