CDS (II) 2023 General Knowledge · Question 43 of 120

Correct — C, Oxalic acid is found in tamarind paste. The question asks which statement is not correct, and this is the one that misplaces the acid. Tamarind's sourness comes from tartaric acid: tartaric acid occurs naturally in many fruits, most notably in grapes but also in tamarinds, bananas, avocados and citrus. Oxalic acid belongs elsewhere on the standard acid-and-source list — it is the acid associated with tomato, and it is also present in spinach and in several other leaves. CDS has itself keyed oxalic acid as the acid found in abundance in tomato, which makes the misplacement here easy to spot for anyone who has met the list before. The other three statements stand. Hydrochloric acid is secreted in the gastric juice of the stomach, where it keeps the contents strongly acidic and activates the protein-digesting enzyme. Acetic acid is the main acid of vinegar — table vinegar is typically no less than about four per cent acetic acid by mass. And lemon and orange juices do carry citric acid and ascorbic acid respectively, ascorbic acid being vitamin C.

• (a)Hydrochloric acid is present in the gastric juice secreted by the stomach. — Correct, so it cannot be the answer to a question asking for the statement that is not correct. Gastric juice contains hydrochloric acid, which is why the stomach lining needs a mucus layer to protect itself.
• (b)Acetic acid is the main constituent of vinegar. — Also correct. Vinegar is a dilute solution of acetic acid in water, typically no less than about four per cent acetic acid by mass, and the acid gives it both its sharpness and its preservative effect.
• (d)Lemon and orange juices contain citric acid and ascorbic acid respectively. — Correct as written, and the statement worth reading carefully. It claims only that these juices contain those acids, which they do — orange juice is a well-known source of ascorbic acid, or vitamin C. Had it claimed that ascorbic acid is the principal acid of orange juice it would have been questionable, since citrus fruits are dominated by citric acid, but 'contain' is a weaker and true claim.

Acids are substances that release hydrogen ions in water, taste sour, turn blue litmus red and react with bases to give a salt and water. Those found in living things are organic acids, generally weak, and each is conventionally associated with a familiar source: citric acid with citrus fruits, tartaric acid with tamarind and grapes, oxalic acid with tomato and spinach, lactic acid with curd, acetic acid with vinegar, methanoic acid with ant stings and nettle leaves, and ascorbic acid with fruits generally as vitamin C. Hydrochloric acid, secreted in the stomach, is the one strong mineral acid that occurs in the body.

This is a list-recall item, and the efficient preparation is to hold the acid-and-source table rather than to reason about chemistry. When a question of this shape appears, check each pairing against the table and stop at the first mismatch. The tamarind entry is the one examiners return to most, because tamarind is intensely sour and 'oxalic' sounds suitably harsh; the correct partner is tartaric acid, and it is the source of cream of tartar as well. A second habit worth building is to read the verb in each statement. Statement (d) says the juices 'contain' those acids, which is a weaker claim than saying they are the main acids, and a claim that survives scrutiny — a candidate who rejects it for naming ascorbic acid rather than citric acid for orange has read more into it than it says.

• Tartaric acid 'occurs naturally in many fruits, most notably in grapes but also in tamarinds, bananas, avocados, and citrus'.
• Oxalic acid is the acid conventionally associated with tomato; CDS keyed it as the acid found in abundance in tomato in an earlier paper.
• Vinegar 'is typically no less than 4% acetic acid by mass', which is why acetic acid is called its main constituent.
• Gastric juice secreted by the stomach contains hydrochloric acid, a strong mineral acid.
• Ascorbic acid is vitamin C; citrus juices contain it alongside citric acid, which is the dominant acid in citrus.

• Pairing tamarind with oxalic acid because both suggest sharpness; tamarind goes with tartaric acid and tomato with oxalic.
• Missing that the stem asks for the statement that is not correct, and marking the first statement that is right.
• Rejecting the lemon-and-orange statement for naming ascorbic acid; it says only that the juices contain it, which is true.

As a which-statement-is-not-correct item over a set of acid-and-source pairings, or as a straight match-the-acid-to-its-natural-source list.

• practice — not a real PYQ

  The sour taste of curd is due mainly to which one of the following acids?

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

  Answer(b) Lactic acid — bacteria ferment the sugar in milk to lactic acid, which both sours the milk and curdles its casein.

• practice — not a real PYQ

  Which one of the following acids is present in vinegar?

  • (a)Acetic acid
  • (b)Ascorbic acid
  • (c)Methanoic acid
  • (d)Sulphuric acid

  Answer(a) Acetic acid — vinegar is a dilute aqueous solution of acetic acid, typically no less than about four per cent by mass.