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An Easy Overhead Projector Demonstration of Surface Tension
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Science Area:  Chemistry

OVERHEAD PROJECTOR ACTIVITY

 

Materials:

A dish with water

Liquid Soap

Pepper

 

Procedure

 

1. Fill a dish with water and place it on the overhead projector.

2. Sprinkle pepper on the water surface.

3. Place a drop of liquid soap in the center of the dish. 

4. Observe the spreading of the pepper. 

 

*Modifications

You can also use either powdered soap or a bar of soap. 

If you use powdered soap, follow steps 1 & 2, then place a flake or two of the soap in the middle of the dish.

If you use a bar of soap, follow steps 1 & 2, then place a corner of the bar of soap in the middle of the dish.

 

 

 

Questions: 

Why is the pepper floating?

 

What made the pepper flakes move away once the soap was added?

 

What is the soap doing to the surface tension of the water?

 

Would other substances cause this same reaction?

 

Explanation:

Water is a polar substance.  This means that one end of the molecule is negative and one end is positive.  The positive end of one water molecule is attracted to the negative end of another water molecule.  This causes all of the water molecules at the surface form a film because they are all attracted to one another. This is known as surface tension.  In this demonstration, the pepper is floating upon the film.  When the liquid soap hits the water it disrupts the attraction of the water molecules. The soap has a positive charge to it and the negative poles of the water molecule are now attracted to the soap.  This disruption causes the water molecules to move taking the pepper along for a ride.