NECO 2001 Chemistry · Question 18

Detergents are better than soaps for washing in hard water because

Options

A.hard water contains insoluble calcium salts.
B.they contain bleaching powder.
C.they contain palmitic acid.
D.they do not form insoluble calcium and magnesium salts.
E.they are organic compounds.

Answer: D. they do not form insoluble calcium and magnesium salts.

Explanation written with AI and checked against the answer key

Step 1: Analyze the question We need the reason detergents work better than soaps in hard water.

Step 2: The concept

Hard water contains dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions.
Soap is the sodium salt of a fatty acid, e.g. sodium stearate, C₁₇H₃₅COONa.
Detergents are usually sodium alkyl sulphonates or sulphates (e.g. sodium dodecylbenzene sulphonate), often called soapless detergents.

Step 3: Solution When soap meets hard water, the Ca²⁺ and Mg²⁺ ions react with it to form insoluble salts called scum:

2C₁₇H₃₅COONa + Ca²⁺ → (C₁₇H₃₅COO)₂Ca↓ + 2Na⁺

This wastes soap, because it has to react with all the Ca²⁺ and Mg²⁺ before it can lather. It also leaves a grey, sticky curd on clothes.

Detergents have sulphonate or sulphate heads. Their calcium and magnesium salts are soluble, so no scum forms. They lather readily and clean well even in hard water.

Step 4: Check the options

A. Wrong. Hard water has soluble calcium salts, and this does not explain why detergents work better.
B. Wrong. Detergents are not defined by bleaching powder.
C. Wrong. Palmitic acid is a fatty acid found in soaps, not what makes detergents better.
D. Correct. Detergents do not form insoluble calcium and magnesium salts (no scum).
E. Wrong. Soaps are also organic compounds.

Answer: D

Tip: Remember "Soap + hard water = scum; detergent + hard water = no scum." If a question asks why detergents are better in hard water, look for the option about no insoluble Ca/Mg salts.

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