Surface tension is a property causing the surface of a liquid to behave like a thin, stretched elastic membrane, resisting external force. This phenomenon results from cohesive forces between molecules, which pull surface molecules inward to minimize the liquid’s surface area. Water is widely known for having an unusually high surface tension compared to most other common liquids. This article explores the molecular reasons for water’s strong surface behavior and compares its properties quantitatively to other substances.
The Unique Role of Hydrogen Bonding
Water’s unusually high surface tension originates from the unique molecular structure of the water molecule (H2O). This molecule is polar: the oxygen atom strongly attracts electrons, giving it a partial negative charge, while the hydrogen atoms carry partial positive charges. This polarity allows a single water molecule to form strong, temporary attractions, known as hydrogen bonds, with neighboring water molecules.
These hydrogen bonds are significantly stronger than the van der Waals forces that dominate cohesive interactions in many other liquids. Deep within a mass of water, each molecule is pulled equally in all directions by these bonds, resulting in a net force of zero. However, molecules at the liquid-air interface are only attracted to molecules below and beside them.
This imbalanced attraction creates a powerful net inward force that pulls the surface molecules toward the bulk of the liquid. The energy required to overcome this cohesive force to increase the liquid’s surface area is what is measured as high surface tension. This mechanism gives the water surface a resilient, skin-like quality, allowing small objects or insects to rest upon it.
Quantitative Surface Tension Comparisons
Water’s surface tension stands at approximately 72.8 millinewtons per meter (mN/m) at 20°C, a value that serves as a high benchmark for most common substances. Liquids that lack the capacity for extensive hydrogen bonding exhibit significantly lower values. For example, acetone, a common organic solvent, has a surface tension of about 23.7 mN/m, while ethanol is even lower at roughly 22.1 mN/m.
These non-hydrogen-bonding liquids rely on weaker intermolecular forces, resulting in much lower cohesive energy at their surfaces. Water’s hydrogen bonds provide more than three times the surface resilience of these common solvents. Other organic liquids, like the hydrocarbon n-hexane, register values as low as 18.4 mN/m, highlighting the weak molecular attraction in non-polar compounds.
Only a few liquids exceed water’s surface tension, most notably mercury. As a liquid metal, mercury’s cohesive forces are generated by strong metallic bonds, not hydrogen bonds. This results in a high surface tension value, nearing 487 mN/m at 15°C, causing it to bead up dramatically on surfaces. Water possesses the highest surface tension of all common non-metallic liquids.
External Factors That Change Water’s Surface Tension
The surface tension of water is a dynamic property that changes predictably in response to external conditions. Increasing the temperature of water causes the kinetic energy of the molecules to rise. This increased motion partially overcomes and weakens the hydrogen bonds, leading to a reduction in the cohesive force at the surface. For instance, the surface tension of water drops significantly between 0°C and 100°C.
Adding a surfactant, such as soap or detergent, drastically lowers surface tension. Surfactant molecules are amphiphilic, possessing a water-attracting (hydrophilic) head and a water-repelling (hydrophobic) tail. These molecules migrate to the air-water interface and align themselves, inserting their hydrophobic tails between the cohesive water molecules. By displacing the strong water-water hydrogen bonds with weaker water-surfactant interactions, the surface film is easily broken, which is why soap is effective for cleaning.
Conversely, dissolving certain salts, such as sodium chloride, can slightly increase the surface tension of water. This effect is known as “salting-out” and occurs because the charged salt ions strongly attract and organize water molecules around them. This intense attraction pulls water molecules away from the surface layer and into the bulk solution. This leaves a more concentrated and tightly-packed layer of water molecules at the interface, resulting in a slight strengthening of the remaining surface’s cohesive forces.