Indium is mainly refined by:

The correct answer is Option (3) → Zone refining

Zone refining rests on the fact that impurities are more soluble in the melt than in the solid metal; it is used for metals needed in very high purity, such as indium.

Why the other options are incorrect

• Option (1) Vapour phase refining — Incorrect. It needs a volatile compound that decomposes easily, as in the Mond process for nickel and the van Arkel method for zirconium and titanium.

• Option (2) Distillation — Incorrect. It is used for low boiling metals such as zinc and mercury.

• Option (4) Liquation — Incorrect. It is used for low melting metals such as tin, which are made to flow away from higher melting impurities.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Zone Refining – “This method is based on the principle that the impurities are more soluble in the melt than in the solid state of the metal. A circular mobile heater is fixed at one end of a rod of the impure metal (Fig. 6.7). The molten zone moves along with the heater which is moved forward. As the heater moves forward, the pure metal crystallises out of the melt and the impurities pass on into the adjacent molten zone. The process is repeated several times and the heater is moved in the same direction. At one end, impurities get concentrated. This end is cut off. This method is very useful for producing semiconductor and other metals of very high purity, e.g., germanium, silicon, boron, gallium and indium.”