Surface tension is caused by the cohesive forces between liquid molecules. Molecules inside the liquid experience balanced attractive forces from all directions, but molecules at the surface lack neighboring molecules above them, resulting in an imbalance of forces. This causes surface molecules to be pulled inward toward the liquid, creating a "skin" or surface film that resists external force and minimizes the surface area of the liquid
. At the molecular level, this inward pull arises because surface molecules have fewer neighboring molecules to bond with compared to those in the bulk. The stronger the cohesive forces, the higher the surface tension. For example, water has a high surface tension due to strong hydrogen bonding between water molecules, which are electrostatic attractions between the hydrogen atom of one molecule and the oxygen atom of another. This network of hydrogen bonds makes water's surface tension much higher than many other liquids
. In summary, surface tension results from the greater attraction of liquid molecules to each other (cohesion) than to the molecules in the air (adhesion), causing the liquid surface to behave like a stretched elastic membrane under tension
