CDS (I) 2020 General Knowledge · Question 6 of 120

Correct — A, A-1 B-2 C-3 D-4. Take the two easy pairs first. Magnesium hydroxide is milk of magnesia, the mild base NCERT names as the standard antacid for indigestion, so C goes to 3. Acetic acid is the acid of vinegar, and NCERT's carbon chapter records that a 5 to 8 per cent solution of acetic acid in water is called vinegar and is used widely as a preservative in pickles, so D goes to 4. That leaves boric acid and citric acid for antiseptic and food preservative. Boric acid is a long-standing mild antiseptic for minor cuts and burns and the active ingredient of boracic lint, so A goes to 1, and citric acid — used across the food industry as an acidifier, flavouring and preservative, carrying the food-additive number E330 — takes 2.

• (b)A-1 B-3 C-2 D-4 — Swaps citric acid and magnesium hydroxide. It would make citric acid an antacid and magnesium hydroxide a food preservative, and milk of magnesia is nobody's preservative.
• (c)A-4 B-3 C-2 D-1 — Sends boric acid to pickles and acetic acid to antiseptics. Boric acid is toxic if swallowed and never goes into food, which rules this out on its own.
• (d)A-4 B-2 C-3 D-1 — Gets citric acid and magnesium hydroxide right but exchanges boric acid and acetic acid, putting boric acid in the pickle jar and vinegar on a wound dressing.

Four substances, each with one everyday use that the school syllabus fixes. Boric acid is a weak acid used dilute as an antiseptic and eye wash. Citric acid is the acid of citrus fruit, made industrially in millions of tonnes a year and used as an acidifier, flavouring and preservative. Magnesium hydroxide is a mild base sold as milk of magnesia and taken as an antacid. Acetic acid is ethanoic acid, the acid of vinegar, and vinegar is the standard pickling preservative.

Match items reward anchoring rather than guessing. Two of these four pairings are locked in by NCERT sentences a candidate has probably already read — magnesium hydroxide as the antacid, and vinegar as the pickle preservative — and once C-3 and D-4 are fixed, only options (a) and (b) survive. The remaining choice is between making boric acid an antiseptic or an antacid, and boric acid is not something anyone swallows. One point of care with the wording of List-II: both entry 2, food preservative, and entry 4, pickle, are preservation uses, so the distinction being drawn is between a preservative added across foods and beverages generally and the specific pickling medium.

• Boric acid is used as an antiseptic for minor burns and cuts and appears in dressings such as boracic lint.
• Citric acid is used widely as an acidifier, flavouring and preservative, and carries the European food-additive number E330.
• NCERT names magnesium hydroxide, milk of magnesia, as the mild base commonly used as an antacid.
• NCERT defines vinegar as a 5 to 8 per cent solution of acetic acid in water, used widely as a preservative in pickles.
• Acetic acid's IUPAC name is ethanoic acid; pure ethanoic acid melts at 290 K, which is where the name glacial acetic acid comes from.

• Pairing citric acid with the antacid entry because both involve neutralising something.
• Treating 'food preservative' and 'pickle' as the same use and then guessing between the two.
• Assuming boric acid, being an acid, must have a food use; it is toxic if ingested.

Almost always as a four-by-four match of a compound to its use or its formula, sometimes reversed into a trade-name against chemical-name grid.

• practice — not a real PYQ

  Which one of the following is the chemical name of the substance sold as milk of magnesia?

  • (a)Magnesium carbonate
  • (b)Magnesium sulphate
  • (c)Magnesium hydroxide
  • (d)Magnesium chloride

  Answer(c) Magnesium hydroxide — a mild base, which is why it works as an antacid.

• practice — not a real PYQ

  Vinegar, used as a preservative in pickles, is a dilute aqueous solution of

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

  Answer(b) acetic acid — vinegar is a 5 to 8 per cent solution of acetic acid, also called ethanoic acid, in water.