The zone refining method is generally used to refine metalloids and ultra-pure metal is obtained. This is based on the idea that impurities are more soluble in the molten state of metal (the melt) than in the solid form. A circular mobile heater is placed at one end of an impure metal rod during the zone refining process. The molten zone of the rod moves along with the heater as it moves. As a result, pure metal crystallizes from the melt, while impurities transfer to the molten zone next to it. Hence gallium is a metalloid so gallium arsenide is purified by zone-refining.