Vinegar is
- (a)5–8% solution of acetic acid in water
- (b)5–8% solution of carbonic acid in water
- (c)5–8% solution of ethanol in water
- (d)10–15% solution of propionic acid in water
Answer
Why
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.
Why the others are wrong
- (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.
Concept
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.
Key facts
- 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.
Study next
Common traps
- 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.
Related PYQs
Assertion (A): Formic acid is a stronger acid than acetic acid. Reason (R): Formic acid is an organic acid.
- (a) Both A and R are true, and R is the correct explanation of A
- (b) Both A and R are true, but R is NOT a correct explanation of A
- (c) A is true, but R is false
- (d) A is false, but R is true
Answer(b) Both A and R are true, but R is NOT a correct explanation of A
The same acid, tested on strength rather than on identity. Formic acid is the stronger of the two because acetic acid's methyl group destabilises its conjugate base: a useful reminder that acetic acid is a weak acid, which is why vinegar is edible.
Match List-I with List-II and select the correct answer using the code given below the Lists : List-I (Compound) A. Boric acid B. Citric acid C. Magnesium hydroxide D. Acetic acid List-II (Use) 1. Antiseptic 2. Food preservative 3. Antacid 4. Pickle
- (a) A-1 B-2 C-3 D-4
- (b) A-1 B-3 C-2 D-4
- (c) A-4 B-3 C-2 D-1
- (d) A-4 B-2 C-3 D-1
Answer(a) A-1 B-2 C-3 D-4
CDS matched acetic acid to pickles four years earlier. That is the same use NCERT names in the sentence this item is built on, so the two questions share a single line of text.
Practice
- 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.