Which one of the following statements about the cleansing action of soap is not true?
- A.The oil and dirt gets collected in the centre of the micelle.
- B.Soap micelles scatter light.
- C.Soaps are ammonium salts of long chain carboxylic acids.
- D.Soap forms insoluble precipitates with the calcium and magnesium ions in hard water.
Correct answer
Explanation
The correct answer is C. Soaps are sodium or potassium salts of long-chain carboxylic (fatty) acids, not ammonium salts.
A soap molecule has a long hydrocarbon tail that mixes with oil and an ionic head that faces water. In water the molecules cluster into micelles: the tails point inwards and trap oil and grease, while the heads stay outside, so rinsing carries the dirt away. Soap is made by heating fats or oils with sodium hydroxide (saponification); potassium soaps are softer.
A is wrong to choose because the oily dirt really does gather at the centre of the micelle.
B is wrong to choose because micelles are large enough to form a colloid, so they scatter light.
D is wrong to choose because calcium and magnesium ions do form insoluble scum with soap, which is why it lathers poorly in hard water.
Exam tip: soap = Na/K salt of fatty acid; micelle traps oil inside; hard water forms scum, so detergents are used there.
Practice Questions
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