What Causes Surface Tension in Water?

The forces that attract water molecules to one another cause surface tension. Water molecules can form hydrogen bonds with each other, and each molecule is attracted to those around it. Molecules at the surface are more attracted to water than the surrounding air, creating surface tension.

The bonds formed by the water molecules at the surface of a volume of water are strong enough to create some unique effects. When water falls through the air in small amounts, instead of falling in a stream, the water tends to form spherical droplets due to the surface tension created by hydrogen bonds. As the water falls through the air, gravity distorts the sphere into the familiar raindrop shape. The same property allows objects that are light enough to float on the surface of the water without breaking the surface. Some insects, for instance, can run across the surface of still water due to the fact that their weight is insufficient to break the tension created by hydrogen bonds on the surface.

The reason hot water seems to wash clothes and dishes more completely than cold water is due to surface tension. Heated water molecules vibrate and move around, reducing the surface tension and allowing the water to soak into soiled areas of clothing or dishes. Adding soap or detergent further decreases the surface tension, allowing the water to serve as a powerful solvent.