BPSC 70th CCE Prelims General Studies · Question 82 of 143

Correct — C, Thermo setting plastic. Plastics divide into two families according to what heat does to them once they have been moulded. A THERMOPLASTIC has long polymer chains that lie alongside one another with only weak forces between them; heat lets the chains slide, so the material softens, can be reshaped, and hardens again on cooling — and it can be put through that cycle repeatedly. A THERMOSETTING plastic is different in structure: during moulding the chains cross-link into a single three-dimensional network held by covalent bonds, and once that network has formed heat cannot undo it. The article can only char or burn; it can never be remelted and remoulded. Melamine belongs to the second family. What is sold as 'melamine' is melamine-formaldehyde resin, made by condensing melamine (C3H6N6, a triazine ring carrying three amino groups) with formaldehyde, and it cures into exactly such a cross-linked network. That structure gives it the properties the question alludes to: it is hard, scratch-resistant, resists fire and tolerates heat better than most plastics, which is why the NCERT science textbook lists its uses as floor tiles, kitchenware and fabrics that resist fire. Note the wording of the option carefully. The examiner is not asking what melamine is chemically; he is asking which CLASS of plastic it exemplifies, and 'thermo setting plastic' is that class. The three other options are all on the wrong side of the divide — one is the opposite class, and the other two are named members of it.

• (a)Thermoplastics — The exact opposite class, and the option that will collect most of the wrong marks because the two words look almost alike on a printed page under time pressure. A thermoplastic softens on heating and can be remoulded again and again — polythene, PVC, polystyrene, nylon. Melamine cannot be remoulded at all once cured.
• (b)Polyethene — Polythene is a specific thermoplastic — the polymer of ethene, used for carry bags, bottles, pipes and packaging film. It is soft, easily melted and reshaped, and would deform under a hot pan, which is exactly why it is not the material of a floor tile or a dinner plate.
• (d)PVC — Polyvinyl chloride is another named thermoplastic, used in pipes, cable insulation, flooring sheet and window frames. It appears here because PVC really is used in flooring, so a candidate reading only the words 'floor tiles' may reach for it — but the question is about the class melamine belongs to, and PVC softens on heating while melamine does not.

Polymers are long molecules built by repeating a small unit, and their bulk behaviour follows from how those long molecules are arranged relative to each other. Linear or lightly branched chains held together by weak intermolecular forces give a thermoplastic: heat overcomes those weak forces, the material flows, and it can be extruded, injection-moulded and recycled. Polythene, PVC, polystyrene, PET and nylon are all in this family, and it is the family that dominates packaging precisely because it can be melted and reformed. Cross-linked chains give a thermoset: the moulding step itself is an irreversible chemical reaction, so the finished article is a single giant molecule that cannot be melted. Bakelite — phenol-formaldehyde — was the first commercial plastic of this kind, and is used for electrical switches and the handles of cookware because it is a poor conductor of heat and electricity; melamine-formaldehyde is the other standard school example, chosen where hardness, heat resistance and a wipeable surface matter. The practical consequences run both ways: thermosets are more durable and heat-resistant but are far harder to recycle, since they cannot be melted down and reformed.

Two habits solve every question of this shape. First, read what is being asked for — a class or a substance. Here the stem names a substance, melamine, and the correct option names a class, so options that name other substances (polythene, PVC) are answering a different question and can be discarded on that ground alone before any chemistry is applied. Second, use the kitchen test for the class itself: if you can imagine the object deforming when something hot is put on it, it is a thermoplastic; if it would char rather than sag, it is a thermoset. A melamine dinner plate does not soften under hot food, and a floor tile does not soften in a warm room, so melamine sits with bakelite among the thermosets. One further association is worth carrying, because it appears in current-affairs questions rather than chemistry ones: melamine is nitrogen-rich, which is why it was criminally added to milk and infant formula in the 2008 Chinese adulteration scandal — standard protein tests measure nitrogen and were fooled by it. That is the same compound, in a completely different context.

• Melamine used in tiles and kitchenware is melamine-formaldehyde resin — a cross-linked thermosetting polymer that cannot be softened or remoulded once cured
• Thermoplastics soften on heating and can be reshaped repeatedly: polythene, PVC, polystyrene, nylon and PET are the standard examples
• Bakelite (phenol-formaldehyde) is the other standard thermoset, used for electrical switches and cookware handles because it conducts neither heat nor electricity well
• NCERT lists melamine's uses as floor tiles, kitchenware and fire-resistant fabrics, noting that it resists fire and tolerates heat better than other plastics
• Thermosets are harder to recycle than thermoplastics precisely because they cannot be melted down; melamine itself is C3H6N6, a nitrogen-rich triazine, the property exploited in the 2008 milk adulteration scandal

• Misreading 'thermosetting' as 'thermoplastic' under time pressure; the two words differ by a few letters and sit next to each other as options here
• Choosing PVC because PVC is genuinely used in flooring — the question asks which class melamine belongs to, not what else floors are made of
• Assuming thermosets are 'stronger plastics' in general; the defining property is irreversible cross-linking, not strength

BPSC asks polymers as a one-line classification with a named material in the stem, and mixes classes and substances among the options so that a candidate who does not read carefully picks a plausible substance instead of the class. UPSC prefers multi-statement questions about a single named polymer — whether PET fibres can be blended with cotton, whether its bottles can be recycled, whether it can be incinerated cleanly — testing the properties and the environmental consequences rather than the textbook category.

• practice — not a real PYQ

  Bakelite, used for electrical switches and the handles of cooking utensils, is an example of

  • (a)a thermoplastic
  • (b)a thermosetting plastic
  • (c)a natural polymer
  • (d)an elastomer

  Answer(b) a thermosetting plastic — phenol-formaldehyde resin, cross-linked and non-remouldable, and a poor conductor of both heat and electricity.

• practice — not a real PYQ

  Which of the following plastics can be softened repeatedly by heating and reshaped ?

  • (a)Melamine-formaldehyde
  • (b)Bakelite
  • (c)Polyvinyl chloride
  • (d)Vulcanised rubber

  Answer(c) Polyvinyl chloride — a thermoplastic, so it softens on heating and can be remoulded; melamine, bakelite and vulcanised rubber are all cross-linked and set irreversibly.