The cleansing effect of soap is low in acidic water because of

Step 1: Analyze the question We need to explain why soap works poorly in acidic water.

Step 2: The concept Soap is the sodium (or potassium) salt of a long-chain fatty acid, e.g. sodium stearate, C₁₇H₃₅COO⁻Na⁺. Its cleansing action depends on the ionized carboxylate head (C₁₇H₃₅COO⁻), which is hydrophilic, and on the long hydrocarbon tail, which is hydrophobic. Together they let soap form micelles that trap grease and dirt.

Step 3: What happens in acid Acidic water contains a high concentration of H⁺ ions. These ions react with the soap anion:

C₁₇H₃₅COO⁻ + H⁺ → C₁₇H₃₅COOH

The product, stearic acid, is a weakly ionized (essentially unionized) fatty acid. It is insoluble in water and forms a scum-like precipitate. This means:

• there is less soap anion available to form micelles and emulsify grease, so the cleansing effect falls.

Step 4: Check the options

• A. Formation of unionized fatty acid: Correct, as shown above.
• B. Increase in the pH of the soap molecules: Acidic water has a low pH, and pH is not a property of soap molecules, so this is wrong.
• C. Anti-biodegradable effect of hydrogen ions: This is not a real concept here.
• D. Hardness of the acidic water: Hardness is caused by Ca²⁺ and Mg²⁺ ions, not by acidity. That is a different problem, where insoluble calcium or magnesium soap scum forms.

Answer: A

Tip: Remember the pair: acid water → H⁺ converts soap to unionized fatty acid; hard water → Ca²⁺/Mg²⁺ form insoluble scum. Both reduce lathering, but the causes are different, and WASSCE likes to test this difference.