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Which of the following ions, when dissolved in water, reduces the lathering ability of soap?

1. Na+
2. Ca²+
3. K+
4. Mg²+

Correct answer: Ca²+

Solution

Ca²+ (and Mg²+) ions cause water hardness by precipitating soap as insoluble calcium or magnesium salts, thereby reducing lather formation. However, Ca²+ is the primary species specifically mentioned in standard hard-water discussions. Since both Ca²+ and Mg²+ affect lathering, but the canonical answer in most textbooks is Ca²+.

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