"Insect surface tension" Return to list of pracs

Aim: This activity is used to demonstrate how surface tension affects objects floating on the surface
of water.

Equipment: The equipment includes...  

Method: Fill a bowl to about the top with water. Place the ping pong ball in the water. Very carefully place the razor blade flat on the surface of the water so that it floats. Note that they both float. Now (very carefully) pour some detergent into the water and note the razor blade sink.

Background knowledge: Objects "float" when the amount of force pushing the object upwards equals the amount of gravity force pulling it downwards. A boat can float because the water underneath is pushing upwards on the bottom of the boat. The amount of water pushed out the way when the boat is place in the water provides the force to allow the boat to float: this movement of water is called displacement.

Another feature of liquids is "surface tension". This is the ability for objects to sit on the surface of the liquid (acting like a tight skin) without penetrating the surface itself. Some insects can skim across the liquid without getting wet or going underwater. They are too light to break through the surface tension.

When you add detergent to the water, it sits on the top of the water and reduces this surface tension. It is like stretching a balloon by hand and then punchering it with a pin ... it then splits open very easily. So the razor blade falls through the surface much more easily and then sinks because it is made of metal. The ping pong ball floats because, being full of air, it is less dense than water.

Density: It is a measure of the amount of matter packed into a set amount of space. A piece of metal the same size as a piece of wood has greater density. There is more mass in the same amount of space than the wood.