Q.Outline the principles of refining of metals by the following methods:

1. Zone refining: a moving molten zone along a metal rod leaves pure solid behind and carries impurities forward.
2. Electrolytic refining: impure metal = anode, pure metal deposits at cathode from a solution of the same metal's salt; more reactive impurities dissolve into solution, less reactive ones fall as anode mud. …

1. Zone refining Principle: impurities present in a metal are more soluble in its molten (liquid) state than in its solid state. Method: a mobile circular heater melts a thin zone at one end of an impure metal rod and is moved slowly along its length. As the heater advances, pure metal solidifies (crystallises) just behind it, while impurities remain preferentially dissolved in the moving molten zone and are carried forward. Repeating the pass (or using several heaters in series) concentrates virtually all impurities at one end of the rod, which is then cut off. Use: obtaining ultra-pure semiconductor elements such as germanium and silicon.
2. Electrolytic refining Principle: the impure metal is made the anode of an electrolytic cell; on passing current, it dissolves anodically into an electrolyte containing a soluble salt of the same metal, while an equal amount of pure metal is deposited at the cathode. Method: a thick block of impure metal is the anode, a thin sheet of pure metal is the cathode, and the electrolyte is an appropriate salt solution of the metal (e.g. acidified $CuSO_4$ for copper). Impurities more reactive than the metal being refined dissolve into solution along with it but do not deposit at the cathode (since the target metal deposits preferentially); impurities less reactive (more noble) than the metal do not dissolve at all and simply fall to the bottom of the cell as anode mud. Use: refining of copper, silver, gold, zinc, aluminium, nickel, etc.
3. Vapour phase refining Principle: the impure metal is converted into a volatile compound by reaction with a suitable reagent; this volatile compound is separated from the (non-volatile) impurities and then decomposed at a different temperature to deposit the pure metal, regenerating the reagent. Requirements: (a) the metal must form a volatile compound with an available reagent, and (b) that compound must be readily/easily decomposable to give back the pure metal. Examples:

• Mond process for nickel: $Ni + 4CO \xrightarrow{330-350\ K} Ni(CO)_4 \xrightarrow{450-470\ K} Ni + 4CO$ …

For each method, state (a) the underlying physical/chemical principle, (b) a brief description of the apparatus/procedure, and (c) a standard example/application — this thre …

• Only naming the method/application without the actual underlying PRINCIPLE (e.g. saying 'zone refining purifies germanium' without explaining the solubility-difference principle that makes it work). …