Short Stories for Teachers
Here are two easy activities for elementary students to show the concepts of Cohesion and Adhesion.
Ask, "How many drops of water do you think you can put on this penny without the it dropping off the edge?" Place the penny flat on a smooth surface. With eyedropper, drip the liquid onto the penny one drop at a time, counting the drops, until it spills off the edge.
Take the same coin and rub detergent on it.
Predict the number of drops of water you can put on the penny now, without it spilling over the edge.
Using the eyedropper, drip the liquid onto the penny and count the drops.
Water molecules have a strong cohesion, or attraction, to other water molecules. This cohesive force of the like-molecules forms the skin-like surface of the liquid, called surface tension. Soap reduces surface tension, causing less drops to stay on the penny with the soap.
"PASS THE PEPPER"
Liquid dish detergent
Fill dish with water and sprinkle some pepper on top.
Ask, "What do you think will happen to the pepper if we add detergent?"
Drop several drops of detergent into the center of the bowl.
Adhesion is the attractive molecular force that holds together UNLIKE bodies in contact. The water is pulling on the pepper evenly from all directions. The detergent reduces the adhesive force between the pepper and the liquid. The liquid around the bowl's edges, untouched by the detergent, still has its full pulling strength.
After working 20 years as an ASCP certified Med Tech, I switched careers to teach science in an inner-city high school. I've written a memoir about the joys of teaching and the obstacles I fought. No Child Left Behind? The True Story of a Teacher's Quest by Elizabeth Blake on http://www.BarnesandNoble.com and Amazon. I loved my job and my students. However, drugs, gangs, a riot, shootings, murdered students, abusive principals - all were obstacles I had to fight and overcome.
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