CDS (I) 2021 General Knowledge · Question 113 of 120

Correct — C, Oxalic acid. School chemistry carries a short table of natural sources of acids, and it is that table the paper is quoting: vinegar gives acetic acid, curd lactic acid, lemon and orange citric acid, tamarind tartaric acid, an ant or nettle sting methanoic acid, and tomato oxalic acid. Each pairing is meant to be learnt as a unit, and this question simply asks for the entry against tomato. The acid is genuinely present in the fruit — measurements of raw tomato give roughly 0.05 per cent oxalic acid by fresh weight — though it is worth being straight about the word 'abundance' in the stem: that figure is small next to spinach at about 0.97 per cent or parsley at about 1.70 per cent, so tomato is not an oxalate-rich food in the dietary sense. What the question is testing is the textbook pairing, and against this option list oxalic acid is the entry the table gives.

• (a)Acetic acid — The acid of vinegar, which is a dilute solution of it. Vinegar is the source the table names, and no version of the list puts acetic acid against tomato.
• (b)Citric acid — The strongest pull among the wrong options, because tomatoes do taste sharp and do contain citric acid. The table assigns citric acid to lemon and orange, and it is that assignment the question is asking a candidate to recall.
• (d)Tartaric acid — The acid the same table gives for tamarind. It also appears in grapes, and it is the acid of the tartrate in baking powder.

Many of the sour foods in a kitchen owe their taste to a weak organic acid, and school chemistry introduces acids through that list of everyday sources. The acids are weak — they only partly release hydrogen ions in water — which is why they are safe to eat while a mineral acid of the same concentration would not be. Oxalic acid itself is the simplest dicarboxylic acid and occurs widely in plants, most heavily in the spinach family, sorrel and rhubarb leaves.

This is a recall item on a fixed table, so the useful work is learning the pairings in one block rather than reasoning towards one of them. Citric acid is the option designed to catch a candidate who reasons from taste instead. Two honest qualifications are worth carrying. The stem's word 'abundance' overstates the position — the measured oxalic acid content of tomato is around one-twentieth of spinach's — and the question is not asking which acid is quantitatively the largest in a tomato, a point this card does not certify either way. What it is asking is which acid the source table names for tomato, and the answer to that is oxalic acid.

• The standard table of natural acid sources pairs tomato with oxalic acid, tamarind with tartaric acid, lemon and orange with citric acid, and vinegar with acetic acid.
• Curd owes its sourness to lactic acid, and an ant sting or nettle sting to methanoic acid.
• Measured oxalic acid content of raw tomato is about 0.05 per cent by fresh weight — real, but low.
• Spinach at about 0.97 per cent and parsley at about 1.70 per cent are the genuinely oxalate-rich vegetables.
• All of these are weak acids, releasing only a small fraction of their hydrogen ions in solution.

• Choosing citric acid from the taste of the tomato rather than from the table.
• Swapping the tamarind and tomato entries, which is the pairing most often reversed.
• Reading 'abundance' as a claim that tomato is among the richest sources of oxalic acid, which it is not.

As a source-to-acid match, either one at a time as here or as a four-row matching list.

• practice — not a real PYQ

  The sour taste of curd is due to

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

  Answer(b) lactic acid — bacteria ferment the lactose of milk into lactic acid, which both sours the curd and sets the protein.

• practice — not a real PYQ

  Which one of the following acids is present in an ant's sting?

  • (a)Methanoic acid
  • (b)Acetic acid
  • (c)Oxalic acid
  • (d)Citric acid

  Answer(a) Methanoic acid — also called formic acid, from the Latin for ant, and the same acid that a nettle sting injects.