Science (086) — AI-generated practice question

(i) Distilled water is soft water. When soap solution is added and shaken, it produces abundant lather, because soap molecules form micelles freely in soft water.

Tubewell water is hard water — it contains dissolved calcium and magnesium salts. When soap is added to it, the soap reacts with these salts to form an insoluble white curdy precipitate (scum) with very little lather:

> Ca²⁺/Mg²⁺ ions + soap → insoluble calcium/magnesium salt (scum)

This scum is the white precipitate observed. Soap is wasted in forming scum, so little lather is produced.

(ii) When detergent solution is used with tubewell (hard) water, it produces good lather with no white precipitate. This is because detergents are sodium salts of sulphonic acids whose charged ends do not form insoluble precipitates with calcium and magnesium ions. Hence detergents remain effective in hard water, unlike soaps.

Source: Chapter 4, Section 4.5 Soaps and Detergents (Activities 4.11 and 4.12)

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• The key distinction examiners want: distilled water = soft, tubewell = hard; soap forms scum with hard water, detergent does not.
• Name the precipitate as insoluble calcium/magnesium salt (scum) — just saying "white precipitate" alone may lose a mark.
• For part (ii), explicitly state two things: lather is formed AND no precipitate forms, then give the reason (charged ends of detergent don't react with Ca²⁺/Mg²⁺).
• Avoid over-explaining micelle mechanism here — it's not asked; focus on hard/soft water behaviour.