BPSC 70th CCE Re-exam Prelims General Studies · Question 61 of 142

Correct — D, None of these. Vinegar is a dilute aqueous solution of acetic acid, and acetic acid is not on the list. That is the whole question. Vinegar typically contains between 4 and 18 per cent acetic acid by volume, the rest being water plus trace flavour compounds, and the acid is produced by a double fermentation: yeast converts simple sugars to ethanol, and acetic acid bacteria then oxidise the ethanol to acetic acid. The very word records the chemistry — vinegar comes from the Old French vyn egre, sour wine. Acetic acid, CH₃COOH, is the second simplest carboxylic acid after formic acid, and its water-free form is called glacial acetic acid because it sets into ice-like crystals a little below room temperature, at 16.6 °C. Every one of the three named acids is a real food acid, but each belongs to a different foodstuff, and none of them belongs to vinegar. Because acetic acid is absent from the options, the escape option is the only truthful answer — and this is the disciplined use of 'None of these': you may take it here because you can name the substance that ought to have been there and show that it is not.

• (a)Citric acid — The acid of citrus fruit — lemon, lime, orange — and the commonest acidulant in soft drinks, where it supplies the tartness. It also gives its name to the citric acid cycle of cellular respiration. It is the most tempting wrong answer here simply because it is the food acid most people can name, but a bottle of vinegar contains essentially none of it.
• (b)Tartaric acid — The acid of grapes and tamarind. In the kitchen it is met as cream of tartar, the potassium salt used in baking powder, and in winemaking it is the acid that precipitates as crystals in the cask. It is genuinely a souring agent, but of imli and of wine — and note that when wine turns to vinegar it is not the tartaric acid that has changed, it is the alcohol that has been oxidised to acetic acid.
• (c)Lactic acid — The acid of curd and sour milk, formed when lactic acid bacteria ferment lactose, and the compound that accumulates in muscle during prolonged anaerobic exertion and is associated with the sensation of fatigue. It sours dahi, not sirka. Both vinegar and curd are products of microbial fermentation, which is exactly why this option is placed here — the process is shared, the acid is not.

Sourness is the taste of hydrogen ions, so every sour food owes its taste to some acid, and school chemistry expects you to hold a short table pairing the common weak organic acids with the foods that carry them. Acetic acid is vinegar. Citric acid is citrus fruit. Tartaric acid is tamarind and grapes. Lactic acid is curd and sour milk. Formic acid is the ant's sting — the name comes from formica, Latin for ant. Oxalic acid is tomato and spinach, malic acid is the apple, butyric acid is rancid butter, and ascorbic acid is vitamin C in amla and citrus. All of these are weak acids: they only partly dissociate in water, which is why they are safe to eat while mineral acids of similar strength on paper are not. Acetic acid's dissociation constant corresponds to a pKa of about 4.76, making it a slightly weaker acid than formic acid at about 3.75 — a comparison UPSC has asked directly.

The right move on any 'which acid causes X' question is to scan the options against the table before deciding anything, because these questions are almost always solved by recognising an absence rather than by identifying a presence. Here all three named acids are attached elsewhere — citrus, tamarind, curd — and the one that belongs to vinegar is missing, which forces the escape. The corroborating route runs through the word itself: vinegar is soured wine, wine is an alcohol, and the microbial oxidation of ethanol yields acetic acid, so no fruit acid can be responsible. One caution about escape options in general: 'None of these' is right only when you can positively name what should have been offered. If you cannot, you are guessing, and on a paper with a one-third negative mark that distinction matters.

• Vinegar is a dilute aqueous solution of acetic acid (ethanoic acid, CH₃COOH), typically 4 to 18 per cent acetic acid by volume.
• The acid is made by a double fermentation — yeast converts sugars to ethanol, then acetic acid bacteria oxidise the ethanol to acetic acid; the word vinegar itself comes from the Old French vyn egre, 'sour wine'.
• Acetic acid is the second simplest carboxylic acid after formic acid; anhydrous acetic acid is called glacial acetic acid because it crystallises at 16.6 °C.
• Food-acid pairings worth memorising: citric acid in citrus fruit and soft drinks, tartaric acid in tamarind and grapes, lactic acid in curd and sour milk, formic acid in the ant's sting, oxalic acid in tomato and spinach, malic acid in apple.
• Acetic acid is coded E260 as a food additive and acidity regulator; concentrated solutions of 25 per cent or more are corrosive to skin.

• Reaching for citric acid because it is the food acid most people can name. Vinegar contains essentially none of it.
• Confusing curd with vinegar because both are fermented. Curd is soured by lactic acid bacteria, vinegar by acetic acid bacteria acting on alcohol.
• Refusing to take 'None of these' when the correct substance is genuinely absent. The safeguard is to name the missing substance first — here, acetic acid.

BPSC asks acid-and-source questions in one line and frequently keeps the true answer off the option list, so the escape option is doing real work rather than padding. UPSC asks the same table but always with the answer present, and prefers the less obvious members of it — which acid an ant injects, whether formic acid is stronger than acetic acid, which acid is used in soft drinks.

• practice — not a real PYQ

  The sour taste of curd is due to

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

  Answer(b) Lactic acid — produced when lactic acid bacteria ferment the lactose in milk. Acetic acid is vinegar, citric acid is citrus fruit, and oxalic acid is found in spinach and tomato.

• practice — not a real PYQ

  Glacial acetic acid is so called because

  • (a)It is obtained by freezing vinegar
  • (b)The anhydrous acid crystallises into ice-like solid at 16.6 °C
  • (c)It is manufactured only in cold climates
  • (d)It is used to preserve ice cream

  Answer(b) The anhydrous acid crystallises into ice-like solid at 16.6 °C — water-free acetic acid sets a little below room temperature, which is where the German name Eisessig, 'ice vinegar', comes from.