Which one of the following statements is NOT correct?
- A.The molecules of soap are sodium or potassium salts of long chain fatty acids
- B.The molecules of soap contain both hydrophobic and hydrophilic ends
- C.Detergents are more effective than soaps in hard water
- D.In micelles the ionic-end of the molecules is towards oil droplet while the other end faces outside
Correct answer
Explanation
The correct answer is D. In a micelle it is the long hydrocarbon tail that points into the oil droplet, while the ionic end faces outward into the water, so statement D reverses the real picture.
A soap molecule has two ends. The hydrocarbon tail is hydrophobic and dissolves in oil and grease; the carboxylate head is ionic and hydrophilic, so it stays in water. In soapy water the molecules cluster into spheres called micelles, tails inside holding the dirt and heads outside facing water. Rinsing then carries the micelles, and the dirt, away.
A is correct: soaps are sodium or potassium salts of long-chain fatty acids such as stearic acid.
B is correct: this two-ended structure is what lets soap link oil and water.
C is correct: detergents (sulphonates) do not form insoluble scum with the calcium and magnesium ions of hard water, unlike soaps.
Exam tip: micelle = tail in (oil), head out (water); detergents work in hard water, soaps form scum.
Practice Questions
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