A sample of “soft soap” contains
- (a)Caesium
- (b)Potassium
- (c)Calcium
- (d)Magnesium
Correct — B, potassium. Soft soaps are the potassium salts of long-chain fatty acids, whereas hard soaps are the sodium salts. Potassium soaps are more soluble and give a softer, sometimes semi-liquid product — which is why they are called 'soft soap'. So a sample of soft soap contains potassium.
- (a)Caesium — Caesium is a rare, highly reactive alkali metal with no role in ordinary soap making.
- (c)Calcium — Calcium salts of fatty acids are insoluble — they form the scum produced when soap meets hard water, not a usable soap.
- (d)Magnesium — Like calcium, magnesium forms insoluble soaps (the scum in hard water); it is not the metal of soft soap.
A soap is a salt of a long-chain fatty acid made by saponification (heating a fat or oil with an alkali). The alkali sets the type: sodium hydroxide gives hard soap (the sodium salt), while potassium hydroxide gives soft soap (the potassium salt), which is more soluble and softer in texture.
Sort the options by solubility. The alkali metals sodium and potassium give soluble soaps; potassium's is the soft one. Calcium and magnesium give insoluble soaps — the scum of hard water — so they cannot be the metal of soft soap.
- Soft soap is the potassium salt of fatty acids; hard soap is the sodium salt.
- Soap is made by saponification — fat or oil heated with NaOH or KOH.
- Potassium soaps are more soluble and softer than sodium soaps.
- Calcium and magnesium soaps are insoluble and form hard-water scum.
- Choosing calcium or magnesium — their soaps are insoluble and cannot form usable soap.
- Confusing soft soap (potassium) with hard soap (sodium).
Asked as 'soft soap contains which metal' — soft = potassium, hard = sodium; calcium/magnesium give only scum.
Consider the following statements: Hard water is not suitable for I. Drinking. II. Washing clothes with soap. III. Use in boilers. IV. Irrigating crops. Which of these statements are correct?
- (a) I and III
- (b) II and III
- (c) I, II and IV
- (d) I, II, III and IV
Answer(b) II and III
Same chemistry — soaps as metal salts of fatty acids. UPSC tests that hard water's calcium and magnesium form insoluble soaps (scum), so it fails to lather; this NDA item tests that soft soap is instead the soluble potassium salt.
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 subject — the physical chemistry of soap. That NDA item tests what soap forms with water (a lyotropic liquid crystal from its fatty-acid salts); this one tests which metal makes the soft variety.
- practice — not a real PYQ
Hard soap is the ______ salt of a fatty acid.
- (a)potassium
- (b)sodium
- (c)calcium
- (d)magnesium
Answer(b) sodium — hard soaps are sodium salts, soft soaps are potassium salts.
- practice — not a real PYQ
The process of making soap by heating a fat with an alkali is called
- (a)hydrogenation
- (b)saponification
- (c)polymerization
- (d)esterification
Answer(b) saponification — the alkaline hydrolysis of a fat to give soap and glycerol.