Q.Explain:

Zone refining: a mobile heater melts a narrow zone of an impure metal rod; as the heater moves along, pure metal crystallises behind it while impurities (which prefer the liquid state) are swept forward, concentrating impurities at one end which is then cut off. …

1. Zone refining Step 1 – Principle: Zone refining is based on the fact that impurities are more soluble in the molten (liquid) state of a metal than in its solid state. Step 2 – Method: A circular mobile heater is fitted around one end of a rod of impure metal, melting a thin zone. The heater is moved slowly along the length of the rod. As it advances, the molten zone moves with it: pure metal crystallises out and solidifies just behind the heater (since the solid rejects impurities), while the impurities remain dissolved in the moving molten zone and are carried forward. Step 3 – Result: Repeating this pass (or using multiple heaters) several times sweeps virtually all the impurity to one end of the rod, which is then cut off and discarded, leaving an extremely pure metal rod behind. Step 4 – Use: This technique is used to obtain ultra-pure semiconductor-grade elements such as germanium and silicon.
2. Column chromatography Step 1 – Principle: Based on the principle that different components of a mixture are adsorbed to different extents on a common adsorbent (stationary phase, e.g. alumina or silica gel). Step 2 – Method: The mixture is loaded at the top of a column packed with the adsorbent. A suitable solvent (the eluant, mobile phase) is passed continuously through the column. Components with weaker adsorption affinity for the stationary phase move down the column faster (are eluted sooner), while strongly adsorbed components move slower, so the mixture separates into distinct bands/fractions as it travels down the column. …

State the underlying physical principle for each method (solubility difference between solid/liquid states for zone refining; differential adsorption for chroma …

• Mixing up the two methods' underlying principles (e.g. describing zone refining in terms of adsorption).
• Omitting the key phrase 'impurities are more soluble in the melt than in the solid' for zone refining, or 'differential …