Food cans are coated with Tin instead of Zinc, because
- (a)Zinc is more expensive than Tin.
- (b)The melting point of Zinc is higher than that of Tin.
- (c)Zinc is more reactive than Tin.
- (d)Zinc is less reactive than Tin.
Correct — C, Zinc is more reactive than Tin.
Zinc is more reactive than Tin because zinc sits above iron in the reactivity series while tin sits below it. A zinc lining would be attacked by the weak acids of fruit, tomato and vegetable preparations and would pass into the food as soluble zinc salts.
Tin is far slower to react with those dilute food acids. A tinned surface therefore stays largely unchanged by the contents and keeps the steel underneath from touching them.
Carry away the reasoning rather than the pair of metals: a protective coating is chosen for how it behaves towards whatever it will be in contact with.
- (a)Zinc is more expensive than Tin. — The comparison runs the other way. Tin trades at several times zinc's price per tonne on the international metal exchanges, so tinning is the costlier coating and cost would have argued for a zinc lining, not against it.
Price does decide metal choices elsewhere: zinc being the cheap one is part of why galvanised steel is the ordinary material for roofing sheets, buckets and fencing. Here the cheaper metal is the one being rejected.
- (b)The melting point of Zinc is higher than that of Tin. — The statement is true — zinc melts near 420 °C and tin near 232 °C. But the stem asks for a reason, and a true fact that explains nothing about the choice is still the wrong option.
Tin's low melting point is what earns it a place in soft solder alloys and in historic hot-dip tinning. It says nothing about whether the lining will dissolve into the contents of the can.
- (d)Zinc is less reactive than Tin. — This inverts the series. Zinc lies above iron and tin below it, so zinc is the more reactive of the two; were the order as this option states, galvanising would not protect iron the way it does.
The same sentence holds for metals that sit below tin — copper and silver are less reactive than tin — but zinc sits on the other side of it.
Metals can be ranked by how readily they give up electrons and enter into reaction. In that ranking zinc lies above iron and tin lies below it, which means zinc corrodes more easily than tin does.
When steel is protected by coating it, the coating metal is picked for the environment it will meet. Against weather and soil, a metal that corrodes in place of the steel is an advantage.
Against food it is a hazard, because whatever dissolves off the coating ends up in what people eat. That single difference decides which of the two metals lines a can.
Steel is the cheap structural metal and it rusts, so most of its everyday uses depend on a coating. Two coatings dominate, and they are picked for opposite reasons.
Galvanising puts zinc on steel for outdoor exposure, where zinc corroding first is exactly what is wanted. Tinning puts tin on steel for cans, where the point is a surface the contents leave alone.
The question is food safety expressed as chemistry: the metal that touches the food should be the one less willing to dissolve into it.
- In the common reactivity series zinc lies above iron and tin lies below it, making zinc the more reactive of the two metals.
- Zinc reacts with dilute acids, including the weak organic acids of fruit and vegetable foods, forming soluble zinc salts.
- Tinplate — thin steel sheet carrying a coating of tin — is the standard material for food cans.
- Galvanising is the coating of iron or steel with zinc, used for roofing sheets, buckets, fencing and structural steel.
- In galvanised steel the zinc corrodes in preference to the iron, which is the basis of sacrificial or cathodic protection.
- Zinc melts at about 420 °C and tin at about 232 °C, so zinc's melting point is the higher of the two.
- Tin costs several times more per tonne than zinc on the international metal exchanges.
- Food cans commonly carry an inner organic lacquer over the tin as a further barrier between metal and contents.
Both coatings go onto ordinary steel. The two highlighted rows are the ones that decide which coating may touch food; the rest describe the same pair of metals without answering the stem.
- A true statement is not automatically a reason. The melting-point option is factually correct and still fails, because the stem asks why tin rather than zinc is used.
- The same fact points both ways. Zinc's greater reactivity is why it is chosen for galvanising and why it is rejected for cans; working out which use is in front of you is the whole task.
- Reversing the series. Tin sits below iron and zinc above it, and carrying that order the wrong way round turns the keyed option into its opposite.
- Arguing from price without checking the direction. Tin is the dearer of the two metals, so a cost argument would have favoured the zinc lining.
- Treating better protection of the steel as the goal. For a can the requirement is a lining that stays out of the contents, which is a different test from keeping the steel intact.
Here the idea arrives as a 'because' item, where the options are all properties of the two metals and only one of them is the reason for the choice.
The same idea can be put in reverse — why zinc is used to galvanise iron — where corroding in place of the steel is the point, or reduced to plain ordering: which of these metals is more reactive than iron.
It can also be turned into application matching, pairing tinplate with food cans and galvanised iron with roofing, buckets and fencing.
No eligible match among this question's citable set.
- practice — not a real PYQ
The process of coating iron or steel with a layer of zinc is known as
- (a)Tinning
- (b)Galvanising
- (c)Annealing
- (d)Anodising
Answerb — Galvanising is by definition the application of a zinc coating to iron or steel.Tinning is the same idea carried out with tin, and is what produces tinplate. Annealing is a heat treatment that softens a metal and relieves internal stress. Anodising thickens the natural oxide film on a metal such as aluminium by electrolysis, and involves no zinc.
- practice — not a real PYQ
Which one of the following metals is more reactive than iron?
- (a)Tin
- (b)Copper
- (c)Zinc
- (d)Silver
Answerc — Zinc lies above iron in the reactivity series, which is why it can corrode in place of iron in galvanised steel.Tin, copper and silver all lie below iron. Tin's position below iron is the reason it is the metal used on food cans, and copper and silver lie lower still, which is why they resist ordinary corrosion well.
- practice — not a real PYQ
Galvanised iron resists rusting even where the coating has been scratched, mainly because
- (a)the zinc layer melts and reseals the scratch
- (b)zinc corrodes in preference to the exposed iron
- (c)zinc is chemically inert
- (d)the scratch fills with iron oxide, which is impermeable
Answerb — Zinc is the more reactive metal, so at a break in the coating it is the zinc that is attacked and the exposed iron is spared. This is sacrificial, or cathodic, protection.Zinc stays solid at everyday temperatures, so (a) describes nothing that happens. Zinc is far from inert — its readiness to react is the whole point — so (c) contradicts the mechanism. Iron oxide is flaky and porous, which is why rusting continues beneath it rather than stopping, ruling out (d).
- practice — not a real PYQ
Tinplate, the material of an ordinary food can, is
- (a)a sheet of tin coated with steel
- (b)a steel sheet coated with a thin layer of tin
- (c)an alloy of tin and zinc
- (d)a sheet of zinc coated with tin
Answerb — Tinplate is thin steel sheet carrying a thin coating of tin. The steel supplies strength at low cost; the tin supplies a surface the contents leave alone.Option (a) reverses the two layers, which would put the cheap structural metal on the food side. Options (c) and (d) bring zinc into the sheet, whereas the coating in a food can is tin, chosen precisely because it is the less reactive metal.