The solubilities of the impurities in the melt and solid states is not same. This principle is applied during the extraction of boron. This method is known as
(a) Poling method This is a method employed in the purification of metal, which contains its metal oxide as an impurity. Copper and tin can be purified by this way, which contains copper oxides and tin oxides as impurities. (b) Zone refining method The principle of zone refining is that the impurities in an ingot or ore of metal are more soluble in the melt state, when compared to the corresponding solid state of the impurities. (c) Liquation method This method is based on the lower melting point of the metal than the impurities and tendency of the molten metal to flow on the sloping surface. (d) van-Arkel's method This method is based on the thermal decomposition of metal components.