Surface Tension

Have you ever seen a water bug walking on top of a pond? It can do this because of surface tension. Water molecules at the surface are pulled together tightly, creating a thin film. This invisible skin is strong enough to support light objects like insects, paper clips, and water droplets.

Why It Happens

Water molecules are attracted to each other. Inside a body of water, each molecule is pulled equally in all directions by its neighbors. But molecules at the surface have no water above them. They are only pulled sideways and downward. This creates a tight layer at the surface that acts like a stretched elastic sheet.

Surface Tension in Action

Surface tension is why water forms round drops instead of flat puddles. It is why you can fill a glass slightly above the rim without spilling. Water strider insects take advantage of surface tension to walk on water. Adding soap to water breaks the surface tension, which is why soap bubbles pop.

Fun Facts

• A carefully placed paper clip can float on water because of surface tension, even though steel sinks.
• Soap reduces surface tension, which is why soapy water spreads more easily than plain water.
• Some lizards can run across water so fast that surface tension helps support them.

Did You Know?

Raindrops are not actually teardrop-shaped as they fall. Surface tension pulls them into nearly perfect spheres. They only flatten out slightly at the bottom due to air resistance as they fall!