Zone Refining and Fractional Distillation: Concept, Process
By Home Academy
Zone refining and fractional distillation are two important separation and purification techniques widely asked in chemistry and engineering examinations. Both methods are based on different physical principles and are used for specific types of substances. From an exam point of view, understanding their definition, principle, applications, and differences is extremely important.
Zone refining is a highly specialized method used for obtaining ultra-pure metals and semiconductors. This technique is mainly used when very high purity is required, especially in electronics and semiconductor industries. Zone refining is based on the principle that impurities are more soluble in the molten state of a substance than in its solid state.
In the zone refining process, a solid rod of impure metal is taken and a narrow region of the rod is heated using a moving heater coil. This creates a small molten zone. As the heater slowly moves along the length of the rod, the molten zone also moves forward. The impurities concentrate in the molten region and move along with it. At the end of the process, impurities collect at one end of the rod, which is then cut off to obtain highly pure metal.
Zone refining is commonly used for the purification of semiconductors such as silicon, germanium, gallium, boron, and indium. The purity achieved by this method can be as high as 99.9999 per cent, which is essential for electronic devices like transistors, diodes, and integrated circuits.
From an exam perspective, it is important to remember that zone refining is suitable for solids, it is based on differential solubility of impurities, and it is mainly used in semiconductor technology. Questions are often asked on its principle, application, and reason for using this method for silicon and germanium.
Fractional distillation, on the other hand, is a separation technique used for separating a mixture of liquids having different boiling points. It is based on the principle that different liquids boil and condense at different temperatures. This method is widely used in petroleum refining, chemical industries, and laboratories.
In fractional distillation, the liquid mixture is heated in a distillation flask. The vapours rise through a fractionating column, which provides repeated condensation and vaporisation. Components with lower boiling points rise higher in the column and condense first, while those with higher boiling points condense lower down. This leads to effective separation of components based on their boiling points.
A classic example of fractional distillation is the refining of crude oil into petrol, diesel, kerosene, LPG, and other fractions. It is also used for separating mixtures such as ethanol and water, liquid air into nitrogen and oxygen, and various organic compounds.
For exams, it is crucial to note that fractional distillation is used for liquids, requires a fractionating column, and works efficiently when the difference in boiling points of components is small. Questions frequently test the principle, apparatus used, and industrial applications of fractional distillation.
In comparison, zone refining is a purification method for solids requiring extremely high purity, while fractional distillation is a separation technique for liquid mixtures. Zone refining depends on solubility of impurities in molten metal, whereas fractional distillation depends on boiling point differences.
In conclusion, zone refining and fractional distillation are conceptually different but equally important techniques in chemistry. Zone refining plays a critical role in modern electronics by providing ultra-pure materials, while fractional distillation forms the backbone of petroleum and chemical industries. Both topics are high-yield for competitive exams and should be prepared with clarity on principles, processes, and applications.
Zone refining is mainly used for the purification of
A. Liquids with close boiling points
B. Gases at low temperature
C. Ultra-pure metals and semiconductors
D. Alloys
Correct Answer: CThe basic principle of zone refining is based on
A. Difference in boiling points
B. Difference in densities
C. Greater solubility of impurities in molten state
D. Electrolysis
Correct Answer: CWhich of the following metals is commonly purified by zone refining?
A. Iron
B. Copper
C. Silicon
D. Aluminium
Correct Answer: CIn zone refining, impurities finally collect at
A. The middle of the rod
B. Both ends of the rod
C. The end of the rod
D. The surface of the rod
Correct Answer: CThe purity of metal obtained by zone refining can be up to
A. 99%
B. 99.9%
C. 99.99%
D. 99.9999%
Correct Answer: DFractional distillation is mainly used for separating
A. Solid–solid mixtures
B. Solid–liquid mixtures
C. Liquid–liquid mixtures with close boiling points
D. Gas–solid mixtures
Correct Answer: CThe apparatus essential for fractional distillation is
A. Condenser only
B. Separating funnel
C. Fractionating column
D. Desiccator
Correct Answer: CFractional distillation of crude oil is carried out based on the difference in
A. Density of hydrocarbons
B. Solubility in water
C. Molecular weight only
D. Boiling points of components
Correct Answer: DWhich of the following is an application of fractional distillation?
A. Purification of copper
B. Separation of silicon
C. Refining of petroleum
D. Electroplating
Correct Answer: CWhich statement is correct?
A. Zone refining is used for liquids
B. Fractional distillation is used for solids
C. Zone refining is important in semiconductor industry
D. Fractional distillation gives ultra-pure solids
Correct Answer: C