Question

What are soaps? Write the structure of a soap molecule. Explain the cleansing action of a soap. Why are soaps not considered suitable for washing clothes in a region where water is hard? How is this problem overcome?

Hint:

Soaps are effective in soft water, but in hard water, they form soap scum. Synthetic detergents are used in hard water as they do not form insoluble salts.

Answer 1

Soaps are sodium or potassium salts of fatty acids. These fatty acids are long-chain carboxylic acids, and soaps are formed by the reaction of these acids with an alkali like sodium hydroxide (NaOH) or potassium hydroxide (KOH). Structure of a Soap Molecule:
The general structure of a soap molecule is shown below: \[ \text{CH}_3\text{(CH}_2\text{)}_{14}\text{COONa} \] - The long hydrocarbon chain (CH\(_3\)(CH\(_2\))\(_{14}\)) is hydrophobic (repellent to water).
- The carboxylate group (COONa) is hydrophilic (water-attracting). Cleansing Action of Soap: Soaps work by emulsifying oils and grease. The hydrophobic tail of the soap molecule interacts with non-polar substances like oil or grease, while the hydrophilic head interacts with water. This results in the formation of micelles, where the oils or grease are trapped in the center of the soap molecules, which are then rinsed away by water. Soap molecules surround and dissolve the grease or oil particles by forming micelles.
The hydrophilic heads of soap molecules interact with water, causing the micelles to be washed away. Problem of Soaps in Hard Water: Soaps do not work efficiently in hard water because hard water contains a high concentration of calcium (Ca\(^{2+}\)) and magnesium (Mg\(^{2+}\)) ions. These ions react with the soap molecules to form insoluble salts (soap scum), which do not dissolve in water and thus reduce the effectiveness of soap in cleaning. Overcoming the Problem: This problem is overcome by using synthetic detergents. Detergents are soap-like substances that work efficiently in hard water because they do not form insoluble salts with calcium or magnesium ions.