Which of the following is an example of herbicide ?

Question

UnlockIAS · Accepted Answer

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fire very easily — are what eventually made it unpopular; several jurisdictions, including the European Union, have withdrawn its approval as a plant-protection product.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.

Why the other options are wrong — (b) 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 (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) 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.

Concept: '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.

State PCS 2019 Q26 — Science & Technology PYQ with Answer

State PCS 2019 Q26 — Science & Technology PYQ with Answer