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Mixtures can be separated into individual components by different processes. Methods of separation depend on the physical characters of individual components of the mixture. Some commonly used laboratory processes for separation of mixtures of substances are as follows:
The process of separating an insoluble solid component from the liquid completely by passing through a porous membrane is called filtration. This principle is based on the fact that solvent molecules and molecules or ions present in the solution can pass through the porous membrane (e.g- filter paper) while suspended particles are retained on a porous membrane. The liquid collected after passing through a porous membrane is called filtrate, whereas insoluble particles left in the porous membrane are called residue. A typical experimental setup for this process is shown in the figure below.
This method is generally employed for the separation of the constituents of the mixture in which one component is liquid and the other component is in the form of coarse solid particles heavier than liquid. This method is based on the effect of gravity. The coarse solid particles being heavy settle down (sedimentation) and the upper clear layer of liquid is poured carefully into another container (decantation). For example- muddy river water is purified by this method. This method is not suitable for the mixture containing very fine particles. The typical setup for this experiment is shown below:
The process of converting solid substances directly into the vapour state without converting it into the intermediate liquid state by applying heat is called sublimation. The vapours on cooling give back solid and it is called sublimate. This method is used to separate the solid components from the mixture which are directly converted into vapours on heating whereas remaining components are not affected by heat. Some of the substances which sublimate on heating are camphor, ammonium chloride, naphthalene, benzoic acid etc. The typical setup for this experiment is shown below:
The process of converting liquid into the vapours state at any temperature is called evaporation. This method is used for recovering soluble solid solute from the solution. In this method, the solution containing solid solute is heated in porcelain basin. The solid solute is left in the porcelain basin, whereas the liquid is evaporated. The typical setup for this experiment is shown below:
This technique is used for obtaining a solid compound in pure and geometrical form. The impure sample of the solid substance is dissolved in the suitable solvent to make the saturated solution at the higher temperature. When such solution is cooled, the substance appears in the form of crystals. The solution left behind is called mother liquor, which consists of all the impurities. The crystals thus obtained can be separated with the help of the spatula.The typical setup for this experiment is shown below:
The process of converting liquid into the vapours in heating and the vapour back to liquid on cooling is called distillation. It is of two types:
• Simple Distillation
This technique is used for separating liquids having boiling points differing by 10-20ºC. The liquid having lower boiling point distills first leaving another liquid behind. For distillation process, the liquid mixture is taken in distillation flask fitted with the thermometer and a condenser. To avoid bumping of liquid, few glass beads are used. The distillation flask is then heated on the sand bath . The liquid having low boiling point boils first and its vapour leaves the flask. These vapours are passed through the condenser and condensed into liquid. This condensed liquid is called distillate, which is collected in the receiver, whereas the liquid having the high boiling point is left behind in the distillation flask. The typical setup for this experiment is shown below:
• Fractional distillation
When the boiling points of two liquids present in the mixture do not differ much or they boil within a narrow range of temperatures, simple distillation doesn't work. Thus, fractional distillation is used. In fractional distillation, vapours of liquid are made to pass through the fractionating column, a glass column packed with glass beads or specially designed column. The vapours of liquid having high boiling point get condensed in this column and return back to the flask, whereas the vapours of liquid having low boiling point leave the column from the exit. These vapours enter the condenser and get condensed, which are collected in the receiver. The liquid collected in the receiver consists of pure more volatile liquid and the less volatile liquid remains in the distillation flask.The typical setup for this experiment is shown below:
Chromatography is an important experimental technique for the separation, purification, and identification of constituents of a mixture. This method is based on the differential distribution of the components of the mixture between two phases commonly called stationary phase and a mobile phase. The mobile phase may be liquid or gas. The components of the mixture have different affinities for two phases and thus move through the system at different rates. The component which has a high affinity for mobile phase moves relatively quickly, whereas a component which has a high affinity for stationary phase moves slowly. Due to the difference in rate, complete separation becomes possible. The common type of chromatography is column chromatography.
Adhikari, Rameshwar; Khanal, Santosh; Subba , Bimala; Adhikari, Santosh; Khatiwada, Shankar Pd. Universal Chemistry XI. First. Vol. 1st. Kathmandu: Oasis Publication, 2069.
Chaudhary, Ganga Ram; Karna, Shila Kant Lal; Sharma, Kanchan; Singh, Sanjay; Gupta, Dipak Kumar. A Textbook of Higher Secondary Chemistry XI. Ed. 2nd. Kathmandu: Vidyarthi Pustak Bhandar, 2069 (2012).