CDS (II) 2022 General Knowledge · Question 11 of 120

Correct — A, Methanoic acid. The stem is very nearly a sentence out of the NCERT Class 10 chapter on acids, bases and salts, which says that the 'stinging hair of nettle leaves inject methanoic acid causing burning pain'. Methanoic acid is the systematic name for formic acid, HCOOH, the simplest carboxylic acid, and the same textbook's table of natural acids lists it against both the ant sting and the nettle sting — the Latin formica, ant, is where the older name comes from. The traditional field remedy given alongside it is to rub the area with a dock leaf, a plant that grows beside nettles and whose juice is basic enough to take the edge off the acid.

• (b)Tartaric acid — The acid of tamarind, and of the cream of tartar used in baking powder. It is a solid dicarboxylic acid found in fruit, not an injectable irritant.
• (c)Hydrochloric acid — A mineral acid, and the one the human stomach secretes. Plants do not make it; the acids of plant and animal origin in this chapter are all organic.
• (d)Sulphuric acid — An industrial mineral acid. At any concentration capable of causing the described sting it would burn the skin chemically rather than produce the temporary irritation a nettle causes.

Acids come from two very different sources in this chapter. Mineral acids — hydrochloric, sulphuric, nitric — are made industrially and are strong, meaning they ionise almost completely in water. Organic acids come from living things and are weak: methanoic acid from ants and nettles, ethanoic acid from vinegar, citric from citrus, tartaric from tamarind, oxalic from tomato, lactic from sour milk. A weak acid still hurts on contact with nerve endings, which is what a nettle exploits.

The nettle's stinging hair is a fine silica-tipped tube. Brushing against it snaps the tip, turning the hair into a hypodermic needle that drives fluid under the skin, and the acid then irritates the nerve endings. Two hooks make this item easy to keep. The first is the ant, because the same acid appears in both places and CDS has asked the ant version separately. The second is the rule that the three named mineral acids in the option list are never the answer to a question about a plant or an insect. It is worth noticing how literally CDS lifts these stems: this one, the milk of magnesia item next to it and the school bell item a few questions later are all close paraphrases of NCERT Class 10 sentences.

• Methanoic acid is the systematic name for formic acid, HCOOH, the simplest carboxylic acid.
• NCERT lists methanoic acid as the acid of both the ant sting and the nettle sting.
• Tartaric acid comes from tamarind, oxalic acid from tomato, citric acid from citrus fruits and lactic acid from sour milk.
• Hydrochloric acid is the mineral acid secreted by the gastric glands of the human stomach.
• The traditional relief for a nettle sting is rubbing the area with a dock leaf, which grows beside nettles in the wild.

• Picking a mineral acid for a biological sting; the acids of plants and insects in this syllabus are organic.
• Mixing up tartaric acid with tamarind's other associations, or oxalic with tomato.
• Assuming a bee sting is the same as an ant sting — a bee sting is treated with a mild base such as baking soda.

As a one-liner naming the sting and asking for the acid, as a match of acid to natural source, or the other way round, asking where methanoic acid is found.

• practice — not a real PYQ

  Methanoic acid is another name for which one of the following?

  • (a)Acetic acid
  • (b)Formic acid
  • (c)Citric acid
  • (d)Lactic acid

  Answer(b) Formic acid — methanoic acid, HCOOH, is the simplest carboxylic acid, named after formica, the Latin for ant.

• practice — not a real PYQ

  Tartaric acid is naturally found in which one of the following?

  • (a)Tomato
  • (b)Tamarind
  • (c)Curd
  • (d)Vinegar

  Answer(b) Tamarind — tomato gives oxalic acid, curd lactic acid and vinegar ethanoic acid.