Soaps & Detergents — Homi Bhabha Class 9 Chemistry MCQs with Solutions
Free Homi Bhabha Class 9 Chemistry Soaps & Detergents MCQs with step-by-step solutions (9 questions). Part of Substances in Daily Use & Applied Chemistry. Practise online on Prepizo — no login needed.
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Questions with solutions
Q1 — Soaps & Detergents · medium · theory
Soap is chemically the sodium or potassium salt of:
A. nitric acid
B. carbonic acid
C. long-chain fatty acids ✓ Correct
D. mineral acids
Solution: Soaps are sodium/potassium salts of long-chain fatty acids.
Q2 — Soaps & Detergents · hard · theory
Soap molecules remove oily dirt by forming tiny clusters called:
A. polymers
B. ions
C. micelles ✓ Correct
D. crystals
Solution: The soap tails dissolve in oil while the heads face water, forming micelles that lift dirt away.
Q3 — Soaps & Detergents · hard · theory
Soaps do not form a good lather with hard water because hard water contains:
A. excess soap
B. no minerals
C. calcium and magnesium salts ✓ Correct
D. only sodium salts
Solution: Ca²⁺ and Mg²⁺ ions react with soap to form an insoluble scum, so little lather forms.
Q4 — Soaps & Detergents · medium · theory
Detergents are preferred over soaps for washing in hard water because detergents:
A. form scum in hard water
B. do not clean
C. dissolve only in oil
D. work well even in hard water ✓ Correct
Solution: Detergents do not form scum with hard-water ions, so they clean effectively in hard water.
Q5 — Soaps & Detergents · hard · theory
The process of making soap by reacting fats/oils with an alkali is called:
A. saponification ✓ Correct
B. neutralisation
C. polymerisation
D. fermentation
Solution: Saponification is the alkaline hydrolysis of fats/oils to produce soap and glycerol.
Q6 — Soaps & Detergents · easy · theory
Chemically, an ordinary bath soap is best described as
A. a long-chain alcohol dissolved in water
B. a simple sugar dissolved in oil
C. the sodium or potassium salt of a long-chain fatty acid ✓ Correct
D. the sodium salt of a strong mineral acid
Solution: Soap is made by heating fats or oils with sodium (or potassium) hydroxide. The product is the sodium or potassium salt of long-chain fatty acids. This reaction is called saponification.
Q7 — Soaps & Detergents · medium · theory
A soap molecule has a long hydrocarbon tail and a charged head. When it cleans oily dirt, the hydrocarbon tail
A. dissolves in the water and keeps away from the oily dirt
B. breaks the oily dirt down into carbon dioxide gas
C. makes the surrounding water evaporate more quickly
D. attaches to the oily dirt while the charged head stays in the water ✓ Correct
Solution: The hydrocarbon tail is water-repelling and dissolves in grease, while the charged head is water-loving and stays in water. Many molecules surround an oil droplet to form a micelle, which is then washed away.
Q8 — Soaps & Detergents · medium · theory
Soap does not give a good lather with hard water because
A. the calcium and magnesium ions in hard water react with soap to form an insoluble scum ✓ Correct
B. hard water contains chlorine that destroys the soap
C. soap can dissolve only in oil and not in hard water
D. hard water is always too hot for soap to act
Solution: Hard water contains dissolved calcium and magnesium ions. These react with soap to form an insoluble precipitate called scum, so a large amount of soap is wasted before any lather appears.
Q9 — Soaps & Detergents · hard · theory
Assertion: Synthetic detergents can be used for washing even in hard water. Reason: The calcium and magnesium salts of detergents are soluble in water. Choose the correct option.
A. The assertion is false but the reason is true
B. The assertion is true but the reason is false
C. Both assertion and reason are true, and the reason correctly explains the assertion ✓ Correct
D. Both assertion and reason are true, but the reason does not explain the assertion
Solution: Unlike soaps, detergents form calcium and magnesium salts that stay soluble, so they do not form scum and keep lathering and cleaning in hard water. The reason therefore correctly explains the assertion.