STPM Chemistry Form 6 Notes – Phase Equilibrium (Part 7)

by BerryBerryTeacher

in Berry Reference (Notes)

This post, Part 7 of Berry Berry Easy short study notes for STPM Chemistry Form 6 on Phase Equilibrium will be focused on a mish-mash of solution properties such as boiling point and freezing point. Also discussed are ways to alter their values and the underlying principle behind the ability to change it. The colligative properties or more easily understood as solution properties that relies on the number of molecules within a fixed solvent volume, will also be discussed. So hop on this post and read this relatively easy subtopic.

[Tips: Colligative properties are dependent on the number of molecules in specified solvent volume and should not be mixed up with the properties of the molecules. This is one frequent mistake by students year after year.]

STPM Chemistry Form 6 Notes – Phase Equilibrium (Part 7)

Fractional Distillation

Fractional Distillation

Boiling Point Elevation

  • Boiling point of a solution increases with the addition of impurities (Increase in B.P., decreases in vapour pressure)
  • Example: Addition of salt in water.

Boiling point elevation is due to two factors:

  • Decrease surface area for the liquid to evaporate.
  • Water has stronger attraction to the ionic impurities. It requires more energy to break ion-dipole interaction than the hydrogen bond between water molecules.

Freezing Point Depression

  • Freezing point of a compound can be altered by varying the intermolecular forces.
  • F.P. decreases as soluble impurities are added.
  • Example: Salt is added to the surface of an ice cube and the water molecules on the surface of the lattice are attracted to the salt. This is an exothermic reaction because addition energy is released to melt the ice. Therefore, salt can be used to help to melt the ice on roads and prevent future freezing.

Freezing point depression is due to factor:

  • Solute particles exhibit attractive forces in solution.

Colligative Properties: Concentration Effects

  • Colligative – properties of a solution that are affected by the concentration of a soluble impurity. This included boiling point elevation (the same as vapour pressure reduction), freezing point depression and osmotic pressure (osmotic pressure is not included in STPM syllabus).
  • The greater the number of impurities, the greater the effect.
  • Impurities are added to solution –> solvent has greater tendency to remain as a liquid – increase intermolecular forces in the solution phase.
  • Water tends to stay in the liquid phase with the increased energy of impurities (rather changes to another phase). The boiling point of an aqueous salt solution increases and the freezing point of an aqueous salt solution decreases.


  • Pure water does not conduct an electrical current (no ions present).
  • The specific conductance of an aqueous salt solution is directly proportional to the concentration of salt on solution.
  • Condosity solution – the molar concentration of an aqueous sodium chloride solution that has the same specific conductance as the aqueous salt solution.
  • Example: the 1.0 mol dm-3 salt (aq) solution has a condosity of 2.0, means 2.0 mol dm-3 NaCl (aq) solution would have the same conductivity as a 1.0 mol dm-3 salt (aqueous) solution.

Finally you have reached the end of this part. The next post, Part 8 of Berry Berry Easy‘s series of notes for STPM Chemistry Form 6 on Phase Equilibrium will be focused on distribution law and partition law.

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