NCERT Solutions
Class 12 Chemistry
General Principles and Processes of Isolation of Elements

Q.4
Explain: (i) Zone refining
(ii) Column chromatography.
(i) Zone refining
Zone refining method is based on the principle that impurities are more
soluble in the molten state of metal (the melt) than in the solid state. The
impure metal is heated with the help of a circular mobile heater at o ne end.
This results in the formation of the molten zone or melts. As the heater is
removed along with the length of the rod, the pure metal crystallizes out of
the melt and impurities pass into the adjacent molten zone. This process is
repeated several times till the impurities are completely driven to the end of
the rod which is then cut off and discarded. The method is very useful for
semiconductor and other metals of very high purity, e.g., silicon, germanium,
boron, and gallium.
(ii) Column chromatography
It is based on the principle that different components of a mixture are
differently adsorbed on an adsorbent. The mixture to be separated is put in a
liquid or gaseous medium which is moved through the adsorbent. Different
components are adsorbed at different levels on the column. Later, the
adsorbed components are removed (eluted) by using suitable solvents
(eluents). There are several chromatographic techniques such as paper
chromatography, column chromatography, gas chromatography, etc.
chromatography is used for the purification of elements which are available
in minute quantities and the impurities are not very different in chemical
properties from the element to be purified.