UPPSC 2019 Prelims GS Paper-I · Question 26 of 150

Correct — A, Sodium chlorate. Sodium chlorate (NaClO3) is a classic non-selective (total) herbicide or weed-killer: it is taken up by roots and foliage and kills essentially every green plant it reaches, which is why for decades it was used to clear vegetation from railway tracks, industrial yards, drains and firebreaks rather than from a standing crop. It also leaves a soil residue, so treated ground stays bare for months. Its other properties — it is a powerful oxidising salt, and chlorate-soaked dry vegetation catches fire very easily — are what eventually made it unpopular; several jurisdictions, including the European Union, have withdrawn its approval as a plant-protection product. But the use the question is testing is unambiguous: sodium chlorate is a weed-killer, and neither of the other two named chemicals is.

• (b)Potassium permanganate — Potassium permanganate (KMnO4, 'Condy's crystals') is a strong oxidising agent used as a disinfectant and antiseptic, in water treatment (to remove iron and manganese and control taste and odour) and as a laboratory reagent. It is not used to kill weeds.
• (c)Bleaching powder — Bleaching powder (calcium hypochlorite / chloride of lime) is a bleaching and disinfecting agent — it chlorinates drinking water and bleaches textiles and paper pulp. Killing weeds is not among its uses.
• (d)None of the above — This would only be right if no listed chemical were a herbicide. Sodium chlorate genuinely is one — a well-documented non-selective herbicide — so option (d) is ruled out.

'Pesticide' is the umbrella term for chemicals used to kill unwanted organisms, and it is divided by target: herbicides (weedicides) kill plants, insecticides kill insects, fungicides kill fungi, rodenticides kill rats, nematicides kill nematodes. Herbicides are further classed as selective (kill only certain plants — 2,4-D against broad-leaved weeds, atrazine in maize) or non-selective/total (kill all vegetation — glyphosate, paraquat, and the inorganic sodium chlorate), and as contact (kill only the tissue they touch) or systemic (translocated inside the plant). Sodium chlorate is the standard textbook example of an inorganic, non-selective, soil-acting herbicide.

The trap is that all three named chemicals are strong oxidising agents, so a candidate who reasons from chemistry alone cannot separate them. Separate them by USE instead: potassium permanganate and bleaching powder both belong to the sanitation shelf (disinfecting water, bleaching cloth), while sodium chlorate belongs to the weed-control shelf. The second trap is option (d): an unfamiliar-looking name tempts candidates to declare that none of the options fits.

• Sodium chlorate (NaClO3) is a non-selective herbicide and defoliant that kills all green vegetation and leaves a residual soil effect; industrially it is also the feedstock for chlorine dioxide used to bleach paper pulp.
• Herbicides are one branch of the pesticide family: herbicide (plants), insecticide (insects), fungicide (fungi), rodenticide (rodents), nematicide (nematodes).
• Selective herbicides such as 2,4-D spare the crop and kill broad-leaved weeds; non-selective ones such as glyphosate, paraquat and sodium chlorate kill whatever they touch.
• Potassium permanganate = disinfectant/oxidising agent; bleaching powder (calcium hypochlorite) = disinfectant and bleach — neither is a weed-killer.
• In India the import, manufacture, sale, transport and use of pesticides, herbicides included, is regulated under the Insecticides Act, 1968.

• Assuming that any strong oxidising chemical must be an agricultural chemical — potassium permanganate and bleaching powder are disinfectants, not weed-killers.
• Using 'pesticide' as if it meant only insecticide; herbicides are pesticides too.
• Choosing 'None of the above' merely because the correct chemical's name is unfamiliar.

UPPSC asks the plain one-line identification ('which of these is a herbicide / insecticide / fungicide') or pairs a chemical with its use; UPSC prefers the applied angle — pesticide residues, herbicide-tolerant crops, neem-coated urea, banned pesticides.

• practice — not a real PYQ

  Which one of the following is a non-selective (broad-spectrum) herbicide?

  • (a)2,4-D
  • (b)Glyphosate
  • (c)Malathion
  • (d)Mancozeb

  Answer(b) Glyphosate — a non-selective systemic herbicide that kills most green plants; 2,4-D is selective for broad-leaved weeds, malathion is an insecticide and mancozeb a fungicide.

• practice — not a real PYQ

  In India, the import, manufacture, sale and use of pesticides — including herbicides — are regulated primarily under which Act?

  • (a)Insecticides Act, 1968
  • (b)Environment (Protection) Act, 1986
  • (c)Factories Act, 1948
  • (d)Food Safety and Standards Act, 2006

  Answer(a) Insecticides Act, 1968 — its definition of 'insecticide' is wide enough to cover the whole pesticide family, herbicides included, and registration is done by the Central Insecticides Board & Registration Committee.