MCQ Detailed View

Soaps are sodium or potassium salts of long-chain fatty acids. When soap is dissolved in pure water, the soap anions remain free and help in emulsifying oils and dirt through micelle formation. However, when soap is added to hard water, which contains calcium (Ca²⁺) and magnesium (Mg²⁺) ions, a chemical reaction takes place.

In hard water, the soap anions such as sodium hexadecanoate (sodium palmitate, C₁₆ fatty acid salt) and sodium octadecanoate (sodium stearate, C₁₈ fatty acid salt) react with Ca²⁺ and Mg²⁺ ions. These cations replace sodium in the soap salt and form insoluble calcium and magnesium salts of fatty acids. These salts appear as a white sticky substance known as scum or curd.

For example:

Na(C₁₆H₃₁O₂) + Ca²⁺ → Ca(C₁₆H₃₁O₂)₂ ↓ (insoluble scum)

This insoluble precipitate reduces the cleaning power of soap because it does not dissolve in water and prevents lather formation. That is why more soap is required in hard water, and cleaning efficiency decreases.

Sodium hexadecanoate and sodium octadecanoate are the main components of natural soaps made from animal fats and vegetable oils. These long-chain fatty acid salts are most affected by calcium and magnesium ions in hard water. Sodium pentadecanoate is less common in soap composition, which is why it is not the correct choice here.

Therefore, both sodium hexadecanoate and sodium octadecanoate are the soap anions that react with Ca²⁺ and Mg²⁺ ions in hard water to form scum.

Understanding this reaction is important in environmental chemistry, as it explains water hardness problems, soap wastage, and the need for synthetic detergents. Unlike soaps, detergents are designed to work effectively even in hard water because they do not form insoluble salts with Ca²⁺ and Mg²⁺.