35. What happens to soap molecules when they are added to water?
[RRB Group D (2025 recruitment cycle), Shift 3 — 25 Aug 2026]
(A) The hydrocarbon tail remains in water while the ionic heads project outwards
(B) Soap molecules dissolve randomly without any specific orientation
(C) The ionic heads remain in water while the hydrocarbon tails project outwards
(D) Both the ionic heads and tails remain completely inside water
Answer: (C) The ionic heads remain in water while the hydrocarbon tails project outwards
Explanation: Soap molecules possess an ionic, water-attracting head and a non-polar, water-repelling tail. At a water–air interface, their heads stay in water while tails point away. In micelles formed within water, however, the tails gather inward and the heads face outward.
34. Why does a soap solution appear cloudy?
[RRB Group D (2025 recruitment cycle), Shift 3 — 20 Aug 2026]
(A) Because soap micelles are large enough to scatter light
(B) Because soap molecules dissolve completely
(C) Because hydrocarbons are soluble in water
(D) Because dust remains suspended in water
Answer: (A) Because soap micelles are large enough to scatter light
Explanation: Above a certain concentration, soap molecules aggregate into micelles, which behave as colloidal particles. Their dimensions permit scattering of visible light, producing a cloudy appearance. This optical phenomenon, called the Tyndall effect, distinguishes colloidal dispersions from perfectly transparent molecular solutions.
33. The cleaning action of detergents is most effective in:
[RRB Group D (2025 recruitment cycle), Shift 2 — 11 Aug 2026]
(A) Neutral medium
(B) Acidic medium
(C) Alkaline medium
(D) Highly acidic medium
Answer: (C) Alkaline medium
Explanation: Many laundry detergents are formulated for mildly alkaline conditions. Alkalinity helps loosen fatty soils, improves certain cleaning additives, and reduces the redeposition of dirt. The precise optimal pH depends on the detergent formulation and fabric; this question refers to conventional alkaline laundry formulations.
32. Why is soap less effective in hard water?
[RRB Group D (2025 recruitment cycle), Shift 1 — 9 Aug 2026]
(A) Soap evaporates quickly
(B) Soap forms insoluble scum
(C) Soap becomes acidic
(D) Soap loses its polarity
Answer: (B) Soap forms insoluble scum
Explanation: Hard water contains calcium and magnesium ions, which react with fatty-acid carboxylate ions in soap. The resulting insoluble calcium or magnesium salts form scum. This reaction consumes soap before it can form cleaning micelles, reducing lather and washing efficiency.
31. How does saponification relate to soap formation?
[RRB Group D (2025 recruitment cycle), Shift 1 — 6 Aug 2026]
(A) Saponification produces only esters used in flavoring
(B) Saponification makes ethanol for cleaning
(C) Saponification is unrelated to soap production
(D) Saponification produces sodium or potassium salts of long-chain carboxylic acids, which are soaps
Answer: (D) Saponification produces sodium or potassium salts of long-chain carboxylic acids, which are soaps
Explanation: Saponification is the alkaline hydrolysis of triglycerides found in fats and oils. Sodium hydroxide or potassium hydroxide breaks ester bonds, producing glycerol and the corresponding fatty-acid salts. These salts are soaps, with long oil-attracting chains and water-attracting ionic groups.
30. In the saponification process, triglycerides are reacted with which alkali to produce glycerol and fatty acid salts called “soaps”?
[DSSSB Assistant Superintendent, Paper I & II, Shift 2 — 5 Jul 2026]
(A) NaOH
(B) Mg(OH)₂
(C) Ca(OH)₂
(D) Ba(OH)₂
Answer: (A) NaOH
Explanation: Sodium hydroxide is the usual alkali for manufacturing solid soap. It cleaves the three ester linkages of a triglyceride, yielding glycerol and sodium salts of fatty acids. Potassium hydroxide can instead produce softer soaps, but it is not among these options.
29. The cleansing action of soap is due to which of its properties?
[RRB Group D, Shift 1 — 23 Dec 2025]
(A) Being a strong acid
(B) Being a strong base
(C) High melting point
(D) Ability to form micelles
Answer: (D) Ability to form micelles
Explanation: Soap molecules aggregate into micelles with non-polar hydrocarbon interiors and water-loving outer surfaces. Grease becomes enclosed within these structures and is carried into the washing water. Micelle formation and emulsification, rather than high acidity or melting point, explain soap’s cleansing action.
28. Soap molecules arrange themselves in which form to perform cleaning action?
[RRB Group D, Shift 2 — 21 Dec 2025]
(A) Micelles
(B) Precipitation
(C) Crystal
(D) Colloid
Answer: (A) Micelles
Explanation: The spherical assemblies formed by soap molecules are called micelles. Their non-polar tails gather inside, where they can hold oily dirt, while charged heads remain exposed to water. Agitation then carries the suspended grease-containing particles away during rinsing.
27. What is the hydrophilic end of a soap molecule like?
[RRB Group D, Shift 3 — 15 Dec 2025]
(A) Ionic and reacts with water
(B) Neutral and non-polar
(C) Non-ionic and soluble in water
(D) Non-ionic and soluble in oil
Answer: (A) Ionic and reacts with water
Explanation: The hydrophilic portion of an ordinary soap molecule is its charged carboxylate head, represented as –COO⁻. It interacts favorably with polar water molecules. The opposite end is a long hydrocarbon tail that associates with oils and other non-polar substances.
26. Detergents can work in which of the following?
[RRB Group D, Shift 3 — 5 Dec 2025]
(A) Only distilled water
(B) Only hot water
(C) Only soft water
(D) Both hard and soft water
Answer: (D) Both hard and soft water
Explanation: Most synthetic detergents retain cleaning effectiveness in both soft and hard water. Their common sulphate or sulphonate surfactants generally do not produce the same insoluble scum as fatty-acid soaps in the presence of calcium and magnesium ions.
25. Which statement is most accurate regarding soap?
[DSSSB PGT Chemistry, Shift 1 — 22 Jul 2025]
(A) Soap micelles have hydrophobic interiors and hydrophilic surfaces
(B) Soap molecules do not contain ionic heads and non-polar tails
(C) Soap is chemically identical to synthetic detergents
(D) Soap works equally well in hard and soft water
Answer: (A) Soap micelles have hydrophobic interiors and hydrophilic surfaces
Explanation: In water, non-polar soap tails orient towards the centre of a micelle, making its interior hydrophobic. The negatively charged head groups remain outside, in contact with water. This organization stabilizes dispersed oily material and permits its removal during washing.