Surface tension is a fundamental physical and chemical property of liquids that arises from the cohesive forces between molecules. It is defined as the property of a liquid surface that enables it to resist an external force, behaving as if it were covered with a stretched elastic membrane. Quantitatively, surface tension (γ or T) is measured as the force acting at right angles to a line of unit length drawn on the surface of the liquid.
- SI Unit: Newton per meter (N/m)
- CGS Unit: Dyne per centimeter (dyn/cm)
Molecular Mechanism of Surface Tension
The phenomenon of surface tension can be fully explained by analyzing the intermolecular attractive forces acting upon molecules at different positions within a liquid.
Bulk Molecules
A molecule deep inside the bulk of a liquid is surrounded by other liquid molecules on all sides. As a result, it experiences equal intermolecular attractive (cohesive) forces in all directions. The net upward, downward, and lateral forces cancel each other out, making the net attractive force acting on a bulk molecule exactly zero.
Surface Molecules
A molecule situated at the surface of a liquid is only partially surrounded by liquid molecules. It experiences downward and lateral cohesive forces from the liquid molecules below and beside it, but virtually no upward attractive force from the sparse gas molecules in the air above. Consequently, surface molecules experience a permanent net inward attractive force pulling them toward the interior of the liquid.
Surface Energy
Because of this perpetual inward pull, the molecules at the surface possess higher potential energy than those in the bulk. This excess energy possessed by the surface molecules per unit area is called surface energy. Thermodynamically, a system always seeks to minimize its potential energy. To achieve the lowest possible energy state, a liquid naturally minimizes its surface area, causing the surface layer to contract and create a state of tension.
Spherical Shape of Liquid Drops
Among all geometric shapes with a given volume, a sphere has the absolute minimum surface area. Because surface tension forces a liquid to minimize its surface area to reach its lowest potential energy state, small liquid drops spontaneously assume a perfectly spherical shape.
- Examples: Free-falling raindrops, spherical beads of liquid mercury on a flat surface, and small oil droplets suspended in water.
Factors Influencing Surface Tension
Surface tension is not a fixed constant for a liquid; it varies significantly based on temperature and the presence of dissolved solutes.
1. Effect of Temperature
Surface tension decreases continuously as the temperature of a liquid rises.
- Mechanism: Increasing the temperature raises the kinetic energy of the molecules, which significantly weakens the intermolecular cohesive forces holding them together. At a specific temperature known as the critical temperature, the intermolecular forces become negligible, and the surface tension of the liquid drops to zero.
2. Effect of Dissolved Solutes
The impact of a solute depends on how it alters the intermolecular forces at the surface layer.
- Highly Soluble Solutes (Electrolytes): When strongly soluble inorganic salts like sodium chloride (NaCl) or copper sulfate (CuSO4) are dissolved in water, the strong ion-dipole attractions between the ions and water molecules increase the overall cohesive forces. This causes the surface tension of the solution to increase.
- Sparingly Soluble Solutes: When weakly soluble organic substances like soap, detergents, phenol, or alcohol are added to water, they concentrate preferentially at the surface layer, disrupting the hydrogen-bonding network of water molecules. This causes the surface tension of the solution to decrease drastically. Substances that significantly lower the surface tension of a liquid are termed surfactants or surface-active agents.
Capillary Action (Capillarity)
Capillary action is the spontaneous movement of a liquid up or down a narrow tube (capillary tube) due to the relative strengths of two competing intermolecular forces: cohesive forces (attraction between like liquid molecules) and adhesive forces (attraction between liquid molecules and the solid walls of the tube).
Capillary Rise (Concave Meniscus)
When adhesive forces are stronger than cohesive forces, the liquid wets the solid wall and crawls upward. The liquid forms a concave (curved downward) meniscus.
- Example: Water rising inside a glass capillary tube or a narrow glass straw.
Capillary Depression (Convex Meniscus)
When cohesive forces are stronger than adhesive forces, the liquid avoids the solid wall and is depressed downward inside the tube. The liquid forms a convex (curved upward) meniscus.
- Example: Mercury placed inside a glass tube.
Real-World and Practical Applications
Cleansing Action of Soaps and Detergents
Pure water has a relatively high surface tension, preventing it from penetrating deep into the microscopic pores of fabrics or wetting greasy stains effectively. Soaps and detergents act as surfactants that drastically lower the surface tension of water. This allows the water to spread smoothly, wet the fabric completely, and emulsify and wash away trapped grease and dirt.
Use of Hot Water for Washing Clothes
Hot water is far more efficient at cleaning dirty clothes than cold water. Raising the temperature decreases the surface tension of the water, allowing it to spread much faster and penetrate deeper into fabric fibers to dissolve and dislodge dirt particles.
Antiseptics and Disinfectants
Medicinal antiseptics (like Dettol or Savlon) and disinfectants are formulated to have exceptionally low surface tensions. This property ensures that when applied to a wound, the liquid spreads rapidly and completely across the irregular surface, penetrating deep into microscopic cuts and crevices to kill bacteria effectively.
Agricultural Water Absorption
Capillary action is responsible for the transport of groundwater up through the microscopic pores of the soil, allowing the root systems of plants and trees to absorb moisture and essential dissolved nutrients. Farmers often plow their fields after rains to break up these soil capillaries, preventing groundwater from rising to the surface and evaporating into the dry air.
The “Walking on Water” Phenomenon
Several light insects, such as the water strider, can run or walk across the surface of a pond without sinking. They can do this because their extremely low body weight does not break the elastic surface film created by the high surface tension of water. Their feet merely create tiny depressions on the water’s surface without puncturing it.
Last Modified: May 25, 2026