CDS (I) 2024 General Knowledge · Question 37 of 120

Correct — A, 5–8% solution of acetic acid in water. NCERT gives the figure exactly: '5-8% solution of acetic acid in water is called vinegar and is used widely as a preservative in pickles.' Acetic acid is the common name for ethanoic acid, the two-carbon member of the carboxylic acid family, and it is what the acetic acid bacteria produce when they oxidise the ethanol in a fermented liquid. The same passage supplies the other fact usually asked with this one: pure ethanoic acid melts at 290 K and so freezes in a cold winter, which is how it came to be called glacial acetic acid.

• (b)5–8% solution of carbonic acid in water — Carbonic acid is what forms when carbon dioxide dissolves in water — the acid of aerated drinks. It is weak, unstable and has nothing to do with vinegar's sharpness or its preservative action.
• (c)5–8% solution of ethanol in water — Ethanol is the alcohol vinegar is made from, not vinegar itself. Acetic acid bacteria oxidise that ethanol to acetic acid, so a dilute ethanol solution is the raw material one step earlier.
• (d)10–15% solution of propionic acid in water — Wrong acid and wrong strength. Propionic acid and its salts are used as preservatives in bread, but vinegar is an acetic acid solution at the 5 to 8 per cent level.

Ethanoic acid, CH3COOH, is the first carboxylic acid most students meet. Its dilute aqueous solution is vinegar, at 5 to 8 per cent by the NCERT figure, and that acidity is what preserves pickles by holding the medium at a pH where spoilage organisms cannot grow. Industrially, vinegar is made by a two-stage fermentation — sugars to ethanol by yeast, then ethanol to acetic acid by acetic acid bacteria — which is why almost any sugar or starch feedstock, from grapes to malt to fermented potato mash, can become a vinegar.

The option list is built so that each wrong answer sits one step away from the right one. Carbonic acid is the wrong acid entirely; ethanol is the right process at the wrong stage; propionic acid is a genuine food preservative at the wrong concentration. Only one option pairs the right acid with the right band, and that band, 5 to 8 per cent, is worth memorising as printed because CDS quotes it verbatim from the textbook. Note also that acetic acid is a weak acid, only partially ionised in water, which is why vinegar is safe to eat while a mineral acid of the same concentration would not be. The glacial acetic acid story — freezing at 290 K, about 17 degrees Celsius — is a favourite follow-up.

• NCERT: '5-8% solution of acetic acid in water is called vinegar and is used widely as a preservative in pickles.'
• Acetic acid is the common name for ethanoic acid, CH3COOH, a carboxylic acid.
• Pure ethanoic acid melts at 290 K and often freezes in cold climates, hence the name glacial acetic acid.
• Vinegar is produced by fermenting sugars to ethanol and then oxidising that ethanol with acetic acid bacteria.
• Acetic acid is a weak acid, only partially ionised in water.

• Choosing ethanol because vinegar is made from an alcoholic liquid; the alcohol has been oxidised away.
• Confusing carbonic acid, the acid of fizzy drinks, with acetic acid.
• Misremembering the concentration band; the textbook figure is 5 to 8 per cent, not 10 to 15.

Asked as a fill-the-definition item quoting the textbook percentage, with near-miss acids and a wrong concentration among the options.

• practice — not a real PYQ

  Glacial acetic acid gets its name because

  • (a)it is manufactured in cold countries
  • (b)pure ethanoic acid freezes in cold weather, melting only at about 290 K
  • (c)it is always stored with ice
  • (d)it looks like crushed glass

  Answer(b) pure ethanoic acid freezes in cold weather, melting only at about 290 K — the ice-like solid gave the name.

• practice — not a real PYQ

  In the manufacture of vinegar, ethanol is converted to acetic acid by

  • (a)yeast
  • (b)acetic acid bacteria
  • (c)hydrochloric acid
  • (d)sodium hydroxide

  Answer(b) acetic acid bacteria — yeast makes the ethanol first, and the bacteria then oxidise it.