Does Soap Affect The Surface Tension Of Water

Furst, John-Nicholas Ehlers, Raymond

Does Soap Affect the Surface Tension of Water? Background: Surface tension refers to water's ability to "stick to itself". Surface tension can be measured and observed by dropping water (drop by drop) onto a penny. The number of water drops that can fit on a penny will surprise you. Question: How does soap affect the water’s surface tension? Hypothesis: I think that the surface tension of soapy water will be less than that of fresh water because H20 has strong polar bonds, and when water is mixed with soap the polar bonds which help to bind the water together are weakened, thus lessening the surface tension. Materials: pipette, liquid soap, water, beakers, penny, graduated cylinder, paper towels. Procedure: 1. I received a beaker filled with tap water. 2. I took a pipette and filled it up with water from the beaker. 3. I began to add drops of water to a penny, which was heads up, every one second from a height of ½ centimeter and counting how many drops I had put on it. 4. After I added the drop that caused the water to flow over, I recorded how many drops stayed on the penny, and I then dried the penny with a paper towel. 5. I repeated steps two through four, four times. 6. I took 100 ml of water and poured it into a beaker. 7. I got 3 ml of soap. 8. I added the 3 ml of soap to the beaker and stirred the solution 40-50 times with a fresh pipette. 9. I filled the pipette with the soapy water solution and began to start adding drops of the solution to the same penny, from the distance of ½ centimeter, at a rate of one drop per second. 10. After adding the drop that caused the soapy water to flow over, I recorded how many drops stayed on the penny, and I washed the penny under running water and dried it with a paper towel. 11. I repeated steps nine and ten, four times.

Data: Drops of Tap and Soapy Water Applied to a Penny Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Drops of Tap Water 35 27 10 15 Drops of Soapy Water 20 16 12 25

39 13

Average 25.2 17.2

Compairison of Soapy Water vs. Tap Water 30

25

Average Number of Drops

20

15

10

5

0 Tap Water

Soapy Water Type of Water

Analysis: I averaged the results I got from my five trials for both tap water and soapy water by using the average function in Microsoft Excel. Conclusion: I have accepted my hypothesis because of the results of the data from this lab. In my hypothesis I stated that the surface tension of water is higher than that of soapy water. My data supports this hypothesis because the average drops of tap water the penny could hold was 25.2 while the average drops of soapy water was 17.2 drops. This shows that soapy water has a lower surface tension, thus making it not able to hold as many drops of soapy water could on the penny. This relates back to the way soapy water and tap water act in everyday situations. The soap causes the surface tension to be less than that of water so if you poured soapy water onto a kitchen counter the solution would disperse as broadly as it could over the entire counter. Tap water on the other hand when spilled on a counter will naturally try to come together in water colonies. The molecules in the tap water will move towards each other and stack on top of each other thus leaving some parts of the counter dry and some parts with canals of water. I noticed in my data that in trials 3 and 4 that the penny held more drops of soapy water than tap water. This was not in accord with my hypothesis, and I think it may have been due to a weakness in the experiment. When I used the pipette to suction water and then drop it onto the penny I

may not have been doing it exactly the same way each time. Human imprecision may have impacted this experiment. For a further understanding of this topic I would suggest redoing the experiment using different solutions, temperatures, surfaces, height of drops and a precise mechanical devices to apply the drops.