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The end of the tension
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Materials

      Glass.
      Three wood toothpicks.
 
Compounds

      Water.
      Detergent.
 
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Pictures:  








Procedure

     1. Fill with water about 3/4 of a glass.

     2. In the center of the water surface place two toothpicks side by side.

     3. Put some detergent in the tip of the third toothpick.

     4. With the wet tip touch the middle of the two flotation toothpicks.

     5. You can verify that the two toothpicks stand back quickly, as if the third toothpick has broken the connection among the two. Great!
 
Why?

The connection was broken when the third toothpick touched the center of the others. This experiment reminds the rope game, in which two people pull the rope in opposite directions. However, to complete the analogy it is necessary to introduce a third person that cuts the rope. The responsible for the connection among the toothpicks is the superficial tension. The molecules of water in the surface suffer strong attractions to the liquid bulk, not suffering the reverse attractions in the opposite direction. When adding the toothpicks to the surface, the water surface tension becomes compensated by them. As we know, the detergent has the function to lower the superficial tension of water to enable the removal of dissolved fats. When the detergent toothpick touches the center of the others, the superficial tension decreases abruptly. The toothpicks are instantly stand back to the periphery of the glass (were the surface tension is higher).
 




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