What is the specific purpose of using potassium hydroxide during the saponification process?
- (a)To obtain soaps which are hard on the skin
- (b)To obtain soaps which are soft on the skin
- (c)To obtain natural fragrance
- (d)To make the saponification very economical
Correct — B, To obtain soaps which are soft on the skin. In saponification a fat or oil is boiled with an alkali. Using potassium hydroxide (KOH) yields the potassium salts of fatty acids — 'soft soaps' that are milder and gentler on the skin — whereas sodium hydroxide (NaOH) gives harder soaps. So KOH is chosen specifically to make soft soaps.
- (a)To obtain soaps which are hard on the skin — Hard soaps come from sodium hydroxide (NaOH); potassium hydroxide gives soft, milder soaps, so this is the opposite of KOH's purpose.
- (c)To obtain natural fragrance — Fragrance is supplied by added perfumes or essential oils, not by the choice of alkali.
- (d)To make the saponification very economical — Potassium hydroxide is generally costlier than sodium hydroxide; it is chosen for the softness it gives, not to cut cost.
Saponification is the alkaline hydrolysis of fats and oils to give soap and glycerol. The metal in the alkali decides the soap's hardness — potassium hydroxide produces soft potassium soaps, while sodium hydroxide produces hard sodium soaps.
The key is to link the alkali to the product: potassium goes with soft soap, sodium with hard soap. Fragrance and cost are distractors that have nothing to do with why KOH in particular is used.
- Saponification takes a fat or oil and an alkali to give soap plus glycerol.
- Potassium hydroxide gives soft (potassium) soaps; sodium hydroxide gives hard (sodium) soaps.
- Soft soaps are milder and are used in shaving creams, liquid soaps and toilet soaps.
- Soap cleans by forming micelles that trap oil and grease so they can be washed away.
- Swapping the roles of potassium hydroxide and sodium hydroxide.
- Thinking the alkali choice adds fragrance to the soap.
NDA/UPSC ask what a particular alkali does in saponification or how soap cleans — remember KOH gives soft soap and NaOH gives hard soap.
No directly related past PYQ was found.
- practice — not a real PYQ
Sodium hydroxide is used in saponification to obtain
- (a)soft soaps
- (b)hard soaps
- (c)liquid detergents
- (d)glycerol only
Answer(b) hard soaps — sodium salts of fatty acids form hard soaps, while potassium salts form soft soaps.
- practice — not a real PYQ
The by-product obtained along with soap during saponification is
- (a)ethanol
- (b)glycerol
- (c)acetic acid
- (d)chlorine
Answer(b) glycerol — hydrolysis of the fat releases glycerol together with the soap.