PHYSICAL CHEMISTRY IN BRIEF

CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 218

(l)

(s 2 )+(l)

(l)+(s 1 )

E

(s 2 )+(s 1 )

0 x 1 1

T

Tf2

Tf1

Obr. 7.11: Dependence of the freezing temperature on composition in a two-component system
whose components are immiscible in the solid phase. Tf, 1 andTf, 2 are the freezing temperatures of
pure components 1 and 2, E is the eutectic point.

4. Of the two substances with a comparable temperature of melting, the one with a
   lower enthalpy of melting will dissolve more.

• For the temperature dependence of the solubility of salts we can write for low values of
  molality

ln

m(l) 2 (T)

m(l) 2 (T′)

=

H

⊗

2

νR

(

1

T′

−

1

T

)

, (7.66)

whereνis the total number of ions to which a salt of the molalitym(l) 2 dissociates in a

given solvent, andH

⊗

2 is its last differential heat of solution.

• Forlowering of the melting temperatureby dissolving a component 2 in a pure
  liquid substance 1 it holds in the ideal solution approximation

∆T=Tf, 1 −T=. KCm 2 =KC

m 2

M 2 m 1

, (7.67)