Assistant Provident Fund Commissioner and Enforcement Officer / Accounts Officer (APFC and EO/AO) 2025 recruitment test. Transcribed from the Commission's own booklet (series A, PENA-A-PTS); answer worked out by our faculty — UPSC has published no answer key for this sitting. This question is in Part B. Answer last reviewed 25 August 2026.

Which one among the following is known as Carbolic acid ?

(a)Phenol
(b)Alcohol
(c)Ether
(d)Acetone

Answer

Why

Correct — A, (a) Phenol. Carbolic acid is the old name for phenol, C6H5OH — a benzene ring with a hydroxyl group attached to it.

Why an alcohol-looking compound is called an acid. In phenol the –OH is bonded directly to the aromatic ring, and the ring pulls electron density away from the oxygen. When the hydrogen leaves, the negative charge on the resulting phenoxide ion is spread around the ring rather than sitting on one atom, so it is far more stable than an ordinary alkoxide. That stabilisation makes phenol give up its proton where an ordinary alcohol will not: phenol turns blue litmus faintly red and dissolves in sodium hydroxide solution to give sodium phenoxide. It is a weak acid — much weaker than carboxylic acids such as acetic acid, and it does not liberate carbon dioxide from sodium bicarbonate, which is the classic laboratory test separating a phenol from a carboxylic acid.

The name has a history worth knowing. Joseph Lister introduced carbolic acid as an ANTISEPTIC in surgery in the 1860s, spraying it on wounds, instruments and dressings, and the fall in post-operative infection was the foundation of antiseptic surgery. Phenol is still the reference substance against which disinfectants are rated by the phenol coefficient, and phenolic disinfectants remain in use. It is corrosive to skin, which is why it was later replaced in direct wound care by gentler agents. Industrially, phenol is the starting material for bakelite and other phenol-formaldehyde resins, for aspirin and other drugs, and for polycarbonate plastics.

Why the others are wrong

(b)Alcohol — A whole class rather than a substance, and the class that phenol is most often confused with. In an alcohol the hydroxyl group is attached to a saturated carbon; in a phenol it is attached to an aromatic ring, and that difference is what gives phenol its acidity. Ethanol has no acidic character to speak of and no historical name of this kind.
(c)Ether — An ether has an oxygen atom BETWEEN two carbon groups, R–O–R, with no hydroxyl and no acidic hydrogen at all. Diethyl ether earned its own place in the history of surgery as an early general anaesthetic, which is probably the association at work here — but anaesthesia and antisepsis are different chapters of the same story, and the antiseptic one is phenol's.
(d)Acetone — Acetone is the simplest ketone, propanone, a common laboratory and industrial solvent. It is neutral, not acidic. It has a genuine connection with phenol in industry — the cumene process produces phenol and acetone together from benzene and propene — but that is a shared origin, not a shared name.

Concept

Phenol, C6H5OH, is a colourless crystalline solid with a characteristic sharp smell, moderately soluble in water and corrosive to skin. It is weakly acidic because the phenoxide ion left after losing a proton is stabilised by delocalisation into the benzene ring — the standard illustration of how an aromatic ring changes the behaviour of an attached functional group. It reacts with neutral ferric chloride to give a violet colouration, a classic identification test. Commercially almost all phenol is made by the cumene process, which yields acetone as a co-product. Its uses run from disinfectants and the manufacture of phenol-formaldehyde resins to intermediates for drugs including aspirin.

Trivial or historical names of common chemicals are standard examination material because they are unambiguous and easy to set — carbolic acid for phenol, formalin for aqueous formaldehyde, quicklime for calcium oxide, blue vitriol for copper sulphate, and so on. There is nothing to derive; the item measures whether a candidate has kept such a list. It is worth building one and adding the substance's formula and one everyday use against each name.

Trivial-name questions are pure recall and cannot be derived, so the only preparation that works is a list — and the list is short enough to be worth making. Carbolic acid, formalin, quicklime, slaked lime, blue vitriol, green vitriol, washing soda, baking soda, caustic soda, plaster of Paris, marsh gas, laughing gas, aqua regia and heavy water between them cover most of what these papers ask. Against each name put the chemical name, the formula and one use. This item is also a reminder to read what the question asks for: three of its options are classes of compound and only one is a substance.

Key facts

Carbolic acid is phenol, C6H5OH — a hydroxyl group attached directly to a benzene ring.
Phenol is weakly acidic because the phenoxide ion is stabilised by delocalisation into the ring; alcohols are not appreciably acidic.
Phenol dissolves in sodium hydroxide to give sodium phenoxide but does not liberate carbon dioxide from sodium bicarbonate, unlike carboxylic acids.
Joseph Lister introduced carbolic acid as a surgical antiseptic in the 1860s, founding antiseptic surgery.
The phenol coefficient rates the effectiveness of disinfectants against phenol as the standard.
Neutral ferric chloride gives a violet colour with phenol, a common identification test.
Industrially, phenol and acetone are produced together by the cumene process.
Phenol is the raw material for phenol-formaldehyde resins such as bakelite and for several drugs.

Study next

Common chemicals and their trivial names, with formulae — a list worth keepingAlcohols, phenols and ethers — how the three classes differ and how they are distinguished in the laboratoryAntiseptics and disinfectants, and the difference between themPolymers from phenol and formaldehyde, and thermosetting plastics generallyThe cumene process and other named industrial processes

Common traps

Answering 'alcohol' because phenol carries a hydroxyl group; the attachment to an aromatic ring is what makes it a different class.
Mixing up the surgical histories of ether (anaesthesia) and carbolic acid (antisepsis).
Treating phenol as a strong acid; it is weaker than carboxylic acids and does not react with bicarbonate.
Assuming a class name can be the answer where the question asks for a specific substance.

One-line chemical identification questions appear in every EPFO paper: a common name, a use, or a formula, with four substances offered. They are answered from a memorised list, so the preparation that pays is a table of trivial names against chemical names, formulae and uses.

Related PYQs

EPFO_EOAO_2020_Q51

Which one of the following substances, when added to water, will not change the pH ?

(a) NaHCO3
(b) NH4Cl
(c) Na2CO3
(d) NaCl

Answer(d) NaCl

A chemistry item of the same one-line type from the earlier paper — which substance, added to water, will not change its pH, which turns on recognising acidic, basic and neutral compounds by name.

Practice

practice — not a real PYQ
Which one among the following is the chemical name of the substance commonly called quicklime ?
- (a)Calcium carbonate
- (b)Calcium oxide
- (c)Calcium hydroxide
- (d)Calcium sulphate
Answer(b) Calcium oxide
practice — not a real PYQ
Phenol is weakly acidic mainly because of which one among the following ?
- (a)The hydroxyl group is attached to a saturated carbon
- (b)The phenoxide ion is stabilised by delocalisation into the benzene ring
- (c)It contains a carboxyl group
- (d)It readily forms hydrogen bonds with water
Answer(b) The phenoxide ion is stabilised by delocalisation into the benzene ring

Why