Penny Popper

    Kids, this column is for you really young ones, ages 5-7 or so. It is about something called surface tension. We will concentrate on water here, because water molecules really like to stick together. An electrostatic-like force attracts them. When they are near each other, they will try very hard to stay together instead of going off on their own. Even though individual molecules are too small to see, you can see how they work by watching their behavior.

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    Note to your adult partners: manipulating materials and equipment are important experiences for young children. In addition, using words to describe observations helps to develop mental muscles. To do this have them answer questions like "How?" "Why?" and "What makes you think so?" throughout this activity.

    First, practice using a medicine dropper by slowly releasing water one drop at a time and counting drop by drop. To an adult partner, predict how many drops of water you can drop onto a penny before the water runs off. (Most children will guess between 2 and 10 drops). If more than one of you are doing this activity, make it a competition to see who can get the most drops on the penny.

    Now it's time to start and test your prediction. Add drops one at a time. We'll bet that you'll be surprised how many the total can be (adults – see below). Look at the water on the penny's surface as it builds up higher. Describe its shape. The reason why so much water can be held there is due to surface tension. Water molecules hold tightly to one another on all sides. At the surface, the molecules don't have any neighbors above them to hold onto. So all of their holding power is used on molecules on their sides and below. That makes the surface act like a strong skin that can hold in a lot of the molecules.

    Try the activity again but first add a drop of dish soap to the water. The penny will hold fewer soapy drops, and the dome will be flatter. Detergent molecules lessen the "pull" between water molecules. Interestingly, a chemical word for soap that helps to describe this action is "surfactant". 


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    February 2002


    Reference:  Faith Brynie, "Water's Molecular Madness" in Scientific American Explorations magazine, Winter 2002, page 34.   Most children are excited to see the total amount go to 20 drops or more on the penny.