If one mixes up ashes with animal fat, the substance received in the crude form is called
- (a)Pheromone
- (b)Soap
- (c)Cement
- (d)Concrete
Correct — B, soap. Wood ash is rich in potassium carbonate, and leaching it with water gives an alkaline liquid — the old word for it is lye. Boil an animal fat or vegetable oil with that alkali and the fat, which is chemically an ester of glycerol with long-chain fatty acids, is split apart: the alkali takes the fatty acid part as its salt, and glycerol is set free. That salt of a fatty acid is soap. The reaction is called saponification, and it is very likely the oldest deliberate chemical synthesis in human history — the ash-and-fat recipe in the question is precisely how it was made for several thousand years before industrial alkali became available.
- (a)Pheromone — A pheromone is a chemical signal that an animal releases to influence the behaviour of others of its species, such as an ant's trail marker. It is a biological messenger, not something you make by boiling fat with an alkali.
- (c)Cement — Cement comes from burning limestone with silica-bearing clay at very high temperature in a kiln, and contains no fat of any kind. The only shared word here is 'ash', and coal ash used in some cements is a completely different material from wood ash.
- (d)Concrete — Concrete is not a chemical product at all but a mixture — cement, sand, coarse aggregate and water. Since it is made from cement, ruling out option (c) rules this one out with it.
Fats and oils are triglycerides, esters formed from glycerol and three fatty acid molecules. Heating them with a strong alkali hydrolyses the ester links and produces the alkali metal salts of the fatty acids, together with free glycerol. That hydrolysis is saponification. The soap molecule that results has two very different ends — a long hydrocarbon tail that will mix with oil and grease, and an ionic head that dissolves in water. When soap is stirred into dirty water, the tails bury themselves in the oily dirt and the heads face outward, forming clusters called micelles that lift the grease away and hold it in suspension.
The question is really testing whether you recognise a chemical reaction described in everyday words. Ash supplies the alkali, animal fat supplies the ester, and the two together give soap — 'in the crude form' because the ash-lye method gives a soft, impure product with the glycerol still mixed in, unlike modern soap made with measured amounts of sodium or potassium hydroxide. Which alkali is used decides the kind of soap: sodium hydroxide gives hard soap, and potassium hydroxide, the alkali that wood ash naturally supplies, gives soft soap.
- Saponification is the alkaline hydrolysis of a fat or oil, giving soap and glycerol.
- Wood ash contains potassium carbonate, and leaching it gives the alkaline solution historically called lye.
- Sodium hydroxide yields hard soap; potassium hydroxide yields soft soap, which is gentler on the skin.
- A soap molecule has a water-attracting ionic head and an oil-attracting hydrocarbon tail, and cleans by forming micelles around greasy dirt.
- Soap fails in hard water because calcium and magnesium ions precipitate it as an insoluble scum; synthetic detergents avoid this by forming soluble salts instead.
- Being distracted by the word 'ashes' into thinking of cement or fly ash; wood ash here is simply the alkali source.
- Confusing saponification with esterification — one breaks the ester with alkali, the other builds it from an acid and an alcohol.
- Assuming all soap is sodium soap; potassium soap is what the ash route actually gives.
NDA returns to soap year after year — sometimes as this history-flavoured recognition item, sometimes on the cleansing mechanism, sometimes on the alkali used — so hold the reaction, both products and the hard-against-soft distinction together.
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
Carries the practical limitation of the soap this question makes — the fatty acid salt is precipitated by the calcium and magnesium in hard water, which is why washing is on the list and drinking is not.
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
Answer(b) To obtain soaps which are soft on the skin
Names the reaction and the alkali choice directly. Since wood ash supplies potassium, it explains why the crude ash-and-fat product of this 2016 item comes out as a soft soap.
A sample of “soft soap” contains
- (a) Caesium
- (b) Potassium
- (c) Calcium
- (d) Magnesium
Answer(b) Potassium
The same hard-against-soft distinction reduced to a one-word recall, and a neat confirmation that the ash route gives potassium soap.
- practice — not a real PYQ
Besides soap, the other product formed when a fat is boiled with an alkali is
- (a)ethanol
- (b)glycerol
- (c)acetic acid
- (d)urea
Answer(b) glycerol — the alkali splits the triglyceride, taking the fatty acids as their salts and releasing glycerol.
- practice — not a real PYQ
Soap does not lather well in hard water because
- (a)hard water is acidic
- (b)calcium and magnesium ions precipitate the soap as an insoluble scum
- (c)hard water has a high boiling point
- (d)soap dissolves only in distilled water
Answer(b) calcium and magnesium ions precipitate the soap as an insoluble scum — synthetic detergents avoid the problem by forming soluble salts with the same ions.