Hostel kitchen audit: five changes, five different explanations
A student science club audits storage in the school hostel kitchen and records five observations. (i) The iron tawa hanging near an open window has reddish-brown flaky patches, and the patches are worst on the side facing the window. (ii) A sealed packet of potato chips three months past its date puffs out a sharp, oily, unpleasant smell when opened; the packet was flushed with nitrogen when packed. (iii) A silver spoon kept in a cupboard adjoining the bathroom has turned black. (iv) A copper-bottomed pan stored beside a crate of lemons has developed green patches. (v) A piece of aluminium foil wrapped around a cut lemon overnight has become thin with tiny holes; when a fresh piece of foil is dropped into a test tube of lemon juice, small bubbles collect, and a burning splinter held at the mouth of the tube gives a small pop. The club must explain each observation and recommend one action per item.
1. Name or describe the chemical change in each of the five observations, and identify the gas produced in (v) together with the reaction type.
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(i) Rusting - iron reacts with both oxygen and moisture to form hydrated iron(III) oxide, Fe2O3·xH2O, which is flaky and reddish-brown. The window side is worse because both air movement and dampness are greater there; rusting needs both oxygen and water.
(ii) Rancidity - the fats and oils in the chips are oxidised by atmospheric oxygen, producing aldehydes and short-chain fatty acids with a sharp smell and changed taste.
(iii) Tarnishing (corrosion) - silver reacts with hydrogen sulphide in the air (bathrooms are a source) to form black silver sulphide, Ag2S: 2Ag + H2S → Ag2S + H2.
(iv) Corrosion of copper - the metal reacts with oxygen, moisture and carbon dioxide to form green basic copper carbonate, CuCO3·Cu(OH)2.
(v) Displacement / metal–acid reaction. Citric acid in lemon juice reacts with aluminium: 2Al + 6HCl-type acid → 2Al³⁺ salt + 3H2 (in practice with citric acid the same gas is released). The gas is hydrogen. It burns with a small pop when a lighted splinter is brought to the mouth of the tube - the standard test for hydrogen. The foil thins and pits because aluminium is being consumed.
2. Why is the chips packet flushed with nitrogen, and why does keeping the same packet in a refrigerator slow the spoilage even after opening?
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Nitrogen is chemically inert under these conditions - it does not react with the fats. Filling the packet with nitrogen displaces oxygen, so the oxidation of the oils cannot proceed. Removing the reactant (oxygen) removes the cause of rancidity. Oxygen absorbers and small amounts of antioxidants added to the oil work on the same principle: prevent or delay the oxidation of fats.
Refrigeration works differently - it does not remove oxygen, it slows the rate of the reaction. The rate of oxidation roughly doubles for every 10 °C rise in temperature, so cooling the packet can cut the rate several-fold. Cold also slows the growth of moulds and bacteria that cause separate spoilage.
So the two measures act at different points: nitrogen removes a reactant, refrigeration reduces the reaction rate. Neither makes the food last forever - once the packet is opened and air enters, oxidation resumes, and refrigeration only buys time. This is the reasoning a candidate should give rather than simply listing 'nitrogen is inert'.
3. Recommend one protective measure for the tawa and one for the silver spoon, and justify each in terms of what it actually prevents.
Read the worked answer
Tawa: coat it with a thin film of oil after washing and drying, or have it painted/enamelled/greased. The coating is a barrier that keeps oxygen and moisture away from the iron surface. Since rusting requires the simultaneous presence of both oxygen and water, blocking either one stops the reaction. Galvanising (a zinc coating) works the same way and has an added advantage: zinc is more reactive than iron, so it corrodes first and protects the iron even where the coat is scratched. Keeping the tawa away from the open window and drying it after washing are supporting measures.
Silver spoon: store it in an airtight, dry container, lined with cloth, away from the bathroom and from rubber, which releases sulphur compounds. The aim is to keep hydrogen sulphide and moisture away from the silver surface, since Ag2S is what forms the black layer. A tightly closed box limits the supply of the reactant gas. Polishing with a mild abrasive removes the layer already formed, but it also removes a little silver each time, so prevention is better than repeated cleaning.
The general principle to state: protective measures either block contact between the metal and the corroding agents or sacrifice a more reactive metal in its place.