The surface tension of water is caused by water molecules expanding and bonding together. Water tension can be changed by the introduction of foreign elements to the water such as common household dish soap. Teaching the physics of water tension to kids can be achieved by simple experiments with minimal materials.
Find a clear piece of plastic that can be easily held by the student. This can be anything such as a Ziplock bag--as long as it is completely transparent. Take a large drop of water (one quarter of a teaspoon) and drop it in the center of the plastic. Do not let the water fall off, but allow it to cup in the center. Have the student move the water above any printed text, such as a newspaper or a book page just an inch away and look through the water. The text will curve to the shape of the water because the tension of "film" on the top of the water bends light to it's own shape.
Fill a clear glass with water. Add a packet of salt and a packet of pepper to the water glass. Let the seasonings float a minute until they are evenly dispersed across the top of the water. Salt and pepper are not buoyant, they will be held on top of the water by the surface tension. This layer holding the seasonings afloat is called the "film" of the water. Add two droplets of dish soap to the glass directly in the middle. The seasonings will quickly glide over the water to the sides. The soap has a much more powerful surface tension and pushes the water molecules out of the way. After a time, the salt and pepper will gradually float down to the bottom as the soap over takes the surface.
Tension in the Air
Every kid likes bubbles, but they can be a great experiment. Bubbles (like those in bubble bath or in a bubble wand) are created from the mixing of soap and water and then allowing air to fill a 360 degree tension area. Fill a bucket with water. Fill a glass with one part dish soap to three parts water. Pour the glass into the bucket to create a number of large bubbles. Dip your hand into the water away from a bubble and lift it up from the underside. Wet a finger from your other hand and very slowly and lightly poke at the bubble. The bubble will move but not break because the soap and water tension levels are higher than just the water on the finger.
- Photo Credit water drop and water rings image by glgec from Fotolia.com
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