MCQ Detailed View

Detergents clean clothes effectively because they reduce the surface tension of water, allowing it to wet fabrics better and loosen dirt particles. Water naturally has a high surface tension due to the strong cohesive forces between water molecules. This high surface tension prevents water from spreading easily on cloth fibers and penetrating stains deeply.

Detergents are surface-active agents (surfactants). When added to water, detergent molecules arrange themselves at the surface with their hydrophilic (water-loving) heads in water and hydrophobic (oil-loving) tails pointing away. This arrangement breaks the cohesive forces between water molecules, thereby lowering water’s surface tension. With reduced surface tension:

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  Water spreads more evenly on the cloth surface.

  
  


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  It penetrates tiny pores in fabric fibers.

  
  


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  It surrounds and lifts away dirt, grease, and oil particles.

  
  

Additionally, detergents form micelles, which are tiny spherical structures that trap grease and oil in their center. These micelles suspend dirt particles in water, preventing them from reattaching to the fabric during rinsing.

The other options do not accurately describe the main cleaning mechanism:

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  Increasing temperature of water: Hot water can improve cleaning but detergents work even in cold water.

  
  


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  Reacting chemically with dirt: Detergents do not chemically react with dirt; they physically remove it.

  
  


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  Dissolving dirt: Water dissolves some dirt, but greasy stains are removed mainly by detergents breaking surface tension and forming micelles.

  
  

Therefore, the key action of detergents is reducing the surface tension of water, enabling better contact between water, detergent molecules, and fabric fibers for efficient cleaning