What Is Surface Tension?
QUICK ANSWER
Surface tension is the cohesive force at a liquid's surface that makes it act like a thin elastic skin. Surface molecules are pulled inward by neighbors below them, creating tension. Water has unusually strong surface tension due to hydrogen bonding between molecules, which lets insects walk on water and droplets form spheres.
Surface tension is one of the more visible properties of water, responsible for everything from water striders walking across ponds to the round shapes of dewdrops. The underlying physics involves attractive forces between water molecules that create a thin 'skin' at the surface. Understanding surface tension reveals important physics about water and explains many everyday observations.
What causes surface tension?
Surface tension arises from the imbalance of forces on molecules at a liquid's surface. Molecules in the middle of a liquid are pulled equally in all directions by their neighbors. Molecules at the surface have neighbors below and to the sides but not above (where the air is), so they're pulled inward and toward each other. This creates tension across the surface, like a stretched elastic film. The stronger the cohesive forces between molecules, the stronger the surface tension. Surface tension is what makes droplets form spherical shapes, since spheres have the smallest surface area for a given volume.
Why is water's surface tension unusually strong?
Water has exceptionally strong surface tension among ordinary liquids due to hydrogen bonding between water molecules. Each water molecule has slightly positive hydrogen atoms and a slightly negative oxygen atom, allowing the molecules to form strong electrostatic bonds with neighboring water molecules. These hydrogen bonds are much stronger than the van der Waals forces between most other liquid molecules. Water's surface tension at 20°C is about 72 millinewtons per meter, much higher than most other common liquids. This high surface tension is part of what makes water unique and essential for life.
What examples show surface tension?
Many everyday observations demonstrate water's surface tension. Water striders and small insects walk on water surfaces because the surface tension supports their weight. Raindrops and water droplets form approximately spherical shapes. Water can be piled slightly above the rim of a full glass without overflowing (the surface tension holds it up). Small objects denser than water (like needles) can float when placed carefully on the surface. Water beads up on waxed surfaces like leaves. Bubbles work because surface tension keeps the thin water films from breaking apart. Plant roots use water's cohesion to draw water upward.
How does soap affect surface tension?
Soap and detergents dramatically reduce water's surface tension by disrupting hydrogen bonding between water molecules at the surface. Soap molecules have water-loving (hydrophilic) and water-fearing (hydrophobic) ends. The molecules concentrate at water surfaces with their hydrophobic ends pointing outward, breaking up the cohesion that creates surface tension. With reduced surface tension, water can spread more easily and penetrate small spaces, helping it clean. This is why soapy water gets between dirt and surfaces effectively, while pure water just beads up on greasy surfaces.
Surface tension is the cohesive force at a liquid's surface that makes it behave like a thin elastic skin, caused by molecules at the surface being pulled inward by their neighbors. Water has exceptionally strong surface tension due to hydrogen bonding between molecules. The property explains insects walking on water, spherical droplets, and many other everyday observations. Soap reduces surface tension, enabling effective cleaning.
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