EPFO APFC 2023 General Studies · Question 88 of 120

Correct — C, (c) Sodium benzoate. The ask is negative and the booklet prints the 'not' in bold italics: three of these four are soaps and one is not. A soap is defined by structure, not by the word 'sodium' in its name — it is the sodium or potassium salt of a long-chain fatty acid, which means a carboxylic acid whose hydrocarbon chain runs to roughly twelve to eighteen carbon atoms. That structure is what gives a soap its two halves: a long non-polar hydrocarbon tail that will not mix with water but dissolves grease, and a charged carboxylate head that water accepts. Sodium benzoate is the sodium salt of benzoic acid, and benzoic acid is an aromatic carboxylic acid consisting of a benzene ring with a single carboxyl group attached. It has a carboxylate head, which is why it is a salt of an acid at all, but no long hydrocarbon tail — just a compact six-carbon ring. Without a tail it cannot do the one thing a soap does. In water, soap molecules gather into micelles, tails inward around the oil droplet and heads outward facing the water, so that the dirt is carried away suspended; a molecule with a stubby aromatic ring in place of a chain has no hydrophobic tail to point inward and forms no micelle, so it has no cleansing action. Sodium benzoate belongs to a different world of use altogether: it is a food preservative, effective in acidic products such as soft drinks, fruit juices and pickles, where it holds back the growth of moulds, yeasts and some bacteria. The other three options name the sodium salts of stearic, palmitic and oleic acids, which are the three commonest fatty acids in the fats and oils that soap is actually made from, and every one of them is a soap in the strict sense.

• (a)Sodium stearate — This is a soap, and it is the most typical one in the list. Sodium stearate is the sodium salt of stearic acid, a straight-chain saturated fatty acid with eighteen carbon atoms, and it is the principal constituent of ordinary hard bar soap. Its long saturated chain packs well, which is what makes soaps rich in stearate firm rather than soft. It illustrates the general rule governing the whole family: sodium salts of fatty acids give hard soaps and potassium salts give soft soaps, which is why toilet and laundry bars are sodium soaps while shaving creams and liquid soaps are commonly potassium ones. A candidate looking for the odd one out should note that this option, like two others in the set, names a fatty acid of the kind found in animal fat or vegetable oil, while only one option names an aromatic acid.
• (b)Sodium palmate — This is a soap. Sodium palmate is the sodium salt of palmitic acid, a saturated fatty acid with sixteen carbon atoms, and it is what palm oil becomes when it is saponified with caustic soda. The name is not a textbook invention: sodium palmate is printed on soap wrappers as an ingredient, alongside sodium tallowate from animal fat and sodium cocoate from coconut oil, and recognising those names as fatty acid salts is worth a mark in itself. Palmitic acid's sixteen-carbon chain sits between the twelve of lauric acid and the eighteen of stearic and oleic acids, comfortably inside the range in which a carboxylate behaves as a surfactant. Every structural test a soap must pass — a long hydrocarbon tail, a carboxylate head, a sodium or potassium counter-ion — this compound passes.
• (d)Sodium oleate — This is a soap. Sodium oleate is the sodium salt of oleic acid, an eighteen-carbon fatty acid with one carbon-carbon double bond, which makes it unsaturated where stearic acid is saturated. Oleic acid is the chief fatty acid of olive oil and is abundant in many other vegetable oils, so sodium oleate is a major component of soaps made from them. The double bond puts a kink in the chain that stops the molecules packing as tightly, which is why oleate-rich soaps are softer and more soluble than stearate-rich ones. That difference in physical properties has no bearing on the classification: saturated or unsaturated, a long-chain fatty acid salt is a soap, and the only compound in this option set that fails the definition is the one built on an aromatic ring instead of a chain.

A soap is the sodium or potassium salt of a long-chain fatty acid, and it is made by saponification: an animal fat or a vegetable oil, which chemically is an ester of glycerol with three fatty acids, is boiled with a caustic alkali such as sodium hydroxide, giving the fatty acid salts and glycerol. The molecule that results has two ends of opposite character. The long hydrocarbon chain is non-polar and hydrophobic, and will mix with oil and grease; the carboxylate group with its metal ion is ionic and hydrophilic, and dissolves in water. Cleaning follows from that duality. Dropped into greasy water, soap molecules arrange themselves into spherical micelles with their tails buried in the oil droplet and their charged heads facing outward, so the droplet acquires a charged surface, is repelled by its neighbours, stays suspended and is carried off with the rinse water. A compound with no long tail cannot form a micelle and is not a surfactant however similar its head group looks. Two extensions of the idea are examined as often as the definition. The first is hard water: calcium and magnesium ions form insoluble salts with the fatty acid anion, so the soap precipitates as scum instead of lathering, which is why synthetic detergents were developed. Detergents are typically sodium salts of long-chain alkyl sulphonic or sulphuric acids, and because their calcium and magnesium salts are soluble, they work in hard water. The second is the alkalinity of soap solution: a soap is the salt of a strong base and a weak acid, so it hydrolyses in water and the solution turns out alkaline, which is why soap feels slippery and why the effluent of a soap works raises the pH of the water receiving it.

The chemistry items in this paper's science block reward definitions held precisely rather than loosely, and this one is a clean example of the difference. A candidate who carries the definition of a soap as 'a sodium salt used for cleaning' has nothing to work with here, because all four options are sodium salts and none of the names announces what it is used for. A candidate who carries it as 'the sodium or potassium salt of a long-chain fatty acid' answers in seconds, because only one of the four acids named is not a fatty acid at all. The option set is built to reward exactly that precision: the three soaps are drawn from the three commonest fatty acids in the oils and fats industry actually uses, so they look like a family, and the intruder is a familiar household chemical whose name has the same shape. The wider habit worth taking from the item is to read a chemical name as a structure. 'Sodium stearate' decomposes into stearic acid plus sodium; 'sodium benzoate' into benzoic acid plus sodium; and the question of whether the compound is a soap is then simply the question of whether the acid has a long chain. That method also answers the neighbouring questions this exam sets — why soap fails in hard water, why detergents do not, and why the effluent from a soap works is alkaline, which this same paper touches on in its environmental item on industrial effluents.

• A soap is the sodium or potassium salt of a long-chain fatty acid, typically of twelve to eighteen carbon atoms. The definition is structural, so the presence of sodium and of a carboxylate group is not enough; the long hydrocarbon chain is what makes the compound a soap.
• Sodium benzoate is the sodium salt of benzoic acid, an aromatic carboxylic acid consisting of a benzene ring bearing one carboxyl group. It has no long hydrocarbon tail, forms no micelle and has no cleansing action; it is used as a food preservative, effective in acidic foods such as soft drinks, juices and pickles.
• The three soaps in this option set are the salts of the three commonest fatty acids in soap-making: stearic acid with eighteen carbons and no double bond, palmitic acid with sixteen carbons and none, and oleic acid with eighteen carbons and one. Sodium palmate, sodium tallowate and sodium cocoate are the names actually printed on soap wrappers.
• Soap is made by saponification: a fat or oil, chemically an ester of glycerol with three fatty acids, is boiled with caustic alkali to give the fatty acid salts and glycerol. Sodium salts give hard soaps and potassium salts give soft soaps.
• Cleaning works through micelle formation. In water the hydrophobic tails turn inward around an oil droplet and the hydrophilic heads face outward, so the droplet is suspended and rinsed away. A molecule without a long tail cannot form a micelle and is not a surfactant.
• In hard water, calcium and magnesium ions form insoluble salts with the fatty acid anion, so soap precipitates as scum rather than lathering. Synthetic detergents, usually sodium salts of long-chain alkyl sulphonic or sulphuric acids, have soluble calcium and magnesium salts and therefore work in hard water.

• Treating any sodium salt of a carboxylic acid as a soap. All four options are sodium carboxylate salts; only three of them have the long hydrocarbon chain that the definition requires.
• Not recognising sodium palmate as a real compound. It is the sodium salt of palmitic acid and is listed by that name on soap wrappers, so its unfamiliarity is not evidence that it is the odd one out.
• Thinking that an unsaturated fatty acid salt is somehow not a soap. Sodium oleate is a soap; the double bond changes how soft the soap is, not what it is.
• Reading past the bold-italic 'not'. Three of the four options are soaps, so a candidate who marks the first recognisable soap he sees has three ways to be wrong and one to be right.
• Confusing sodium benzoate with benzoyl peroxide or with sodium benzenesulphonate. The first is a preservative, the second an antiseptic and bleaching agent, and only a long-chain sulphonate salt would be a detergent.

Soaps and detergents appear in this exam in four familiar shapes. The first is this one: four names, three of them fatty acid salts and one an impostor, asked negatively. The second asks for the mechanism — what a micelle is, how it removes grease, why the solution needs both a hydrophobic and a hydrophilic end. The third asks about hard water and is answered by the insolubility of calcium and magnesium salts of fatty acids, with the follow-up that detergents escape the problem because their corresponding salts are soluble. The fourth reaches into environment and asks why detergent-rich sewage causes foaming or eutrophication, which brings in phosphate builders. All four are covered by holding one structural definition, one mechanism and one comparison table between soap and detergent. Expect the option set to be built from names that all share a prefix, so that reading the second half of each name — which acid it is the salt of — is what actually decides the question.

No directly related past PYQ was found.

• practice — not a real PYQ

  Soaps used for washing are chemically best described as the sodium or potassium salts of which one of the following classes of compounds?

  • (a)Long-chain fatty acids
  • (b)Aromatic carboxylic acids
  • (c)Long-chain alkyl sulphonic acids
  • (d)Amino acids

  Answer(a) long-chain fatty acids — the long hydrocarbon chain provides the hydrophobic tail and the carboxylate group the hydrophilic head, and both are needed for micelle formation and cleansing. An aromatic carboxylic acid such as benzoic acid has the head but no tail, and the sodium salts of long-chain alkyl sulphonic acids are synthetic detergents rather than soaps, which is precisely why they continue to work in hard water.

• practice — not a real PYQ

  Soap does not lather satisfactorily when used with hard water. The principal reason is that hard water contains

  • (a)dissolved carbon dioxide, which neutralises the soap
  • (b)calcium and magnesium ions, which form insoluble salts with the soap anion
  • (c)sodium chloride, which precipitates the soap by the common ion effect
  • (d)suspended clay particles, which adsorb the soap

  Answer(b) calcium and magnesium ions, which form insoluble salts with the soap anion — the fatty acid anion is precipitated as an insoluble calcium or magnesium salt, seen as scum, so it is consumed before it can lather or clean. Synthetic detergents avoid this because their calcium and magnesium salts are soluble, which is the historical reason detergents were developed in the first place.