Key Concepts & Self-Assessment18 Key Facts
Review key Surface Tension: Cohesive Forces, Liquid Interfaces & Water-Walking Mechanics exam facts and rate your mastery to track revision.
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#1
Surface tension is the elastic tendency of a fluid surface that causes it to contract into the minimum possible surface area.
#2
In SI units, surface tension is measured as force per unit length in Newtons per meter (N/m) or energy per unit area in Joules per square meter (J/m²).
#3
Surface tension explains why falling liquid droplets naturally form spheres, as a sphere minimizes surface area for a given volume.
#4
The phenomenon arises from unbalanced cohesive intermolecular forces pulling surface molecules inwards toward the bulk liquid.
#5
Bulk liquid molecules experience equal cohesive forces in all directions, yielding a net intermolecular force of zero.
#6
Surface molecules lack liquid neighbors above them, resulting in a persistent net inward force that creates surface tensile stress.
#7
Water possesses an unusually high surface tension (~0.0728 N/m at 20°C) due to its extensive intermolecular hydrogen bonding network.
#8
Among common room-temperature liquids, only liquid mercury possesses a higher surface tension (~0.485 N/m) than water.
#9
Surface tension decreases progressively with increasing temperature, as thermal kinetic motion disrupts intermolecular cohesive bonds.
#10
Water striders (Gerridae) walk on water by distributing their weight across widely splayed legs without puncturing the surface film.
#11
Water strider legs are coated with grooved microscopic hairs (microsetae) that trap air cushions, imparting superhydrophobicity.
#12
The contact angle on a water strider's leg exceeds 150°, allowing it to rest upon elastic depressions formed in the water meniscus.
#13
Surfactants, such as soaps and detergents, dramatically reduce surface tension by inserting hydrophobic tails between water molecules.
#14
Adding a small drop of detergent to water causes floating insects and paperclips to sink instantly as cohesive surface forces collapse.
#15
Capillary action, the spontaneous rising or falling of a liquid within a narrow tube, is driven by the balance between cohesion and adhesion.
#16
Jurin's Law quantifies capillary rise, showing that liquid elevation is directly proportional to surface tension and inversely proportional to tube radius.
#17
Pulmonary surfactant produced in mammalian alveoli reduces alveolar surface tension, preventing lung collapse during exhalation.
#18
The Young-Laplace equation describes the pressure differential maintained across curved fluid interfaces due to surface tension.
Subject Specialist Commentary
Analytical perspective & practical exam advice from the Master10 academic board
Surface tension is the tendency of a liquid surface to behave like a stretched elastic sheet, contracting to the smallest possible surface area. Inside the fluid, cohesive forces pull molecules equally in all directions, canceling out. At the surface, however, molecules lack liquid neighbors above, experiencing a net inward pull. Water has exceptionally strong surface tension due to hydrogen bonding, allowing water striders to glide across ponds on water-repellent, hair-covered legs without sinking.
For UPSC, SSC, and State PSC science papers, surface tension explains diverse everyday phenomena. Examiners frequently test why raindrops form spherical drops, why hot soup tastes better, and how detergents wash clothes; heating or adding soap reduces surface tension by disrupting cohesive bonds. Expect questions on capillary rise under Jurin's law and pulmonary surfactants preventing lung collapse. Remember the molecular rule: "Cohesion attracts identical molecules, whereas Adhesion joins different substances."
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