(i) Manoj has studied that certain metals like iron react with air or water in the environment to form undesirable compounds (usually oxides) and the process is called corrosion. In case of iron, corrosion is called rusting. Therefore, the normal iron hinges will rust after certain time. Manoj has also learnt that iron is protected from rusting by galvanisation. The galvanised iron articles have coating of zinc on their surfaces, which protects them from rusting. If some scratches occur on the protective film on coated iron, even then iron will not be rusted. This is due to the fact that because of scratches, both zinc and iron get exposed to oxidation but zinc undergoes oxidation in preference to iron. Thus, galvanised iron hinges will have longer life and lustre.
(ii) Rusting is a phenomenon which damages the iron articles. It destroys iron bridges, buildings, automobiles etc. and we must prevent rusting of iron at any cost. So, Manoj gave a good suggestion to his father to buy galvanised iron hinges though these are costlier. In longer term, these will prove to be cheaper.

(a) As a student of chemistry, we would prefer to buy the battery with cadmium plates, though it is costlier than the battery with lead plates. This is because
(i) Lead battery will result into harmful effects on human beings as well as on the environment because it causes environmental pollution.
(ii) A cadmium plated battery will have sustained long life benefit than short term gains with regard to health and money.
(b) Knowledge is useful only when put to practice; awareness of a lead-free petrol-legal aspect of pollution free environment. Less usage of lead batteries to keep ourselves and the environment healthy.

Many food products are available in the markets in tin cans, which are made from steel with a very thin coating of tin. the thin layer, which is only a few thousands of a millimeter thick, prevents the steel from rusting. If the coating gets scratched, the can will soon corrode. This will allow bacteria from the air to cause food poisoning. Thus, the tin can with scratches have chances of getting spoiled. Hence, the decision of Jayant is justified.