Amazing facts of phenol water system phase rule


  1. In today’s article, we are going to study about ‘ phenol water system phase rule ‘ How beneficial it is and what is the phenol water system. We will also read about some more topics like immiscible liquids, partially miscible liquid phase diagram, phase diagram of phenol water system, explanations of curves, etc in detail.  So, take your notebooks in you hand and get ready to study physics in an easy and sorted way.

Phenol water system phase rule

  1. Phenol and water are partially miscible at ordinary temperature.
  2. Therefore, when these two liquids are shaken with each other two saturated solutions of different compositions, are of phenol in water and the other of water in phenol, are obtained.
  3. Such solutions of different compositions coexisting with one another are termed conjugate solutions.
  4. When the temperature is increased, mutual solubility of two liquids increases. Therefore, the concentration of phenol in water as well as that of water in phenol goes on increasing with rise of temprature.
  5. And finally at a certain temprature the two conjugate solutions change into one homogeneous solution.
  6. The variation of mutual solubility of water and phenol with temprature is shown graphically in fig. PS represents the solubility of phenol in water and QS represents the solubility of water in phenol.

Phenol water system phase rule

Point A and B shows the phenol water system composition at a temperature T. At A phenol forms homogeneous solution with water whereas at ‘B’ water forms homogeneous solution with phenol. Any point ‘C’ in between AB shows the liquid state whose composition is shown by A and B respectively.

Following equation gives the quantitative ratio of two layers :

Mass of phenol layer/ Mass of water layer = Distance CA / Distance CB

As the temprature increases, mutual solubilities of phenol and water increases and points A and B approach to S. At highest point S (Maximum) both layers have identical composition and solution becomes homogeneous. This point is known as critical solution point. Temperature at this point is known as critical solution temprature (60°C) and percentage of phenol is 33% (By weight). Above 60°C water and phenol are miscible in all proportions.

Few other examples of such solutions are given in table.

Partially miscible liquid System phase diagram

  • There are certain liquid pairs which are partially miscible at room temperature. After a certain limit of solubility a stage is reached when no more solubility takes place and two liquid layer are formed.
  • These two solutions in equilibrium with each other are called conjugate solutions.
  • Composition of these conjugate solutions changes with temperature since mutual solubility of two liquids changes with temperature. On this basis these solutions are classified into two types.
  • (a) The liquid pairs in which the partial miscibility increase on increasing the temperature and there is a certain temperature above which the pair of partially miscible liquids becomes miscible in all proportions. This temprature is called critical solution temperature or consolute temperature for the pair. Since this temperature is above the normal temperature. Sometimes it is also called as Upper consolute temperature. Examples of such liquid pairs are methyl alcohol-carbon disulphide, phenol-water, aniline-water, aniline-hexane, cyclohexane-methanol etc.
  • Those in which partial miscibility increase on lowering the temperature. At certain and lower temperature they are completely miscible with each other. Since this temperature is below the normal temperature, it is called lower critical solution temperature or lower consolute temperature. Nicotine-water, meta toludiene-glycerol are the examples of this types.

Immiscible liquids

  1. In a mixture of two immiscible liquids, each liquid behaves independently of the other.
  2. Each component has its own Vapour pressure and therefore total pressure of the mixture is equal to the summation of vapour pressure of individual pure components.
  3. If PA and PB are the vapour pressure of two components A and B in a mixture, then the total pressure is given by- P= PA+ PB …(1)
  4. The mixture of two immiscible liquids will boil at the temperature when the total vapour pressure of the mixture equals the atmospheric pressure i..e PA+PB= atmospheric pressure.
  5. It means the boiling of the mixture will be less than that of the other components.

Relative amounts of two components can be determined by using gas laws for a distillate. If V is the volume of Vapour, then,

pA V = nA RT

And pB V= nB RT

pA/ pB= nA/nB    …(2)

Where nA and nB are the number of moles of A and B respectively in vapour state.

Further, we know that

nA = WA/ MB and nB= WB/ MB

Therefore, pA/pB= (WA/MA)/(WB/MB) = (WA/ WB)× (MB/MA)

WA/ WB= PA MA / PB MB     …(3)

Thus, masses of the two components in the distillate deepens upon the molar masses and the vapour pressures of the pure components. Equation (3) is the basic principle of steam distillation.


So our article is finished and after completely reading this article, one can easily tell what an phase diagram of phenol water system phase rule is. And how we can explain it.

‌So one can say that they got a detailed information about phase diagram of phenol water system phase rule.

Must read : 

Application of phase diagram : water system, Application of phase diagram : carbon dioxide system and Application of phase diagram : sulphur system in earlier articles. You can visit there for more information.

IT was an article based on ‘phenol water system phase rule’. Your suggestions are invited for more topics of articles.

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