• Drinking glass
• Sewing needle
• Spoon
• Piece of facial tissue
• Liquid dishwashing soap

1. Take a piece of facial tissue that is slightly larger than a sewing needle and lay it on the surface of a glass of water.
2. Once the facial tissue is soaked and laying flat on the surface, quickly but carefully lay the needle on the facial tissue
3. Now, using either the handle of a spoon or your finger (if you’re careful), poke down at the each corner of the facial tissue so it sinks to the bottom. What happened to the needle? What’s keeping it from sinking?
4. Now put a tiny drop of dishwashing soap into the water... what happened to the needle and why?

The surface of any body of water behaves as though it is covered with an invisible skin which allows small, light insects to walk across it. 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 – a sphere. This is why water droplets and surfaces are curved.

Detergents lower surface tension and allow the water to coat the needle. The wet needle is now heavier than water and sinks to the bottom of the container.

Many household cleaners contain detergents. Sometimes, these detergents and others can end up in nearby bodies of water. With enough water pollution of this kind, the surface tension of these bodies of water may reduce significantly. A variety of insects and animals, like water striders, depend on the surface tension of water to survive. This is one of the many reasons why water pollution should be monitored and reduced!