Surface Tension

This activity is designed for students to observe water's cohesive forces pulling the pepper from all directions and what happens when surface tension breaks. Students will use critical thinking skills and investigation in this activity and can be used to various extents, depending on the level one is teaching. Students will be investigating and learning around the concept of surface tension. Another concept students will understand is how cohesion works. Vocabulary words such as surface tension, molecules, atoms, cohesion, hydrogen bonds, and surfactants will become a part of students' vocabulary and will be discovered throughout the lesson.

This lesson can be used in various grades. The activity goes well when students are learning about surface tension, characteristics of water, molecules, and cohesion/bonds. It is an easy enough experiment that students at any grade level should be able to do it on their own. For best results, it is better if students or partners can see it happen by doing it themselves. Depending on what standard is linked with the lesson, the length will vary. The equipment for the lesson can be found in most households. This lesson can be used as an introduction to surface tension or it could be used as a review to previous concepts about surface tension and bonds.

What you need: bowl or plate, water, ground black pepper, and liquid soap.
Steps:
1. Place water in bowl/plate. Fill dish about ¾ full.
2. Shake pepper over the entire surface of the water.
3. Put a drop of liquid soap on the center of the water and pepper mixture.
For primary grades, I would introduce the concept by using glue or tape and asking my students what these two things have in common. I would hope for responses around that they are sticky and hold things together. From there I would introduce water and how it can be sticky or cohesive with other substances. However, I would put my finger in the water and show that it is not actually a sticky substance. Next, I would either demonstrate, or have pairs try the experiment. What happens in the experiment is that when the liquid soap is placed in the water/pepper, the pepper appears to scatter or run away from the soap. While students are working in pairs, I would ask them to draw the steps and what happened at each step. I would then ask them to hypothesize about why the pepper wanted to run away. After they experiment, I would have them share their results. Everyone would hopefully have the same result. I would do the experiment one more time as a class to make sure we all have the same experience. As I do the experiment, I would explain key terms and point out various terms. One thing I might point out while explaining the surface tension is how some insects can walk on the water. Next, students would have the opportunity to choose other products and try the same experiment. As a closure, I would bring everyone back to a group setting to share their results. While they are speaking, I would ask them to use as many terms as possible that they learned today.

One rule of safety in the classroom would be to wear, if available, goggles. Students should know that if they touch any of the materials, they need to wash their hands after. This way, students will not get soap or pepper in their eyes. With this experiment, one can also bring in various concepts of measurement. For instance, students can measure various amounts of pepper or water to see if that has any effect on the experiment. This experiment works because at the surface all of the water molecules are pulling toward one another and a "skin" forms at the surface of the water. Water is made up of hydrogen and oxygen molecules. The water molecules are attracted to each other, which is a hydrogen bond. This is what we call surface tension. The surface tension breaks by using soap because the soap molecule and water molecule stick, so there is no longer a chain of water molecules along the top. The soap is a surfactant, which means it weakens the hydrogen bonds, which leads to a break in surface tension. The other water molecules (except the ones around the soap) are still pulling the other water molecules, so the pepper is quickly pulled to the side.
I have not done this experiment with my class, so one thing to remember is if I want students to try the experiment more than one time with the same plate/bowl, they need to rinse it very well and start with fresh water, or the experiment will not work.

For individual assessment, I would have each student hand in their drawing of the experiment. After we learn and review the terms I would ask them to place words such as surface tension, bond, break, and other terms/concepts in their drawings. By looking at their drawings, I will be able to see if they have an understanding of the concepts.

This lesson can be done at various grades. In the primary grades (K-2) this lesson fits with I. History and Nature of Science B1. The student will observe, describe, and raise questions about the natural world and make careful observations, and seek answers. It also fits with III. Earth Science. AI. The student will observe and describe water.