Pepper Tension

    Kids, can you make pepper flakes, paper clips, and needles “walk on water”?  Indeed you can, and here are the things you will need to make it happen: a bar of soap or liquid detergent, water, three shallow bowls or dishes, pepper flakes or talcum powder, a small string, paper clip, fork, needle, and a small plastic berry basket. 

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

    Sprinkle pepper or talcum powder on the surface of cold clean water in a shallow dish.  Allow the particles to spread out and cover the surface.  Touch a bar of soap or put a drop of liquid detergent on the water's edge at the side of the dish.  Observe what happens to the particles on the water surface.  What did the pepper flakes do when you added the drop of soap?  Did they (a) clump into a ball, (b) sink to the bottom, or (c) spread out toward the rim of the dish?  You should see (c) happen, where the pepper spreads out toward the rim of the dish.

    What’s happening here?  The water molecules on the surface are strongly attracted to each other and pull each other in all directions at once.  This creates a strong but flexible "skin" on the water's surface that is called surface tension.  As the soap or detergent dissolves, it breaks the surface tension of the water in the area of the soap.  Surface tension results from the tight bonds between the water molecules.  When these bonds are disrupted by the soap, the surface tension is broken.  The pepper flakes act as a visual indicator of this process as it is happening, as they “walk on water” towards the rim of the dish.

    In the second shallow dish, float a small loop of string in the middle of the surface of water.  Touch a bar of soap or put a drop of liquid detergent inside the loop. Observe what happens to the loop of string. The surface tension inside the loop of string should weaken by the soap but the surface tension outside the string should have pulled the string outward.

    In the third shallow dish, lower a paper clip and a needle flat onto the water surface using the fork. In this same dish, carefully set the berry basket on the surface of the water.  All three of these items should float on the water surface.  Touch a bar of soap or put a drop of liquid detergent to the water surface.  Observe what happens to the floating items.  The paper clip, needle and even the plastic basket should have floated on top of the water due to the water's surface tension.  As soon as the tension was broken by the soap, these items should have sunk to the bottom.

    What else besides soap do you think could break the surface tension?  Try different things!  NOTE: the website version of this article will show a figure of the pepper experiment. 


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    June 2006