3 marksShort AnswerSection A

What is the difference between the molecules of soaps and detergents, chemically? Explain the cleansing action of soaps.

Q: What is the difference between the molecules of soaps and detergents, chemically? Explain the cleansing action of soaps.

### Chemical Difference **Soaps** are sodium or potassium salts of long-chain carboxylic acids (fatty acids). For example, Sodium Stearate. **Detergents** are generally ammonium or sulphonate salts of long-chain carboxylic acids. Unlike soaps, detergents do not form insoluble precipitates (scum) with the calcium and magnesium ions present in hard water. ### Cleansing Action of Soaps Soap molecules have two ends with different properties: 1. **Hydrophilic end:** A polar/ionic head (e.g., $-COO^-Na^+$) that dissolves in water. 2. **Hydrophobic end:** A long hydrocarbon tail that dissolves in oil or dirt. When soap is added to water containing oily dirt, the molecules arrange themselves into clusters called **micelles**. In a micelle, the hydrophobic tails attach themselves to the dirt/oil droplet at the center, while the hydrophilic heads face outwards into the water. This traps the dirt in the center of the micelle. When the water is agitated, the micelles (carrying the dirt) are pulled away from the surface and washed away.

Carbon and its Compounds

Soaps and Detergents

Official Answer

Chemical Difference

Soaps are sodium or potassium salts of long-chain carboxylic acids (fatty acids). For example, Sodium Stearate.

Detergents are generally ammonium or sulphonate salts of long-chain carboxylic acids. Unlike soaps, detergents do not form insoluble precipitates (scum) with the calcium and magnesium ions present in hard water.

Cleansing Action of Soaps

Soap molecules have two ends with different properties:

Hydrophilic end: A polar/ionic head (e.g., $-COO^-Na^+$ ) that dissolves in water.
Hydrophobic end: A long hydrocarbon tail that dissolves in oil or dirt.

When soap is added to water containing oily dirt, the molecules arrange themselves into clusters called micelles. In a micelle, the hydrophobic tails attach themselves to the dirt/oil droplet at the center, while the hydrophilic heads face outwards into the water. This traps the dirt in the center of the micelle. When the water is agitated, the micelles (carrying the dirt) are pulled away from the surface and washed away.

Sodium saltsSulphonate saltsHydrophilicHydrophobicMicelleHard waterScum

Marking Scheme

1Chemical difference (Soaps: Na/K salts of fatty acids; Detergents: Sulphonate/Ammonium salts) (1 mark)
2Explanation of hydrophilic and hydrophobic ends (1 mark)
3Description of micelle formation and dirt removal (1 mark)

Hint

Focus on the 'water-loving' and 'water-fearing' parts of the soap molecule and how they form a sphere.

Quick Oral Answer

Soaps are sodium salts of fatty acids, while detergents are usually sulphonates. Soaps clean by forming micelles where the hydrophobic tail traps oil and the hydrophilic head stays in water.

Analysis & Explanation

The cleansing action relies on the dual nature of the soap molecule. Because the ionic heads are negatively charged, the micelles repel each other and stay suspended as an emulsion in the water. This prevents the dirt from re-depositing on the clothes. Detergents work on the same principle but are more effective in hard water because their polar heads do not react with the minerals in hard water to form scum.

Common Mistakes

1Swapping the definitions of hydrophilic and hydrophobic.
2Stating that soaps work well in hard water.

Interesting Facts

The word 'hydrophobic' comes from the Greek words 'hydro' (water) and 'phobos' (fear).

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2015?

This question carries 3 marks in the CBSE Class 10 Science 2015 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Carbon and its Compounds" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Soaps and Detergents" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2015 paper?

This is a Short Answer question from Section A in the CBSE Class 10 Science 2015 paper.