Science (086) — AI-generated practice question

Soap molecules are sodium or potassium salts of long-chain carboxylic acids. When added to hard water (containing dissolved Ca²⁺ and Mg²⁺ salts), the soap reacts with these ions to form insoluble calcium and magnesium salts of the fatty acid. This insoluble substance appears as a white curdy precipitate (scum), and lather is not formed until all the Ca²⁺/Mg²⁺ ions are used up.

Detergents are sodium salts of sulphonic acids or ammonium salts with chloride/bromide ions. The charged ends of detergent molecules do not form insoluble precipitates with calcium and magnesium ions in hard water. Therefore, detergents produce lather readily in hard water and remain effective as cleansing agents.

Source: Chapter 4, Section 4.5 – Soaps and Detergents

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• The key contrast examiners look for: soap → insoluble precipitate with Ca²⁺/Mg²⁺ vs. detergent → no such precipitate.
• Mention the chemical nature of both (soap = salt of carboxylic acid; detergent = salt of sulphonic acid) for full marks.
• Avoid vague phrases like "detergent is better" without the chemical reason — that's where students lose marks.