Answer: Calcium and magnesium salts of sulphonic acid detergents remain soluble in water, unlike the calcium and magnesium salts of fatty acids found in soap.
- A Hard water ions react with detergents to form a precipitate that helps rather than hinders their cleaning action as frequently documented in most reference accounts
- B Calcium and magnesium salts of sulphonic acid detergents remain soluble in water, unlike the calcium and magnesium salts of fatty acids found in soap
- C Detergents carry a positive charge on their hydrophilic head, which limits how readily they react with calcium or magnesium ions under normal conditions
- D Detergents have a shorter hydrophobic hydrocarbon chain, which weakens their ability to form micelles in water as generally observed in typical laboratory settings
Correct answer: B. Calcium and magnesium salts of sulphonic acid detergents remain soluble in water, unlike the calcium and magnesium salts of fatty acids found in soap
Explanation: Soap's calcium and magnesium salts are insoluble and form scum, but the corresponding calcium and magnesium salts of detergents' sulphonate groups remain soluble in water, so detergents keep working in hard water.
The hydrophobic hydrocarbon tail of each soap/detergent molecule dissolves into an oily dirt droplet, while the hydrophilic (ionic) head stays in the surrounding water - trapping the grease inside a micelle that rinses away with water.
Concept context
Drugs and how they interact with biological targets, chemicals in food, and cleansing agents (soaps and detergents) - chemistry applied directly to daily life and healthcare.