• Pennies
• Water
• Water droppers

1. Using a dropper carefully drop as many drops of water as possible on a penny.
2. When the water flows over the edge, start again!

The surface of the water curves because the top layer of the water acts like an invisible skin over the water, which keeps the water from flowing over the edge of the penny. This skin effect is caused by surface tension. Like molecules are attracted to each other – they like to “stick together”- and this attractive force is known as cohesion. Forces, or attraction, between unlike molecules is called adhesion.

Molecules within a liquid are attracted to all other neighbouring molecules equally so that the resultant force on any molecule is zero. Molecules at the surface of the liquid have the same molecules on one side (water) but different molecules (air) on the other side. The cohesive forces between water molecules is far greater than the adhesive forces between water and air molecules this results in a net force on the surface towards the centre of the liquid. As a result of this force the liquid assumes a shape that has the smallest surface area – that of a sphere and so the surface of the water on the penny is curved. Eventually gravity will overcome the cohesive forces and the water will flow over the edge of the penny.

Soaps and detergents interfere with the cohesive forces between water molecules, reducing overall surface tension when they are mixed with water. Pollution of water with detergents and household cleaners can significantly affect the survival of animals, insects and plants that rely on surface tension to live. For example, some plants absorb water by exploiting the surface tension of water. Some insects, like water striders, rely on the surface tension of water for locomotion.