CDS (I) 2022 General Knowledge · Question 9 of 120

Correct — C, ant stings. Methanoic acid, HCOOH, is the systematic name for what is far better known as formic acid, and the older name is the giveaway — it comes from formica, the Latin word for ant. It is the simplest carboxylic acid, with a single carbon carrying the −COOH group, and it occurs in most ants; wood ants of the genus Formica can spray it from the tip of the abdomen at prey or at anything disturbing the nest. The burning that follows an ant sting is that acid acting on the skin, which is why the traditional remedy is a mild base such as baking soda, applied to neutralise it. The same acid is carried in the stinging hairs of the nettle plant, and it is what those hairs inject on contact.

• (a)muscles — The acid that builds up in working muscle is lactic acid, produced when the cells respire anaerobically and cannot oxidise pyruvate for want of oxygen. It is a three-carbon hydroxy acid, quite unlike methanoic acid.
• (b)urine — Urine carries urea as its main nitrogenous waste, along with uric acid and traces of others. Methanoic acid is not among the substances the kidney is excreting.
• (d)human brain — Nothing in normal brain chemistry stores methanoic acid. It does appear in the body in one dangerous circumstance — methanol poisoning, in which methanol is metabolised to methanoic acid — but that is a toxic accident, not where the acid is 'normally found'.

Carboxylic acids are organic acids carrying the −COOH group, and the first two members of the family are the ones school chemistry meets by name: methanoic acid, HCOOH, with one carbon, and ethanoic acid, CH₃COOH, with two. Both are weak acids, meaning they ionise only partly in water, unlike the mineral acids. Several of them are known from where they occur in nature rather than from the laboratory — methanoic acid from ants, ethanoic acid from vinegar, lactic acid from sour milk and from exercising muscle, citric acid from lemons, oxalic acid from tomatoes and spinach, and tartaric acid from tamarind.

This is a recall item, and the name itself carries the answer for anyone who has met the older name. Formic acid and methanoic acid are the same substance under two naming systems, and formica is Latin for ant. That link makes the question almost self-answering, which is presumably why examiners keep returning to it — the CDS and NDA papers have asked for the acid in ant stings and in nettle hairs at least half a dozen times across the last decade. The remaining three options are all places where some other acid is genuinely found, so the item doubles as a check on lactic acid and uric acid.

• Methanoic acid, HCOOH, is the simplest carboxylic acid; its common name, formic acid, comes from the Latin formica, meaning ant.
• It occurs in most ants, and wood ants of the genus Formica spray it in defence.
• The stinging hairs of the nettle, Urtica dioica, inject the same acid, which is why the burning feels similar.
• Rubbing a mild base such as baking soda on an ant sting works by neutralising the acid.
• Lactic acid belongs to muscle, urea and uric acid to urine — the acids the other options are reaching for.

• Failing to connect methanoic acid with formic acid and treating them as two different substances.
• Attaching lactic acid to ant stings because both involve a burning sensation.
• Assuming the acid in a nettle sting must be different from the one in an ant sting; it is the same.

Either from the acid to its source, as here, or from the source to the acid — the standard version asks what a nettle or an ant injects.

• practice — not a real PYQ

  The acid that accumulates in muscles during vigorous exercise and causes cramp is

  • (a)methanoic acid
  • (b)lactic acid
  • (c)citric acid
  • (d)oxalic acid

  Answer(b) lactic acid — when oxygen runs short the muscle respires anaerobically and converts pyruvate to lactic acid rather than oxidising it fully.

• practice — not a real PYQ

  Which one of the following pairs of an acid and its natural source is correctly matched?

  • (a)Oxalic acid — tamarind
  • (b)Tartaric acid — spinach
  • (c)Citric acid — lemon
  • (d)Lactic acid — vinegar

  Answer(c) Citric acid — lemon. Oxalic acid belongs to spinach and tomato, tartaric acid to tamarind, and vinegar is a solution of ethanoic acid.