Explain the cleansing action of soap. Why do soaps not work in hard water ? (2 marks)
Cleansing action of soap : The cleansing action of soap is due to the fact that soap molecules form micelles around the oil droplets in such a way that hydrophobic part of stearate ions is in the oil droplet and hydrophilic part projects out of the grease droplet like the bristles. Since the polar groups can interact with water, the oil droplet surrounded by stearate ions is now pulled in water and removed from the dirty surface. Thus soap helps in emulsification and washing away of oils and fats.
Reason : Hard water contains calcium and magnesium ions. These ions form insoluble Ca and Mg salts. These salts act as scum. The insoluble scum sticks on the clothes, so the cleaning capacity of soap is reduced when Na or K soaps are dissolved in hard water.