CDS (II) 2021 General Knowledge · Question 24 of 120

Correct — A, Methanoic acid. The stinging hairs of the nettle leaf inject methanoic acid, and that is what produces the burning pain. Methanoic acid is the systematic name of what is more often called formic acid, and the older name records where it was first noticed — formica is Latin for ant, and an ant's bite delivers the same compound. The hairs themselves work like tiny hypodermic needles: brush against the leaf and the brittle silica tip snaps off, leaving a sharp open tube that pierces the skin and releases the irritant fluid beneath. Because the injected substance is an acid, the traditional remedies are all mild bases — rubbing the spot with the leaf of the dock plant, which often grows beside the nettle, or dabbing on baking soda. Neutralising an acid with a base is the same chemistry used against a bee sting, which is also acidic.

• (b)Citric acid — The acid of lemons and oranges. It is a mild edible acid, not an injected irritant, and nothing in a nettle secretes it.
• (c)Tartaric acid — Found in tamarind and used in baking powder. It has no role in plant defence of this kind.
• (d)Acetic acid — The acid of vinegar, also written as ethanoic acid. Its everyday familiarity makes it a tempting pick, but the compound in nettle hairs is the one-carbon acid, methanoic, not the two-carbon one.

Many acids occur naturally, and the standard table of them pairs each with its source: vinegar with acetic acid, orange and lemon with citric acid, tamarind with tartaric acid, curd with lactic acid, tomato with oxalic acid, and both an ant sting and a nettle sting with methanoic acid. Some of these are defensive. Nettle and bee both deliver an acid into the skin, and the standard first aid is a mild base, which neutralises it.

The item is decided by one pairing from that table, and the three wrong options are its neighbours, all of them acids you eat rather than acids that are injected into you. Sorting the list by function rather than by name is the quickest safeguard: citric, tartaric and acetic are food acids, while methanoic is the sting acid shared by ants, some bees and the nettle. The pair of names is worth fixing too, because papers switch between them without warning — methanoic acid and formic acid are the same substance, just as ethanoic acid and acetic acid are. Note also that modern work on nettles identifies several irritants in the sting fluid alongside the acid, including histamine and serotonin, but the school syllabus and this item both name methanoic acid as the cause.

• The stinging hairs of nettle leaves inject methanoic acid, which causes the burning pain.
• Methanoic acid and formic acid are two names for the same compound; the older name comes from formica, Latin for ant.
• An ant sting also delivers methanoic acid, and a bee sting leaves an acid that a mild base such as baking soda relieves.
• The traditional remedy for a nettle sting is to rub the area with the leaf of the dock plant, which often grows beside the nettle.
• In the standard table of natural acids, vinegar gives acetic acid, orange and lemon citric acid, tamarind tartaric acid, curd lactic acid and tomato oxalic acid.

• Choosing acetic acid because vinegar is the acid most people know; the nettle acid has one carbon, not two.
• Treating methanoic and formic acid as different substances when a paper switches names.
• Assuming the sting is alkaline and reaching for an acid remedy; the fluid is acidic and needs a base.

As a name-the-acid item on a sting or a food, as a match list of natural sources against acids, or as a first-aid reasoning question on why baking soda helps.

• practice — not a real PYQ

  Which acid is present in tamarind?

  • (a)Oxalic acid
  • (b)Tartaric acid
  • (c)Lactic acid
  • (d)Methanoic acid

  Answer(b) Tartaric acid — oxalic acid belongs to tomato, lactic acid to curd, and methanoic acid to ant and nettle stings.

• practice — not a real PYQ

  Rubbing baking soda on the site of a bee sting gives relief because baking soda

  • (a)is a mild acid that dissolves the sting
  • (b)is a mild base that neutralises the injected acid
  • (c)cools the skin by evaporating
  • (d)kills the bacteria carried by the sting

  Answer(b) is a mild base that neutralises the injected acid — the sting fluid is acidic, so a base counteracts it.