Enforcement Officer / Accounts Officer (EO/AO) 2017 recruitment test. Transcribed from the Commission's own booklet (series A, GRS-A-TNS); answer as per the official UPSC final answer key. This question is in Part B. Answer last reviewed 24 August 2026.

Bleaching powder contains

(a)nitrogen
(b)iodine
(c)chlorine
(d)bromine

Answer

Why

Correct — C, (c) chlorine. Bleaching powder is made by passing chlorine over dry slaked lime, calcium hydroxide, and the school textbook writes it exactly that way: bleaching powder is produced by the action of chlorine on dry slaked lime, and is represented as CaOCl2. The chlorine is not merely a reagent used in the manufacture and then lost — it stays in the product, and it is the chlorine that does the bleaching.

The formula itself is worth reading. Calcium oxychloride, CaOCl2, contains calcium, oxygen and two chlorines in different oxidation states — one behaving as chloride and one as hypochlorite. That mixed arrangement is what makes the compound useful: it is stable enough to be stored as a dry powder, yet releases its active chlorine readily on contact with water, acid or even the carbon dioxide of the air. The chlorine liberated, and the hypochlorite that carries it, oxidise coloured organic compounds to colourless ones, which is what bleaching is. The same available chlorine is what kills bacteria in drinking water.

So the answer follows from the name of the compound as well as from its chemistry, and it can be reached from either end: from the manufacture, chlorine plus slaked lime; from the formula, CaOCl2; or from the uses, which are all uses of available chlorine — bleaching cotton, linen and wood pulp, disinfecting drinking water, and acting as an oxidising agent in the laboratory and in industry.

One further connection is worth carrying. Chlorine for this purpose comes from the chlor-alkali process, the electrolysis of brine, which yields chlorine at the anode, hydrogen at the cathode and sodium hydroxide in solution. The same textbook chapter that gives bleaching powder gives that process and the other products made from common salt, which is why examiners tend to ask about them together.

Why the others are wrong

(a)nitrogen — Nitrogen has no place in bleaching powder at all. It is not present in the compound, plays no part in its manufacture and has no bleaching action; molecular nitrogen is notably unreactive at ordinary temperatures, which is the opposite of what a bleaching agent has to be. The option is on the page as a plain distractor rather than as a near miss, and it is safely eliminated by remembering that a bleaching agent works by oxidising, while nitrogen in its elemental form does nothing of the kind.
(b)iodine — Iodine is a halogen and shares chlorine’s general chemistry, which makes it the family-resemblance guess. But it is not in bleaching powder, and its practical uses run the other way: tincture of iodine is an antiseptic, iodine is used in the laboratory as a test for starch, and potassium iodide is added to salt against goitre. Iodine is also the least reactive of the common halogens and a much weaker oxidising agent than chlorine, so it would be a poor bleaching agent even in principle. Bleaching power in the halogen family falls as you go down the group, which is the reason chlorine and not iodine is the one industry uses.
(d)bromine — Bromine, like iodine, is a halogen and is a stronger oxidising agent than iodine but weaker than chlorine, and it is not a constituent of bleaching powder. It is a liquid at room temperature, corrosive and expensive relative to chlorine, and its industrial uses lie elsewhere — flame retardants, and formerly photographic silver bromide. If the halogen family is the source of the confusion here, fix it with the trend: fluorine, chlorine, bromine, iodine, in falling order of reactivity and oxidising power, with chlorine the one that is both strong enough to bleach and cheap enough to make in bulk from brine.

Concept

Bleaching powder, calcium oxychloride, CaOCl2, is one of the small family of products made industrially from common salt, and the school chemistry chapter that covers acids, bases and salts treats them together. Electrolysis of brine — the chlor-alkali process — gives chlorine at the anode, hydrogen at the cathode and sodium hydroxide in the solution. Chlorine passed over dry slaked lime gives bleaching powder. Sodium hydroxide with chlorine gives sodium hypochlorite, the household liquid bleach. Sodium hydrogencarbonate, made from brine, ammonia, carbon dioxide and water, is baking soda; heating it and recrystallising gives washing soda, sodium carbonate decahydrate. Plaster of Paris comes from a different starting material, gypsum, heated to drive off most of its water of crystallisation. The property that makes bleaching powder useful is its available chlorine: the compound is a stable dry solid but releases chlorine on contact with water, dilute acid or atmospheric carbon dioxide, and the liberated chlorine and the hypochlorite it comes with oxidise coloured organic matter to colourless products. Its uses follow directly — bleaching cotton and linen in the textile industry, bleaching wood pulp in the paper industry, disinfecting drinking water, and serving as an oxidising agent in chemical manufacture.

This is a two-word stem with four one-word options, and it is as short as a question in this paper gets. Items of that kind are testing whether a named substance has been learnt with its composition attached, and the efficient preparation is to keep a short list of common compounds with formula, method of preparation and principal use: bleaching powder, baking soda, washing soda, plaster of Paris, caustic soda, common salt. Each of them can be asked in three directions — what is it made of, how is it made, what is it used for — and one line covering all three answers every version. Where the options are members of one chemical family, as the three halogens are here, the discriminating knowledge is the trend within the family rather than a fact about any one member.

Key facts

Bleaching powder is calcium oxychloride, represented as CaOCl2.
It is produced by the action of chlorine on dry slaked lime, calcium hydroxide.
Its bleaching and disinfecting action comes from the available chlorine it releases on contact with water, dilute acid or atmospheric carbon dioxide.
It is used for bleaching cotton and linen in the textile industry, for bleaching wood pulp in the paper industry, for disinfecting drinking water, and as an oxidising agent.
Chlorine for the purpose is obtained from the chlor-alkali process, the electrolysis of brine, which also yields hydrogen and sodium hydroxide.
Sodium hypochlorite, the liquid household bleach, is the product of chlorine and sodium hydroxide.
Bleaching by chlorine is an oxidation, and oxidising power in the halogen family falls from fluorine through chlorine and bromine to iodine.
Bleaching powder was historically known as chloride of lime, a name that records the same composition.

Chlorine is in the name, in the formula, in the manufacture and in the use

(c) Chlorine→Bleaching powder is produced by the action of chlorine on dry slaked lime, calcium hydroxide. The chlorine is not merely a reagent used in the manufacture and then lost — it stays in the product, and it is the chlorine that does the bleaching.

What the formula says→Calcium oxychloride, CaOCl2, contains calcium, oxygen and two chlorines in different oxidation states, one behaving as chloride and one as hypochlorite. That mixed arrangement is what makes it stable enough to store as a dry powder yet ready to release its active chlorine on contact with water, dilute acid or even the carbon dioxide of the air.

And what the uses say→Bleaching cotton and linen in the textile industry, bleaching wood pulp in the paper industry, disinfecting drinking water, and acting as an oxidising agent. Every one of them is a use of available chlorine, which oxidises coloured organic compounds to colourless ones — and that is what bleaching is.

(b) Iodine and (d) Bromine→Halogens, which makes them the family-resemblance guess. But oxidising power falls down the group — fluorine, chlorine, bromine, iodine — so iodine would be a poor bleaching agent even in principle, and bromine is a corrosive liquid, expensive relative to chlorine. Iodine’s uses run the other way: tincture of iodine as an antiseptic, the laboratory test for starch, potassium iodide in salt against goitre.

(a) Nitrogen→Not present in the compound, no part in its manufacture, no bleaching action. Molecular nitrogen is notably unreactive at ordinary temperatures, which is the opposite of what a bleaching agent has to be.

Where the chlorine comes from→The chlor-alkali process, the electrolysis of brine, which yields chlorine at the anode, hydrogen at the cathode and sodium hydroxide in the solution. Chlorine with sodium hydroxide gives sodium hypochlorite, the household liquid bleach — which is why the products made from common salt tend to be asked about together.

Study next

The chlor-alkali process and the products obtained from common saltBaking soda, washing soda and plaster of Paris — formulae, preparation and usesChlorination of drinking water, residual chlorine and the alternatives to itThe halogens as a group — trends in reactivity, oxidising power and physical stateOxidation and reduction, and why bleaching is an oxidation

Common traps

Guessing a halogen at random because all three of iodine, bromine and chlorine are on the page. The trend within the group is the discriminator.
Confusing bleaching powder, CaOCl2, with sodium hypochlorite, the liquid bleach; both work by chlorine but they are different compounds.
Forgetting that bleaching powder is made from dry slaked lime, not from quicklime or from limestone.
Attributing the bleaching to the calcium or to the oxygen in the formula. It is the chlorine that bleaches.

Named-compound items recur in every EPFO general ability paper and they are among the cheapest marks available, provided the compound has been learnt as a package rather than as a name. Build each package as four lines — common name, chemical name, formula, use — and revise them as a table. Expect one such item per paper, and expect the options to come from a single chemical family so that a half-memory of the right area does not by itself produce the right answer.

Related PYQs

EPFO_EOAO_2017_Q94Open & attempt →

Which one of the following gases has the highest solubility in water?

(a) Chlorine
(b) Ammonia
(c) Carbon dioxide
(d) Nitrogen

Answer(b) Ammonia

Asks which gas has the highest solubility in water and offers chlorine as a distractor — the item printed immediately before this one, and the other chlorine question in this paper’s chemistry run.

Practice

practice — not a real PYQ
Bleaching powder is produced by the action of chlorine on
- (a)quicklime
- (b)dry slaked lime
- (c)limestone
- (d)sodium carbonate
Answer(b) dry slaked lime
practice — not a real PYQ
The electrolysis of an aqueous solution of sodium chloride yields chlorine at the anode, sodium hydroxide in solution and, at the cathode,
- (a)oxygen
- (b)sodium
- (c)hydrogen
- (d)chlorine
Answer(c) hydrogen

Why