Soaps clean surfaces on the principle based on
- (a)Viscosity
- (b)Floatation
- (c)Elasticity
- (d)Surface tension
Correct - D, surface tension. Soaps and detergents clean by lowering the surface tension of water. Ordinary water's high surface tension keeps it beading up and unable to wet greasy surfaces; a soap molecule, with a water-attracting head and an oil-attracting tail, reduces that surface tension so the water spreads and wets the surface, and its tails surround grease and dirt into tiny droplets (micelles) that rinse away. Reducing the surface tension is the underlying principle.
- (a)Viscosity — Viscosity is a fluid's resistance to flow; soap does not clean by changing how thickly water flows.
- (b)Floatation — Floatation - separating particles by making them float, as in mineral ore dressing - is a different process, not how soap lifts dirt from a surface.
- (c)Elasticity — Elasticity is a solid's tendency to regain shape after deformation and has no role in the cleaning action of soap.
Soaps and detergents are surfactants - surface-active agents - each with a hydrophilic (water-loving) head and a hydrophobic (oil-loving) tail. By collecting at the water surface they lower its surface tension, which improves wetting; their tails then dissolve grease and pull it into micelles that stay suspended in the water and wash off.
The keyword is that soap cleans surfaces by a wetting and detergent action, which is a surface-tension effect. Do not confuse it with viscosity or floatation, which describe unrelated properties.
- Soaps and detergents are surfactants that lower the surface tension of water.
- A soap molecule has a hydrophilic head and a hydrophobic hydrocarbon tail.
- Grease is trapped inside micelles and rinsed away with the water.
- Lower surface tension lets water wet and spread over surfaces more effectively.
- Thinking soap raises surface tension - it lowers it.
- Mixing up surface tension with viscosity, which is a different liquid property.
Asked by naming the property behind soap's cleaning action, or by describing how a surfactant works.
The tendency of a liquid drop to contract and occupy minimum area is due to
- (a) viscosity
- (b) surface tension
- (c) density
- (d) vapour pressure
Answer(b) surface tension
Same property — surface tension, the cohesive pull that minimises a liquid's surface. Soap cleans by lowering exactly this surface tension so water can wet and lift grease.
Which one of the following statements about surface tension is correct?
- (a) It increases when temperature increases
- (b) It decreases when temperature increases
- (c) It will remain same with increase or decrease of temperature
- (d) It will always decrease with increase or decrease of temperature
Answer(b) It decreases when temperature increases
Same surface-tension concept. Soap cleans by lowering water's surface tension so it wets surfaces; that question tests how the same property falls as temperature rises.
- practice — not a real PYQ
Detergents and soaps clean mainly by
- (a)increasing the surface tension of water
- (b)lowering the surface tension of water
- (c)increasing the viscosity of water
- (d)floatation
Answer(b) lowering the surface tension of water, so it wets surfaces and lifts grease.
- practice — not a real PYQ
The oil-attracting part of a soap molecule is its
- (a)hydrophilic head
- (b)hydrophobic tail
- (c)ionic charge
- (d)water of crystallisation
Answer(b) hydrophobic tail - the hydrocarbon end that dissolves grease.