Group 1 Group 2 Group 3 Group 4 Average Tap Water Soapy Water Are there any groups that had significantly larger numbers than what you were able to get? Oil will not pile up much at all, as students discover. Compare your original prediction with the number of pennies you were able to add. This is a copy of student data for the viscosity lab portion: Unfortunately, due to the inconsistencies of the viscosity lab portion, some students struggle to see a relationship between the surface tension data and the viscosity data. Instead, a hypothesis is simply a statement that speculates about the reason for an observed phenomenon, based on some prior knowledge of events or mechanisms related to the phenomenon. Repeat the experiment with other liquids. The relative strength of the film-like surface on a liquid due to the cohesive forces of its molecules. I remind students that when they are done with the graphs, to rank their substances from highest to lowest for the property.
Sadly, they are not in full color, nor could I figure out how to put the pictures in in the appropriate places : , due to some issues transferring from my computer to WordPress. It is important to check that 1 their hypotheses are consistent with their data, and 2 their tests fit their hypotheses. This should be a much more accurate measurement of viscosity, and provide much better data for the students. This lesson can serve as an introduction to the unique chemical properties of water. Life would be different without pennies. Yes No Thanks for your feedback! As a consequence, the oxygen end of the molecule is negatively charged on average, and the hydrogen ends are positively charged.
In a water molecule, the two hydrogen atoms attach to the oxygen atom at an angle of about 105 o Figure 1. The water molecules at the surface of a puddle of water have more attraction to each other than they do to the air molecules above them, so they create a dome shape as they try to stick together. You can do this by adding soap, which is a surfactant a material that decreases the surface tension of a liquid. Grade 6 Do you agree with this alignment? The left sidebar shows related lessons and curricula that link to this lesson, or that the author has explicitly recommended, as well as modified copies of the lesson if any. Yes No Thanks for your feedback! Grades 9 - 12 Do you agree with this alignment? When oil is dropped onto a flat, nonporous surface, it quickly spreads and forms a thin layer coating considerably more surface area than would a drop of water.
Surface tension is the reason you can fit so much water on the penny. Controlled aspects could include the use of same coin type and side of the coin, all tests were done at the same temperature, humidity, time of day, etc. Record the value left in your syringe in your lab notebook. They will repeat this four times with tap water, and four times with soapy water, drying the penny off between trials. Intermolecular cohesive forces cause liquid molecules to be attracted to each other and they pull liquid molecules towards each other. Have one team member hold the string so it stays submerged in the water.
Image available at through the. An important feature of a hypothesis is that it must be testable if it is to be useful. How does the droplet of water stay together? Surface tension can be measured and observed by dropping water drop by drop onto a penny. Keep careful count of the number of drops. Seal the larger bag also and leave it until the water inside the quart bag freezes. I highly recommend them for anyone looking for a little more detail. For all students, the acetone is consistent in having both the lowest surface tension and the lowest viscosity.
The paper clip will sink. The molecules will tend to align according to the diagram in Figure 2. The answer to this lies in the structure of the water molecule itself. Ask an Expert The Ask an Expert Forum is intended to be a place where students can go to find answers to science questions that they have been unable to find using other resources. Attraction of the water molecules holds the surface of the water together until the amount of water is too great and spills over the edge. Initial Observation: Observe surface tension by seeing how many drops of water can fit on a penny. Now I can tell the difference among them and explain why.
Eventually it gets big enough to reach the edges of the penny, and finally it spills over the edge. Questioning: Now that you have an idea about how soap affects surface tension. They might wonder, for instance, if other substances can affect the surface tension or if temperature has an effect. One question that arose in each class this year was if it matters which way the penny is facing. Cohesive forces are present for polarized molecules, where one end of the molecule is negatively charged and the other end is positively charged, thereby making a charge dipole. Notice the lack of rankings here, because without the averages the student wasn't sure which trial to use when ranking. This is a reasonable hypothesis, and one that is also easily tested, for example, by counting the number of drops needed to fill a small graduated cylinder to the 5 ml mark.
If you enjoy hands-on work, then you might be interested in the career of a chemical technician. In the absence of other forces, surface tension will keep a liquid together in one single volume. And if they change it to 95 cents, they will lose profit. Under the force of gravity, surface tension influences how far a liquid will stretch before it pinches off to form a drop, and is an important factor in determining the drop size. Ask an adult for help with these calculations if you need it. If you do not have any shiny new pennies, look up directions online for how you can clean pennies with vinegar. Last modified: January 9, 2019.