Potassium nitrate is not used :
- (a)as a fertilizer
- (b)in medicine
- (c)in making gunpowder
- (d)as a preservative of meat products
Correct — D, as a preservative of meat products. Potassium nitrate, the saltpetre of the older books, is one of those compounds with a long list of uses, so this item works by elimination rather than by recall of a single fact. It is a genuine fertiliser, supplying two of the three major nutrients at once, nitrogen and potassium. It is the oxidiser in gunpowder, the component that supplies oxygen so that the charcoal and sulphur can burn without air. And it has a real medicinal use, as the active ingredient in toothpastes for sensitive teeth. None of those three can be dislodged, which leaves the fourth. Meat curing today is done with sodium nitrite rather than with the nitrate, because the nitrite acts directly while the nitrate works only slowly and unevenly. The honest qualification, worth knowing rather than hiding, is that saltpetre was for centuries the traditional curing salt and is still permitted in a few dry-cured products, so this option is sound as a statement about present-day practice and not as a statement about history.
- (a)as a fertilizer — This is a real and important use. Potassium nitrate supplies nitrogen and potassium together, two of the three major plant nutrients, and dissolves readily, which makes it valuable where a quick-acting, chloride-free source is wanted.
- (b)in medicine — Also a real use. Potassium nitrate is the active ingredient in toothpastes formulated for sensitive teeth, where it acts on the nerve response inside the tooth, and it had older medicinal uses besides.
- (c)in making gunpowder — The oldest use of all. Gunpowder is a mixture of potassium nitrate, charcoal and sulphur, and the nitrate is the oxidiser that supplies oxygen so the mixture burns rapidly without needing air from outside.
Potassium nitrate, KNO3, is an ionic salt that is highly soluble in water and acts as a strong oxidising agent when heated, releasing oxygen. Those two properties account for almost everything it is used for: solubility makes it a fertiliser, and the oxygen it gives up makes it the heart of gunpowder and of fireworks. It is also used in heat-treating metals, in toughening glass and as a heat-storage medium in solar power plants.
Items phrased as which use does NOT belong are best attacked from the other end — establish the uses that are certain, and take what remains. Here three options are beyond dispute, so the fourth must be the intended answer even before a candidate has thought about meat at all. It is still worth understanding what the item is really pointing at, because the chemistry is neat. Curing salts work by supplying nitrite, which fixes the red colour of cured meat and, more importantly, suppresses the organism that causes botulism. A nitrate has to be converted to nitrite by bacteria before it can do any of that, which is why it acts slowly and unpredictably, and why the food industry moved to sodium nitrite directly. The nitrate has not vanished from the field altogether — it survives in a few long-cured products — so the statement is true of the modern kitchen rather than of the historical salt-house.
- Gunpowder is potassium nitrate with charcoal and sulphur, the nitrate acting as the oxygen supplier.
- As a fertiliser, potassium nitrate delivers nitrogen and potassium in one soluble salt.
- Potassium nitrate is the active ingredient of toothpastes for sensitive teeth.
- Modern meat curing uses sodium nitrite, which acts directly; the nitrate must first be converted to nitrite by bacteria and so works slowly and inconsistently.
- Potassium nitrate survives as food additive E252 in a few dry-cured products such as salami and dry-cured ham.
Three uses are unarguable, which is what makes the fourth the answer.
- Answering from the word saltpetre alone; the historic curing salt and the modern curing salt are not the same compound.
- Assuming a compound has one use; potassium nitrate has many, which is precisely why the item is set as an exception.
- Confusing potassium nitrate with sodium nitrate, the Chile saltpetre used mainly as a fertiliser.
Usually as a use-to-substance match, or as an odd-one-out on uses; gunpowder and fertiliser are the two hooks that recur across UPSC, CDS and NDA.
Match List I with List II and select the correct answer using the codes given below the lists: List I — I. Potassium bromide, II. Potassium nitrate, III. Potassium sulphate, IV. Mono-potassium tartrate; List II — A. Fertiliser, B. Photography, C. Bakery, D. Gunpowder
- (a) I-B, II-D, III-A, IV-C
- (b) I-B, II-D, III-C, IV-A
- (c) I-D, II-B, III-C, IV-A
- (d) I-D, II-B, III-A, IV-C
Answer(a) I-B, II-D, III-A, IV-C
The same compound placed in a four-way match, where it goes to gunpowder. Note how the fertiliser slot there goes to potassium sulphate — several potassium salts are fertilisers, so the distinguishing use of the nitrate in that item is the explosive one.
- practice — not a real PYQ
In gunpowder, potassium nitrate performs which one of the following roles?
- (a)It acts as the fuel
- (b)It acts as the oxidising agent
- (c)It slows the rate of burning
- (d)It gives the mixture its colour
Answer(b) It acts as the oxidising agent — it releases oxygen on heating so that the charcoal and sulphur can burn rapidly without air from outside.
- practice — not a real PYQ
Which one among the following salts is chiefly used today for curing and preserving meat?
- (a)Potassium nitrate
- (b)Sodium nitrite
- (c)Calcium carbonate
- (d)Ammonium chloride
Answer(b) Sodium nitrite — it acts directly on the meat, whereas a nitrate has to be converted to nitrite by bacteria first and so acts slowly and unevenly.