6.4 Explain: (i) Zone refining (ii) Column chromatography.

<p><strong>Zone refining </strong>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 one end. This results in the formation of the molten zone or melt. 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.</p><p><strong>Column chromatography&nbsp;</strong>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).</p><p>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.</p>

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