An exploration of the properties of water would be incomplete without a discussion about the surface tension of water. Surface tension is a difficult concept for students to understand. Students often ask, “How can something liquid act like a solid?” The answer is in the behavior of the water molecule at the surface as compared to that of a water molecule within the liquid. Within the liquid, the molecule exerts a force in all directions as does all the other molecules, allowing them to “roll over” one another exerting a net force of zero (Chaplin 2010). This moving around is what makes water a liquid; it pours.
Conversely, at the surface, water exerts a force toward itself only. I do not say downward, because, in small enough quantities, water will form a complete sphere or drop. This force is exerted inward over the entire surface of the drop.
The water molecule is slightly electronegative and will attract the hydrogen end of another water molecule. This attraction is called a hydrogen bond. Since each molecule of water is attracted to another, the result is strong surface tension. So much so that water has a stronger surface tension than most other molecules (Chaplin 2010). This bond has led scientists to believe that water may not be a liquid at all but a continuum between a liquid and a solid as the hydrogen bonds between molecules break and reform (Yarris 2005). This weak bonding of the hydrogen ends of the water molecule occurs most often at the surface where there are unequal forces upon the molecules. The unequal forces push the molecule toward the solid state. It is when the molecules are on the continuum toward solid that produces the surface tension.
To break the surface tension of water, an application of sufficient force to break the weak hydrogen bonds between the molecules would be necessary. This can be done with a physical or chemical force or temperature difference sufficient to break the bonds.
To test the surface tension of water, I chose to apply a chemical force on the hydrogen bonds. An easy way to show the distribution of that force is a simple lab called “Paper Boats.” This lab is usually conducted at the elementary level to introduce the property of surface tension without an in-depth explanation of the unique bonding between water molecules. My hypothesis is that an application of dish soap to the surface of water will break the tension. The molecules of water will move to exert an equal force in all directions with the soap creating a new surface thus changing the bonds between the water molecules. This movement will create a force on the paper boats across the surface of the water propelling them forward. The distance traveled by the boat will be in direct relationship to the amount of drops of dish detergent applied.
The materials needed are a rectangular container 13.5 in. L x 11 in. W x 5.25 in. H, enough room temperature water to fill the container with ~6 cm deep of water, sharpie marker, 24 cm ruler, 2 eye droppers, additional sample of water, dish soap, 3 paper boats constructed of ordinary card stock paper (construction paper will suffice) cut into a rectangle with a triangular end. The opposite end has a small rectangle cut out in the center making the entire thing look a lot like a house.
I began my experiment by marking the dry bottom of the container at 2cm intervals beginning at the 6 cm distance from the narrow end and continuing across the container bottom. The next step is placing the boats in the water on the narrow end of the container. The boats were situated with the point end toward the center of the container and equidistant to each other. Care was taken to release a single drop of dish soap into the area directly behind the rectangular cut out portion of two of the paper boats. One boat received drops of water from the same source as the water in the container. Drops were applied to the area behind the boats in direct succession. Distance traveled by the boats was recorded once the boats stopped moving. Additional drops were applied in succession until the boats “arrived” at the other end of the container.
The boats that received the dish soap drops traveled the length of the container with movement at approximately 4-5 cm per drop. The boat that received the water drops did not move forward at all. My hypothesis was correct. The dish soap broke the surface of the water tension and propelled the boats forward.
This experiment was quite fun to do. Although it was elementary level, the kid in me really liked playing with the boats. Afterward, we tried dropping the soap at different locations around the boat to see if we could make them travel on a specific path. That would be an interesting guided inquiry extension to present to students.
The challenge I faced was that my family wanted to become involved. My spouse and children had difficulty being patient enough for me to gather my data before they began experimenting with the steering of the boats via dish soap.
In presenting this to a class as a structured inquiry, it will be important that spills are cleaned up immediately to avoid a slipping/falling hazard. Playing in soapy water is fun for a large number of students. Care should be made that the amount of soap applied to the water is not too extreme and that suds are not produced. Once the surface becomes saturated with soap, the surface tension will be completely broken and the boats will no longer move. An alternative control of the amount of soap would be to have the students just gently touch a soapy glass stir rod to the surface behind the boats.
The main concern I have about a guided inquiry with this activity, is the limited amount of class time I have. After attendance, instructions, lab set up, and lab clean up; the time for actual lab work for 38 students is approximately 20 minutes. After each student conducts the lab and records their results, there isn’t much time for exploration unless it is done during a subsequent lesson. Unfortunately, that is a luxury I do not have with my curriculum constraints. I could have smaller groups. Smaller groups mean additional supplies that tax my already limited budget. It would increase the lab set up and clean up times. I would be in the same predicament with lack of time. The crux of enrichment and extension activities is that in the “real world” classroom, time is a luxury.
Presenting the exploration of the properties of water as an open inquiry is a wonderful idea provided that students understand those properties. The weak bonding of the hydrogen ends of water molecules to each other to create a substance that behaves on a continuum between a solid and a liquid is beyond the level of the majority of my students. It is a subject of continual study in the Berkley Science lab. Until we can actually witness the behavior of these molecules, not likely because molecules vibrate beyond the light spectrum, what constitutes the classical definition of water, beyond 2 hydrogen and one oxygen, is still up for debate (Yarris 2005). And the explanation of its properties with how they relate to that molecule is a major focus of study at the collegiate level. Students below that level can only give basic responses to why when they are left to explore on their own. Its fun, but what are they really learning? That soap can move a boat? Knowing the “How” is where learning takes place. Open inquiries work best when the students know the “how” and can use that knowledge to proof that knowledge.
References
Chaplin, M. (2010, April 6). Anomalous properties of water. Retrieved April 16, 2011,
from London Southbank University London , England
Yarris, L. (2005, October 27). Water: Dissolving the Controversy. Retrieved April 16,
2011, from Lawrence Berkeley National Labroatory website: http://www.lbl.gov/Science-Articles/Archive/sabl/2005/October/03-water- contoversy.html
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