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Miscibility is often applied to liquids, and the term is meant by the extent of two or more liquids dissolve in each other. Miscibility can be categorized into miscible, partially miscible and immiscible. Miscible liquids are completely dissolved in each other and form a homogenous solution. Water and ethanol are miscible because they mix in all proportions. Two liquids may be partially miscible if they mix with each other in limited proportions, for example, ether-water and phenol-water. Oil and water are said to be immiscible due to the formation of two layers of liquids after mixing In general, both liquids become more soluble as the temperature increases until a critical solution temperature is reached. Below the critical solution temperature, the liquids will be immiscible by forming two layers. Above or at the critical solution temperature, miscible liquids are formed with only one layer. The composition for two layers of liquids in equilibrium state is constant and does not depend on the relative amount of these two phases below the critical solution temperature. Apart from temperature and pressure factors, the presence of a third component can influence the mutual solubility of the pair of partially miscible liquids
Phase rule is useful for relating the effect of the least number of independent variables like temperature, pressure and concentration upon the various phases (solid, liquid and gaseous) that can exist in an equilibrium system containing a given number of components. Phase rule can be expressed as F=C-P+2 where F is the number of degrees of freedom in the system, C is the number of components and P is the number of phases present. A system consists of phenol and water is one of systems that exhibit partial miscibility. The maximum temperature at which the two phase region don’t exists is called the critical solution temperatureIn this experiment, we have two component system (phenol and water) where the number of phases depend on the experiment conditions. When phenol and water become miscible, the degree of freedom, F = 2 − 1 + 2 = 3. Since this is a condensed system which means the pressure of the system is fixed, F will be reduced to 2. This means that we need to fix both temperature and concentration to define this system. When phenol and water are immiscible with each other, the degree of freedom, F = 2 − 2 + 1 = 1. This means that we need to fix the temperature only to completely define the system.