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How does soap remove oily dirt?

Class 10 ScienceNCERT chapter: Carbon and its Compoundsइसे हिंदी में पढ़ें

Exam-ready answer

Soap molecules have two ends: a hydrophobic (water-repelling) hydrocarbon tail that dissolves in oil, and a hydrophilic (water-attracting) ionic head that dissolves in water. Many soap molecules surround an oil droplet, tails inward and heads outward, forming a micelle. This emulsifies the oil, which is then washed away with water.

Explanation

Soap is a sodium or potassium salt of a long-chain carboxylic acid (fatty acid). Its molecule has two different ends. The long hydrocarbon chain is hydrophobic—it repels water but dissolves in oil and grease. The ionic end (–COO⁻Na⁺) is hydrophilic—it attracts water. This dual nature makes soap act as an emulsifying agent.

When soap is added to water containing oily dirt, the hydrophobic tails dissolve in the oil droplet while the hydrophilic heads stay in water. Many soap molecules arrange themselves around the oil droplet, forming a spherical aggregate called a micelle. In a micelle, the oil is trapped inside and the ionic heads face outward.

The negatively charged heads on the micelle surface repel each other, so micelles do not come together. They remain suspended in water as an emulsion. When the cloth is rinsed, the micelles carrying the oily dirt are washed away with water. Thus soap removes oily dirt by micelle formation.

Points to remember

  1. Soap is a sodium/potassium salt of a long-chain carboxylic acid.
  2. Soap molecule has a hydrophobic hydrocarbon tail and a hydrophilic ionic head.
  3. Micelle: a spherical cluster of soap molecules with oil trapped inside and ionic heads outside.
  4. Micelles stay suspended in water due to repulsion between negatively charged heads.
  5. Soap works by emulsification; it forms micelles that trap oily dirt and are rinsed away.

Exam tip

Draw a labelled diagram of a micelle showing the oil droplet, hydrophobic tails, and hydrophilic heads. Write the definition of soap and micelle clearly.

Questions students also ask

What is a micelle?
A micelle is a spherical aggregate of soap molecules in water. The hydrophobic tails point inward, trapping oil or dirt, while the hydrophilic ionic heads point outward into water. Micelles keep the dirt suspended so it can be washed away.
Why does soap not work well with hard water?
Hard water contains calcium and magnesium ions. These react with soap to form an insoluble scum (calcium/magnesium salt of fatty acid). So soap is wasted and cleaning becomes difficult. Detergents work better in hard water.
How is soap different from detergent?
Soap is a sodium/potassium salt of fatty acids and forms scum with hard water. Detergents are sodium salts of sulphonic acids or ammonium salts; they do not form scum and work well even in hard water.

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