Surface tension is a property of liquids that allows the surface to resist an external force, almost as if the liquid were covered by a thin, stretched elastic membrane. This phenomenon is a direct result of the cohesive forces operating between the liquid’s molecules. Surface tension is often viewed as the energy required to increase the surface area of a liquid. Compared to most familiar substances, water exhibits a remarkably high surface tension, which is responsible for many of its unique behaviors in nature and chemistry. This characteristic places water in an exceptional category when compared to other common liquids.
Water’s Unique Position Among Liquids
Water’s surface tension at room temperature is approximately 72.8 millinewtons per meter (mN/m), a value that stands near the top of the scale for non-metallic liquids. This high measurement indicates that water molecules hold onto one another with a powerful inward pull at the surface layer. A few substances do possess a higher surface tension than water, most notably the liquid metal mercury. Mercury, which is held together by strong metallic bonds, demonstrates a surface tension of nearly 500 mN/m, far exceeding water’s value. However, among the general class of organic and non-metallic liquids encountered daily, water remains the substance with the highest surface resistance.
The Molecular Basis for High Surface Tension
The high surface tension of water originates from its molecular structure and the resulting strong intermolecular forces called hydrogen bonds. A water molecule is distinctly polar, meaning the oxygen atom pulls electrons toward itself, creating a slight negative charge on the oxygen side and slight positive charges on the two hydrogen sides. This uneven distribution of charge allows the positive end of one water molecule to be strongly attracted to the negative end of a neighboring water molecule. These electrostatic attractions, known as hydrogen bonds, are significantly stronger than the cohesive forces present in most other liquids. At the surface, molecules have no neighbors above them, resulting in an unbalanced net force pulling them inward and sideways. The strong network of hydrogen bonds resists any attempt to separate the surface molecules or increase the surface area.
Comparison Examples: Water vs. Common Liquids
The difference in surface tension becomes clear when comparing water to common organic solvents, whose molecular structures do not allow for the formation of extensive hydrogen bond networks. For instance, acetone, a common solvent found in nail polish remover, has a surface tension value around 23 to 25 mN/m at 20°C. Isopropyl alcohol exhibits a similarly low value, around 22 to 23 mN/m. These organic liquids have significantly lower surface tensions because they primarily rely on weaker intermolecular forces for cohesion, such as van der Waals forces. Their surfaces are easily disrupted, which is why liquids like alcohol spread out quickly and wet surfaces much more readily than water. The quantitative difference between water’s 72.8 mN/m and the 23 mN/m range of these solvents illustrates that water’s surface is approximately three times stronger.
Practical Effects of High Surface Tension
The high surface tension of water leads directly to several easily observable phenomena in the natural world and in daily life.
Surface Resistance and Droplet Formation
One of the most famous examples is the ability of small insects, like the water strider, to walk across the surface of a pond without sinking. Their weight is spread out over their specialized legs, which do not exert enough downward force to break the cohesive molecular layer. Surface tension is also responsible for the characteristic spherical shape of water droplets, such as morning dew or rain beading up on a freshly waxed car. The strong inward-pulling cohesive forces attempt to minimize the surface area of the liquid. A sphere is the geometric shape with the smallest surface area-to-volume ratio, which is why droplets form this way. This same force allows water to form a meniscus.
Capillary Action and Detergents
Capillary action, the process by which water moves up narrow tubes or porous materials against the force of gravity, is partially driven by surface tension. This mechanism is especially important in biology, as it helps draw water upward from the soil into the roots and stems of plants. Even the use of soaps and detergents relies on this property. These cleaning agents are formulated to reduce water’s high surface tension, allowing the water to more easily penetrate and lift away dirt and grease.