Science — CBSE Class 10 board question

Soaps: Soaps are sodium or potassium salts of long-chain carboxylic acids (fatty acids). Example: sodium stearate (C₁₇H₃₅COONa).

Structure of soap molecule:
A soap molecule has two parts:

• A long hydrophobic hydrocarbon tail (interacts with oil/grease)
• A hydrophilic ionic/carboxylate head (interacts with water)

Structure: CH₃–(CH₂)ₙ–COO⁻Na⁺

Cleansing action: The hydrophobic tail of soap molecules dissolves in oily dirt, while the hydrophilic ionic end faces the water. The molecules cluster around oil droplets forming structures called micelles, with the hydrocarbon tails inside (towards oil) and ionic ends outside (towards water). These micelles keep dirt suspended as a colloid and are rinsed away easily.

Problem with hard water: Hard water contains calcium and magnesium salts. Soap reacts with these ions to form an insoluble white precipitate called scum. This reduces lathering and wastes soap.

Solution: This problem is overcome by using detergents. Detergents are sodium salts of sulphonic acids or ammonium salts. Their charged ends do not react with Ca²⁺ or Mg²⁺ ions, so they remain effective in hard water.

Source: Chapter 4, Section 4.5 — Soaps and Detergents

---

• Examiners expect four distinct parts: definition, structure, cleansing action (micelle), hard water problem and its solution — cover all for full marks.
• Draw or describe the soap molecule showing the two ends clearly; labelling hydrophobic/hydrophilic is key.
• The word micelle must appear with a correct explanation of orientation.
• Scum is the keyword for hard water reaction; name detergents as the solution and briefly explain why they work.
• Avoid writing long paragraphs — crisp labeled points score better in board exams.