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Exploring Surface Tension
by Anjali Amit

Have you ever seen a paper clip float on water? Sounds impossible, doesn't it? The answer lies in surface tension.

Surface tension is caused by the water molecules on the surface sticking together to form an invisible skin. Each water molecule bonds with its neighbors to the right and to the left, above and below. The molecules on the surface, however, have only air above them, so they cohere to each other even more strongly. This forms the elastic skin on the surface.

Here are some experiments to demonstrate surface tension.

Experiment One
     What you need:
     A bowl of water
     A metal paper clip (not plastic-coated)
     Any liquid soap

What to do:
     Fold the center part of the paper clip so that it is vertical.
     Shape the bottom of the paper clip into a complete loop. Make sure that it is smooth and flat.
     Hold the paper clip by the vertical end.
     Lower it gently. Take care that your fingers do not touch the water surface.

What Happens:
     The paper clip floats.

Why:
     Surface tension! The "stretchy skin" keeps the paper clip on the surface.

This website demonstrates the experiment:
http://www.expertvillage.com/interviews/surface-tension.htm

Extension of the Experiment
     Put a drop of liquid soap in the water.

What Happens:
     The paper clip sinks.

Why:
     The soap breaks the bond between the molecules. This lowers the surface tension, and the paper clip sinks.

Experiment Two
What you need:
     A tub or sink of water
     Index card
     Scissors
     Liquid soap
     A dropper and a small cup

What to do:
     Out of an index card, cut a small rectangle about ¾" inches wide and 1½" inches long.
     Cut out a 'V'-shaped notch in the center of any side.
     Fill the small cup with liquid soap. Insert the dropper into the cup and fill it with the liquid soap.
     Place the small rectangle onto the water. Surface tension will keep it afloat.
     Gently put a drop of liquid soap at the notched opening.

What Happens:
     The paper zips across the water like a boat.

Why:
     The liquid soap breaks the surface tension. The soap molecules try to spread but are, in the beginning, confined to the opening. As they stream out the back, the paper is propelled forward.

Experiment Three
What you will need:
     A can or large bottle
     A tool (like an awl) for making holes
     Sink with water faucet

What to do:
     Make three to five holes in a straight line, closely spaced, on the bottom of the bottle or can.
     Hold the bottle or can over the sink and turn on the faucet to fill it. Water will flow out in separate streams from the holes.
     Now draw your hand across the water flow. Pretend you are pinching the water together.

What Happens:
     The separate streams join together so the water now flows out as one stream.

Why:
     Remember "sticky water"? When you draw your hand across the water, the molecules from one stream are attracted to the molecules from the other.
     They continue to flow together as a single stream.

Conclusion
     We may think of surface tension as the binding agent that holds the water molecules together. Surface tension makes water sticky and stretchy-skinned. It causes water to behave in distinctly "unwater-like" ways.

Bibliography
Haslam, Andrew. Science Fairs: Ideas and Activities. Chicago: World Book, 1998.

Internet
http://physics.about.com/od/physicsexperiments/a/surfacetension_4.htm
http://www.hwd.com/kids/tension.pdf
http://www.exo.net/~jillj/activities/surfacetension.pdf
http://www.epa.gov/OWOW/NPS/kids/TENSION.HTM

Anjali is a children's book author whose first book,"Folk Tales From Rajasthan", was published when she was in college. Upon graduation she "sold her soul to Mammon"-went to work for a bank. She writes fiction, non-fiction, and edits technical documents. Her articles have appeared in various magazines. "Bedtime Stories from Around the World", her second book, was published a few years ago.