Soaps clean surfaces on the principle based on

Viscosity

Floatation

Elasticity

Surface tension

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This question was previously asked in

UPSC NDA-2 – 2019

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Soaps are surfactants, meaning they reduce the surface tension of water. Water molecules have strong cohesive forces, leading to high surface tension. This makes it difficult for water to spread and penetrate fabrics or surfaces. Soaps interfere with these forces, lowering surface tension and allowing water to wet surfaces more effectively. Additionally, soap molecules have a hydrophilic (water-attracting) head and a hydrophobic (oil-attracting) tail. The hydrophobic tails surround grease and dirt particles, forming tiny structures called micelles. These micelles, with their hydrophilic heads facing outwards, are soluble in water and can be easily washed away, carrying the dirt with them. This cleaning action is primarily based on the reduction of surface tension and the emulsification/dispersion of dirt through micelle formation, both consequences of the surfactant properties related to surface tension effects.

– Soaps are surfactants.
– Surfactants reduce the surface tension of water.
– Reduced surface tension allows water to spread and penetrate effectively.
– Soap molecules emulsify grease and dirt by forming micelles.
– The fundamental principle involves the interaction of soap at surfaces (related to surface tension).

Viscosity is the resistance of a fluid to flow. Floatation is based on buoyancy. Elasticity is the ability of a material to return to its original shape after deformation. While these properties exist, they are not the primary principles behind the cleaning action of soaps. The ability to lower surface tension is key to the wetting and emulsification processes central to how soaps remove dirt.

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