Why does ordinary soap lose its cleansing effectiveness and form an insoluble greyish precipitate (scum) when used with hard water?
Answer and explanation
Correct answer: C. Soap reacts with dissolved calcium and magnesium ions to precipitate insoluble salts
- Step 1: Analyze hard water composition: hard water contains high concentrations of dissolved multivalent mineral cations, primarily and .
- Step 2: Soluble sodium stearate () reacts with these ions to form insoluble calcium and magnesium stearates (scum): , precipitating out and wasting soap.
Answer
Soap reacts with dissolved calcium and magnesium ions to precipitate insoluble salts.
Why not the others: 'Hard water contains dissolved sodium chloride that dissolves the soap molecules completely' is false as hard water is characterized by calcium/magnesium, not sodium; 'Hard water is acidic and converts soap molecules into volatile flammable gases' is chemically incorrect; 'Hard water freezes the hydrophobic hydrocarbon tails into rigid crystalline lattices' is false.
Practise more Nuclear Energy, Carbon Compounds, Polymers, and Industrial Chemistry.
Free questions with an explanation for every one, and full timed SPM Malaysia mock tests.
More Nuclear Energy, Carbon Compounds, Polymers, and Industrial Chemistry questions
- What nuclear process occurs when a heavy Uranium-235 nucleus is bombarded by a slow-moving thermal neutron, splitting into two lighter daugh…
- Which physical conditions are required to initiate nuclear fusion between light isotopes of hydrogen (deuterium and tritium)?
- What is the function of boron or cadmium control rods in a commercial nuclear fission reactor?
- What is the function of the moderator, such as heavy water or high-purity graphite, inside a thermal nuclear reactor?
- In Einstein's mass-energy equivalence equation , what physical quantity does the symbol denote?
- Which radioactive isotope is widely utilized in medical oncology to deliver targeted radiotherapy to destroy malignant cancer cells?