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

by BerryBerryTeacher

in Berry Reference (Notes)

Immiscible liquid or even miscibility will be terms which are new to most Berry Readers. However, if we were to use layman’s terms, miscibility will simply mean the ability of molecules to dissolve in other molecules. However, we’re learning chemistry, so we ought to use more accurate terms. Hence, it’ll be better to use the proper terms. So in this post, Part 6 of Berry Berry Easy short study notes for STPM Chemistry Form 6 on Phase Equilibrium, we’ll be focusing on immiscible liquids. with immiscible liquids come the difficult subtopic of vapour pressure of immiscible liquids and Dalton’s law of partial pressures. It’ll be useful to revisit the concept of vapour pressure if you have forgotten about it. So please pay attention to the post below as it’ll be a deceptively difficult subtopic to master.

[Tips: For those who wants to know the proper term of miscibility (for liquids), it is actually the property of liquids to mix in all proportions to form homogeneous solution. It is also useful to know examples of immiscible liquids, such as the common oil and water. Oil and water are immiscible liquids (relative to each other) as they do not mix and the oil layer floats on top of the water layer.]

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

Fractional Distillation

Fractional Distillation

Immiscible Liquid

  • Immiscible liquid – liquids do not dissolve in one another
  • Two of the immiscible liquids (A & B) are mixed, it form two separate layers
  • Lower density liquid – top layer
  • Higher density liquid – bottom layer
  • The intermolecular forces of attraction between their molecules are different
  • Example: benzene and water, mercury and water, chlorobenzene and water, nitrobenzene and water
  • Water is a polar molecule
  • Benzene, chlorobenzene and nitrobenzene are non-polar molecule with van der Waals forces of attraction and mercury atoms are bonded to each other with metallic bond.

Vapour Pressure of Immiscible Liquids

  • The total vapour pressure of the mixture is the sum of the vapour pressures of the pure components
  • PT = P˚A + P˚B where PT = Total pressure of the liquid mixture, P˚A = Vapour pressure of pure liquid A, P˚B = Vapour pressure of pure liquid B
  • Method of distillation and purification: Steam distillation and followed by separation process using separation funnel
  • Steam distillation is used to extract fragrant oils from plants (making perfume) and purify organic compound
  • To carry out this process, the liquid must be immiscible with water, have a relatively high RMM and high vapour pressure (100˚C)

Dalton’s Law of Partial Pressures

  • A = XA • PT and P˚B = XB • PT where XA = mole fraction component A in vapour and XB = mole fraction component B in vapour
  • XA = nA / (nA + nB) and XB = nB / (nA + nB) where nA = number of moles A and nB = number of moles B
  • (Dividing P˚A and P˚B) P˚A / P˚B = nA / nB
  • Number of moles (n) = mass in gram (m) / molecular mass (M)
  • Final equation, mA / mB = P˚A•MA / P˚B•MB


Water and chlorobenzene are boiled at a pressure of 101 kPa, the temperature is 90˚C and the ratio of the mass of chlorobenzene to that of water is 2.46. The vapour pressure of water at 90˚C is 72.3 kPa, calculate the relative molecular mass (RMM) of chlorobenzene.


  • Use formula mA / mB = P˚A•MA / P˚B•MB
  • Vapour pressure of chlorobenzene at boiling point, 101 – 72.3 = 28.7 kPa
  • 2.47 = (28.7 x Mchlorobenzene) / (72.3 x 18)

Answer: Relative molecular mass of chlorobenzene = 112

So there you go, the end of this part. The next post, Part 7 of Berry Berry Easy‘s series of notes for STPM Chemistry Form 6 on Phase Equilibrium will be related to boiling point elevation, freezing point depression, colligative properties and conductance.

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