Explain the cleansing action of soap, with a labelled diagram of a micelle. Why are detergents better in hard water?
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define in one line.Soap is the sodium or potassium salt of a long-chain carboxylic acid.
2 · Describe the molecule. It has an ionic, hydrophilic head that dissolves in water, and a hydrocarbon, hydrophobic tail that dissolves in oil.
3 · Explain how a micelle forms. In water, the tails gather around the oily dirt and the heads face the water, forming a micelle. Rinsing washes the dirt away.
4 · Draw the micelle. Draw the dirt in the centre with soap molecules around it, tails inward and heads outward. Label: oily dirt, hydrophobic tail, hydrophilic head, water, micelle.
5 · Hard water and detergents. Calcium and magnesium ions in hard water form scum with soap. Detergents do not form insoluble compounds with them, so they work in hard water.
6 · Link to real life. Homes with hard borewell water use detergent powders, and washing hands with soap removes oily dirt and germs.
7 · Close in one line. So soap cleans by trapping oil inside micelles, and detergents keep working in hard water.
Scoring tip: The labelled micelle carries 1 to 2 marks. Make sure the tails point into the dirt.
