What makes water molecules stick together to form a drop of water?
What allows those cool water bugs to walk on water?
But what is surface tension and how to you teach it to children?
According to Wikipedia, surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in the floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects (e.g. water striders) to run on the water surface. This property is caused by cohesion of similar molecules, and is responsible for many of the behaviors of liquids.
Well, I wouldn’t get more than 10 words of that definition spoken before my children’s eyes would glaze over. So I decided to forget the formal definitions and instead do a hands-on demonstration!
To demonstrate surface tension, we used a penny and found a squirt bottle that we could easily control the rate of water droplets. An medicine dropper or pipette would be ideal. We filled the top of the penny with water drops….then we added more drops….and more drops.
As my children worked on adding drops of water to their pennies, I explained to them what surface tension is.
The water molecules hold on tightly to each other. They don’t want to separate. They especially cling to each other at the surface because there is no water molecule on the other side of them to grab on to. The water molecules on the surface holds on to each other so tightly that a “skin” seems to form on the surface. The water droplets keep building on top of each other until a small dome of water forms on the top of the penny.
Of course, the moment eventually comes when the water molecules just can’t hold on any longer and they spill over on to the table.
My children were absolutely amazed with the dome of water they created on their pennies. However, they repeated the demonstration over and over again not to see the dome but to see which one of them would make the water spill on to the table first. What fun we had!