Detergents MCQs Quiz | Class 10

This quiz is designed for Class: X students of Subject: Science (Code 086), covering Unit: Unit I: Chemical Substances – Nature and Behaviour, specifically on the Topic: Detergents. It focuses on Cleansing action and the advantage of detergents in hard water. Attempt all 10 multiple-choice questions, then click ‘Submit Quiz’ to view your score and detailed answer review. You can also download a PDF of your responses for future revision.

Understanding Detergents: Cleansing Action and Hard Water Advantage

Detergents are powerful cleaning agents that have become indispensable in our daily lives. They are synthetic substances often derived from petroleum, designed to overcome the limitations of traditional soaps, especially in hard water conditions. Their unique chemical structure allows them to effectively remove dirt, oil, and grease from various surfaces.

What are Detergents?

Detergents are generally ammonium or sulphonate salts of long-chain carboxylic acids. Unlike soaps, which are sodium or potassium salts of fatty acids, detergents have a different chemical backbone that makes them more versatile. They are broadly classified into anionic, cationic, and non-ionic detergents based on the nature of the charge on their hydrophilic part.

The Cleansing Action of Detergents

The cleansing action of detergents relies on their dual nature: they have both a water-attracting (hydrophilic) part and an oil/grease-attracting (hydrophobic) part. This allows them to bridge the gap between water and oil, which normally do not mix.

  1. Dual Nature: Each detergent molecule has a long hydrocarbon chain (hydrophobic tail) which is oil-soluble, and an ionic group (e.g., -SO3–Na+ or -COO–Na+) which is water-soluble (hydrophilic head).
  2. Micelle Formation: When detergent is added to water containing oily or greasy dirt, the hydrophobic tails of the detergent molecules orient themselves towards the oil/grease, dissolving into it. The hydrophilic heads remain outside, facing the water. This arrangement leads to the formation of spherical structures called micelles.
  3. Emulsification: The micelles trap the oil or grease droplets within their core. Because the outer surface of the micelle is hydrophilic, these dirt-laden micelles can be suspended in water and easily rinsed away. This process of breaking down large oil droplets into smaller ones, suspended in water, is called emulsification.
  4. Lowering Surface Tension: Detergents also lower the surface tension of water, allowing it to penetrate fabrics and surfaces more effectively, thereby enhancing the cleaning process.

Advantage in Hard Water

This is where detergents significantly outperform traditional soaps. Hard water contains dissolved mineral salts, primarily calcium (Ca2+) and magnesium (Mg2+) ions. These ions react with soaps to form an insoluble precipitate called ‘scum’ or ‘lather’. This scum clings to clothes, leaves residues, and reduces the soap’s effectiveness.

Detergents, however, do not form insoluble precipitates with calcium and magnesium ions. Instead, their sulphate or sulphonate groups form soluble compounds with these metal ions. This means that detergents remain effective in hard water, produce adequate lather, and do not lead to the formation of scum, ensuring cleaner laundry without mineral residues.

Comparison: Soaps vs. Detergents
Feature Soaps Detergents
Chemical Nature Sodium or potassium salts of long-chain fatty acids. Sodium salts of long-chain benzene sulphonic acids or long-chain alkyl hydrogen sulphates.
Action in Soft Water Effective, produces good lather. Very effective, produces good lather.
Action in Hard Water Ineffective, forms insoluble scum (calcium/magnesium stearate). Effective, forms soluble salts, no scum formation.
Biodegradability Generally biodegradable. Some early detergents were non-biodegradable, modern ones are largely biodegradable.
Raw Material Animal fats and vegetable oils. Petroleum fractions.

Quick Revision Points

  • Detergents are synthetic cleaning agents effective in both soft and hard water.
  • They have a hydrophobic (oil-attracting) hydrocarbon tail and a hydrophilic (water-attracting) ionic head.
  • Cleansing action involves micelle formation and emulsification of dirt.
  • In hard water, detergents form soluble salts with Ca2+ and Mg2+ ions, preventing scum.
  • Soaps react with hard water ions to form insoluble scum, making them less effective.

Practice Questions

  1. Explain why soap is not suitable for washing clothes in hard water, while detergent is.
  2. Describe the structure of a detergent molecule and label its hydrophilic and hydrophobic parts.
  3. What is a micelle? How does its formation help in the cleaning process?
  4. List two advantages of using detergents over soaps for household cleaning.
  5. Give one example of a synthetic detergent and explain why it is preferred over natural soap.