Which one of the following statements about the cleansing action of soap is not true?
- (a)The oil and dirt gets collected in the centre of the micelle.
- (b)Soap micelles scatter light.
- (c)Soaps are ammonium salts of long chain carboxylic acids.
- (d)Soap forms insoluble precipitates with the calcium and magnesium ions in hard water.
Correct — C, 'Soaps are ammonium salts of long chain carboxylic acids.' This is the statement that is not true. Soaps are the sodium (or potassium) salts of long-chain fatty (carboxylic) acids — for example sodium stearate — not ammonium salts. The other three statements correctly describe how soap cleans, so the exception the question wants is this one.
- (a)The oil and dirt gets collected in the centre of the micelle. — This is true. Each soap molecule has a water-repelling tail and a water-attracting head; the tails point inward and trap oil and dirt at the centre of the micelle, so this correctly describes cleansing and is not the exception.
- (b)Soap micelles scatter light. — This is true. Soap in water forms micelles large enough to scatter a beam of light (a Tyndall-type effect), making the solution look slightly cloudy, so this statement is correct.
- (d)Soap forms insoluble precipitates with the calcium and magnesium ions in hard water. — This is true. In hard water soap reacts with Ca²⁺ and Mg²⁺ ions to form an insoluble scum, which is exactly why soap lathers poorly in hard water, so it is correct.
A soap molecule is amphipathic — it has a long hydrophobic hydrocarbon tail and a hydrophilic –COO⁻ head. In water many such molecules cluster into a spherical micelle with tails pointing inward and heads outward. The inward tails dissolve greasy dirt at the centre, while the outward heads keep the whole droplet suspended in water so it can be rinsed away.
Three of the four statements are standard textbook facts about micelles and hard water. The false one quietly swaps the correct metal (sodium/potassium) for 'ammonium'. Knowing that soaps are metal salts of fatty acids, made by saponifying fats with sodium hydroxide, exposes the error at once.
- Soaps are sodium or potassium salts of long-chain fatty (carboxylic) acids, e.g. sodium stearate.
- A soap molecule has a water-repelling hydrocarbon tail and a water-attracting carboxylate head.
- In water the molecules form micelles — tails inward trapping oil/dirt, heads outward facing the water.
- In hard water soap forms an insoluble scum with Ca²⁺ and Mg²⁺ ions, wasting soap and reducing lather.
- Assuming all four statements are meant to be true, and missing the deliberately false one.
- Forgetting that soaps are sodium/potassium (metal) salts, not ammonium salts.
Asked as 'which statement about the cleansing action of soap is not true?' or by testing the composition of soap and the structure of a micelle.
Assertion (A): Synthetic detergents can lather well in hard water. Reason (R): Synthetic detergents form soluble calcium and magnesium salts with hard water.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually 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(a) Both A and R are individually true and R is the correct explanation of A
Same concept — soaps versus detergents and hard-water behaviour. Soap forms an insoluble scum with Ca²⁺/Mg²⁺ (statement d here), whereas detergents form soluble salts and still lather.
Soap with water forms:
- (a) Metallotropic liquid crystal
- (b) Thermotropic liquid crystal
- (c) Homogeneous solution
- (d) Lyotropic liquid crystal
Answer(d) Lyotropic liquid crystal
Same concept — what soap forms in water (micelles), the structure behind its cleansing action tested here.
A sample of “soft soap” contains
- (a) Caesium
- (b) Potassium
- (c) Calcium
- (d) Magnesium
Answer(b) Potassium
Directly reinforces the composition point — soaps are potassium/sodium salts of fatty acids, not the 'ammonium salts' claimed in the false option.
- practice — not a real PYQ
Soaps are chemically the sodium or potassium salts of
- (a)long-chain carboxylic acids
- (b)mineral acids
- (c)amino acids
- (d)nucleic acids
Answer(a) long-chain carboxylic acids — for example sodium stearate.
- practice — not a real PYQ
Soap does not lather well in hard water because it forms an insoluble scum with
- (a)sodium and potassium ions
- (b)calcium and magnesium ions
- (c)chloride and sulphate ions
- (d)hydrogen ions
Answer(b) calcium and magnesium ions — the ions that make water hard.