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The surface tension of the water “seals” off the small holes in the mesh



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The surface tension of the water “seals” off the small holes in the mesh

Now pour over water with detergent in it. What happens?



The detergent mixture will fall through the mesh.

Why does adding detergent make it easier for the water to go through the mesh?



The detergent lowers the surface tension of the water so it is no longer “strong” enough to “seal” the holes in the mesh.
Student activities:

1. Surface Tension

Put the students into groups of at least two. Give each group an eyedropper and a five cent coin. Ask them to predict how many drops of water/soapy water will fit on the coin without spilling, they will also investigate another liquid: milk, . Distribute plastic cups of water and water+detergent (it doesn’t really matter how much detergent) and give each group two eye droppers and two five cent coins. First have the student find out if the plain water will spill off the five cent coin. After the students have a chance to observe the surface tension of the water on the coin; the surface tension of the water will form a “dome” of water over the coin – get them to draw this in their experimental note books or on the worksheet, ask them to count the number of drops of water and of soapy water they can put on the five cent coin making sure that the drop on the end of the eye dropper does not touch the liquid on the five cent coin. One student can drop and the other can count and observe. Get them to write this number in their notebooks and to repeat this 3 times, switching roles if they want. Were they surprised at how many drops they could add?
Ask them what would happen if they added a drop of detergent to the plain water on one of the five cent coins. Get them to test their prediction.




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