Which one of the following statements is NOT correct?
- (a)The molecules of soap are sodium or potassium salts of long chain fatty acids
- (b)The molecules of soap contain both hydrophobic and hydrophilic ends
- (c)Detergents are more effective than soaps in hard water
- (d)In micelles the ionic-end of the molecules is towards oil droplet while the other end faces outside
Correct — D, the statement about micelles. In a micelle it is the non-polar hydrocarbon tail (the hydrophobic end) that points inward and dissolves in the oil or grease droplet, while the ionic, water-loving (hydrophilic) end faces outward into the water. Option (d) states this the wrong way round — putting the ionic end towards the oil — so it is the statement that is NOT correct.
- (a)The molecules of soap are sodium or potassium salts of long chain fatty acids — This is correct — soaps really are sodium or potassium salts of long-chain fatty acids, such as sodium stearate, so it is not the wrong statement.
- (b)The molecules of soap contain both hydrophobic and hydrophilic ends — This is correct — a soap molecule is dual-natured, with a hydrophobic hydrocarbon tail and a hydrophilic ionic head, which is exactly why soap cleans.
- (c)Detergents are more effective than soaps in hard water — This is correct — detergents work better than soaps in hard water because their calcium and magnesium salts stay soluble and still lather, whereas soap forms insoluble scum.
A soap molecule has two ends — a long non-polar hydrocarbon tail and an ionic head. In water the molecules cluster into tiny spheres called micelles, trapping oily dirt inside so it can be washed away.
The three true statements are textbook facts; the trap simply flips the orientation of the micelle. Remember 'like dissolves like': the oily tail goes into the grease, the ionic head into water.
- Soaps are sodium or potassium salts of long-chain fatty acids; detergents are salts of sulphonic acids.
- In a micelle the hydrophobic tails point inward into the grease and the ionic heads point outward into the water.
- Detergents lather in hard water because their calcium and magnesium salts remain soluble; soaps form scum.
- The dual hydrophobic-plus-hydrophilic structure is what lets soap emulsify oily dirt.
The tail goes into the oil and the ionic head into water; option (d) has it backwards.
- Reversing the micelle orientation — the ionic head faces water, the tail faces oil.
- Thinking soaps outperform detergents in hard water — it is the other way round.
A 'which statement is NOT correct' item where the wrong option reverses the micelle's structure.
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 — soap and detergent chemistry in hard water, which is statement (c) of this NDA question. Detergents lather because their Ca/Mg salts stay soluble, unlike soap scum.
Soap with water forms:
- (a) Metallotropic liquid crystal
- (b) Thermotropic liquid crystal
- (c) Homogeneous solution
- (d) Lyotropic liquid crystal
Answer(d) Lyotropic liquid crystal
The same soap-in-water chemistry: this earlier NDA item names the micellar (lyotropic) phase soap forms in water, the very structure whose orientation the 2025 question tests.
- practice — not a real PYQ
The cleansing action of soap is due to the formation of
- (a)Micelles
- (b)Isotopes
- (c)Esters only
- (d)Colloidal metals
Answer(a) Micelles — clusters that trap grease inside and disperse it in water.
- practice — not a real PYQ
Soaps are chemically the sodium or potassium salts of
- (a)Long-chain fatty acids
- (b)Sulphonic acids
- (c)Carbonic acid
- (d)Nitric acid
Answer(a) Long-chain fatty acids (detergents are salts of sulphonic acids).