In this activity you will see how soap decreases the surface tension of water by putting water droplets on top of a penny. Observations and results You should find that plain tap water produces a much larger, stable drop of water on top of the penny than the soapy water does. This is because plain tap water has higher surface tension, so the surface is "stronger" and can hold together a larger drop.
Making water molecules stick together less is what helps soaps clean dishes and clothes more easily. This activity brought to you in partnership with Science Buddies.
Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. Key concepts Chemistry Molecules Surface tension Introduction Have you ever noticed on a rainy day how water forms droplets on a window?
Materials Penny Medicine dropper or eyedropper Glass, cup or small bowl Tap water Dish soap Dish towel or paper towel Flat, level surface that can get wet, such as a kitchen counter Paper and pencil or pen optional Preparation Place your penny on a flat, level surface that can get a little wet, such as a kitchen counter.
Fill a glass, cup or small bowl with tap water. The other end shuns water hydrophobic but attaches easily to grease. In washing, the "greasy" end of the soap molecule attaches itself to the grease on your dirty plate, letting water seep in underneath.
The property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of its molecules.
Basically, the water molecules are more attracted to each other than they are the air. This property is caused by cohesion of similar molecules, and is responsible for many of the behaviors of liquids.
Because water molecules are attracted to each other, this causes the shape of a liquid droplet. They want to be spherical to be as close to each other as possible, but when dropping gravity starts pulling them away from each other.
In the case of water bugs and other insects like water striders who are able to run on the water surface, their weight is not enough to penetrate the surface of the water caused by tension.
An object like a safety pin or needle can carefully be placed on the surface of the water and float even though it is quite heavy in relation to the water. To demonstrate surface tension , we used a penny and found a squirt bottle that we could easily control the rate of water droplets. We filled the top of the penny with water drops…. As my children worked on adding drops of water to their pennies, I explained to them what surface tension is in my own words:.
The water molecules hold on tightly to each other. What are some of the primary conditions that affect surface tension? The surface tension characteristics of a fluidic substance stay basically stable, but can be changed by temperature variations, chemicals that modify the bonding characteristics of the molecules, oxidation and the presence of impurities.
As temperature decreases, surface tension increases. Conversely, as surface tension decreases strong; as molecules become more active with an increase in temperature becoming zero at its boiling point and vanishing at critical temperature. Adding chemicals to a liquid will change its surface tension characteristics.
The effect of adding an unrelated chemical to a substance, and thereby changing its surface tension, is demonstrated by the example of putting soap a surfactant in water to reduce the surface tension, which allows the dirt on your hands to more easily mix with the water.
Oxidation directly affects surface tension.
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