Question

(a) What is a soap? Name one soap.
(b) Describe the structure of a soap molecule with the help of a diagram.
(c) Explain the cleansing action of soap. Draw diagrams to illustrate your answer.

Answer 1

(a) Soap is a mixture of sodium or potassium salts of various natural fatty acids and oils, which when used with water show cleansing properties. A soap contains a large, non-ionic hydrocarbon group and an ionic group COO^-Na⁺.
An example for a soap is sodium stearate (C₁₇H₃₅COO^-Na⁺). It is a salt of sodium of fatty acid called stearic acid with a non-ionic hydrocarbon group C₁₇H₃₅, and an ionic carboxylate group COO^- Na⁺.

(b) A molecule of soap has a tadpole like structure and is made up of two parts, a long hydrocarbon part and a short ionic part. The hydrocarbon part of the soap molecule is soluble in grease and oil but remains insoluble in water (hydrophobic), whereas the short ionic group is soluble in water (hydrophilic) because of the polar nature of the water molecules.
The structure of a soap molecule is as follows:


(c) Cleansing action of soap: Soap forms a colloidal suspension when dissolved in water. The soap molecules group together to form spherical micelles. When a soiled cloth is immersed in soap water, the hydrocarbon ends of the soap molecules in the micelles entrap the oil and grease on cloth's surface. The ionic ends of the soap molecules in the micelles (having negative charges) are attached to the water. As the soiled cloth gets agitated in the soap solution, the oil and grease entrapped by the soap micelles get dispersed into the water and the soap water becomes dirty while the cloth gets rid of the dirt and becomes clean. Cleansing of the cloth is then done by thoroughly rinsing it in clean water.

The following diagram illustrates the cleansing action of soap: