Read the following and answer the questions that follow:
A chemistry teacher showed students two test tubes — one containing distilled water and another containing hard water collected from a hand pump. To each test tube, she added a few drops of soap solution and shook them vigorously. The distilled water produced a large, stable lather, while the hard water produced very little lather and instead formed a white curdy precipitate. She then repeated the experiment using detergent solution instead of soap. This time, both test tubes produced good lather and no curdy precipitate was observed.
(i) What is the chemical nature of the white curdy precipitate formed when soap is used with hard water? (1 mark)
(ii) Why does distilled water produce lather easily with soap but hard water does not? (1 mark)
(iii) Explain why detergent is effective in hard water but soap is not. (1 mark)
(iv) A student suggests using soap solution (instead of detergent) to test whether a given water sample is hard. Is this a valid test? Give a reason. (1 mark)
Generated by claude-sonnet-4-6 · 2026-06-26 01:08 · grounding rag
Model Answer
(i) The white curdy precipitate is an insoluble calcium or magnesium salt of the fatty acid (soap). It is formed when soap reacts with calcium and magnesium salts dissolved in hard water.
(ii) Distilled water does not contain calcium or magnesium salts, so soap lathers easily. Hard water contains these salts, which react with soap to form insoluble scum, leaving little soap available to form lather.
(iii) Detergents are sodium salts of sulphonic acids. Their charged ends do not form insoluble precipitates with calcium and magnesium ions in hard water, so they remain effective and produce good lather, unlike soap.
(iv) Yes, it is a valid test. Soap produces a white curdy precipitate (scum) with hard water and no stable lather, while it forms a good lather with soft water. Detergent cannot be used for this test since it lathers equally in both types of water.
Source: Chapter 4, Section 4.5 — Soaps and Detergents
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Explanation
- (i) Examiners expect you to identify the precipitate as insoluble calcium/magnesium soap (salt of fatty acid). Saying just "scum" without the chemical nature may lose the mark.
- (ii) The key contrast is presence vs. absence of Ca²⁺/Mg²⁺ ions — link these directly to scum formation.
- (iii) The critical point is that the charged ends of detergent molecules do not react with Ca²⁺/Mg²⁺ to form precipitates — state this explicitly.
- (iv) The textbook itself asks "Would you be able to check if water is hard by using a detergent?" — the answer is no for detergent but yes for soap. Make sure you give a reason, not just yes/no.