Why oil is not stored in galvanized iron pot?

Question

UnlockIAS · Accepted Answer

Correct — D, All of above. Galvanised iron is ordinary iron or mild steel carrying a metallic zinc coat, usually laid down by hot-dipping the cleaned and pickled article in a bath of molten zinc at about 450 °C, which bonds a series of iron–zinc alloy layers metallurgically on to the base metal. Zinc is chosen because it is anodic to iron — the standard electrode potential of the Zn²⁺/Zn couple is −0.76 V against −0.44 V for Fe²⁺/Fe — so in any corrosion cell the zinc is consumed first and the iron underneath survives. That is sacrificial (cathodic) protection, and it is why GI sheets roof houses and GI buckets outlast plain iron ones. But the very reactivity that makes zinc a good sacrificial anode disqualifies it as a container for edible oil, because zinc is a chemically active metal and edible oil is not the inert medium it looks like. Stored oil slowly hydrolyses, so it always carries free fatty acids — the trade measures them as the acid value, in milligrams of KOH per gram — and free fatty acids are weak carboxylic acids. With the traces of moisture that oil also picks up, they attack the coating: Zn + 2RCOOH → (RCOO)₂Zn + H₂, yielding zinc soaps such as zinc stearate, zinc oleate and zinc palmitate. Those are precisely the 'toxic zinc salt' of the Commission's own published remark on this question, so option (a) is true. They dissolve and disperse into the oil, cloud it and taint it, so option (b) is true. And once the sacrificial coat has been locally eaten away the iron beneath is bared and rusts, while the iron ions released are classic pro-oxidant catalysts that accelerate the peroxidation of the oil into rancidity — which is the corrosion and discolouration the Commission's remark ends on, so option (c) is true as well. Three printed effects, not one of them false: the only complete option is (d).

Why the other options are wrong — (a) True, but only one-third of the story, so it cannot be the answer to a question that also offers 'All of above'. The zinc soaps formed are a genuine food-safety hazard — zinc is an essential trace metal, with a US Recommended Dietary Allowance of 11 mg a day for adult men and 8 mg for adult women, yet toxic effects such as nausea, cramps and vomiting are reported above roughly 50 mg in one oral dose, and sustained high intake induces copper deficiency. A candidate who stops at (a) has read the chemistry correctly and the option list carelessly.  (b) Also true and also incomplete. Metal contact degrades oil on two fronts: the zinc soaps themselves cloud it and impart a metallic off-taste, and the iron exposed after the coating is breached releases Fe²⁺/Fe³⁺, which decompose lipid hydroperoxides in Fenton-type reactions and so speed rancidity, off-odour and darkening. It is the option a food-technology student gravitates to, and it is exactly the trap the item is built on.  (c) True as well, and the one the Commission names last: its remark ends with 'corrosion and discolouration of the pot'. The acid attack strips zinc, exposes iron, and rust follows. The reason (c) fails as the answer is arithmetic, not chemical — it accounts for damage to the vessel but says nothing about the toxic salt or the spoilt oil, both of which the Commission also asserts.

Concept: A container is never chemically neutral; it is a reagent that happens to be shaped like a pot. Galvanising exists to solve one specific problem — atmospheric rusting of iron — and it solves it by putting a more reactive metal on the outside so that the more reactive metal corrodes instead. Zinc exposed to air forms zinc oxide, which then takes up carbon dioxide to form a dull grey zinc carbonate patina that seals the surface; the American Galvanizers Association puts the recommended maximum continuous service temperature of hot-dip galvanised steel at 200 °C, above which the zinc peels at the intermetallic layer. All of that protection is designed against air and rainwater. Nothing in it protects against an organic acid medium, and this is the general principle the question tests: a coating is protective only against the environment it was engineered for. Food-contact metals are therefore chosen for inertness rather than for strength or cheapness — tin-plated steel for cans, austenitic stainless steel of Type 304 (roughly 18% chromium and 8% nickel, which grows a passive chromium-oxide film), lacquered or anodised aluminium, or plain glass and glazed earthenware. Galvanised steel belongs to the other list — roofing sheets, water buckets, culverts, transmission towers — and Indian households have always stored oil in glass, stoneware or tinned vessels for exactly this reason.

State PCS 2025 Q136 — Science & Technology PYQ with Answer

State PCS 2025 Q136 — Science & Technology PYQ with Answer