Terry M. Bella
Activity: Cohesive Nature of Water
This is a modeling activity. You want to begin by discussing and drawing the water molecule. Noting and labeling the polar nature of the molecule. The two hydrogens are bonded to the oxygen in the middle at an angle of 104˚. The hydrogen end has a slight positive charge and the oxygen end has a slight negative charge. This occurs because the oxygen is more electronegative and is able to pull the electrons shared in each of two covalent bonds with the hydrogens closer to itself. These electrons being more closely associated with the oxygen thus results in the oxygen having a slightly negative charge the polarity of the molecule. Students should not that opposites attract and adjacent water molecules will bond to each other via the slight negative charge of one end and the slight positively charged hydrogen of another molecule. Because of the bond angle within water this can result in a very complex three dimensional bond structure in ice called the crystalline lattice. Once the students have an understanding of how water molecules bond to each other you want to have them apply this to situations.
You can have the students apply their understanding of how water bonds to water with a simple activity involving a penny and a pipette or dropper. Have the students place the penny on the lab bench and begin adding water to the penny one drop at a time. Instruct them stop once a bubble is formed. Up to 40 drops of water may accumulate, creating a convex shape, like a dome. The students then have to determine how the dome shape is achieved by the water. This is because the water drops have a higher affinity for each other than other surfaces. The attractive force of other water molecules keeps the molecules stuck together despite the force of gravity and the attractive forces of surrounding materials. Instruct the students to draw what they think the molecules of water look like and how they must be arranged.
Cohesive and adhesive behavior can also be modeled in the hallways and stairwells of the building. With each student acting as water molecule you begin at the bottom the stairs. A student climbs the stairs with one hand on the hand rail and the other holding the shirt of another student behind them. Their hands represent hydrogen atoms and their bodies are oxygen atoms. This patter can be repeated. As they climb the stairs in a sort of chain they are modeling the movement of water through a plant, against the opposing force of gravity. The adhesive and cohesive attractive forces are greater than that of the pull of gravity.
Activity: Water Potential
A water potential lab is easy to do with some dialysis tubing and common chemicals. The dialysis tubing will allow for the passage of water molecules as well as ions, but will not allow for large molecules such as sucrose to diffuse through. This is a semi-permeable membrane that will allow your students to experiment with water potential.
What you need are some solution options and about 30cm of dialysis tubing per student group. The solution options are: distilled water; 1M NaCl solution; 1M glucose solution; and 1M sucrose solution. There are other options as well, but these are the ones that I use in my class because of ease of accessibility and safety. The students work with 10cm sections of tubing, having been pre-soaked in distilled water they tie off one end, fill the tubing with a solution and tie off the other end, creating something similar to a sausage. This is then massed and immersed in a beaker of different solution for about 30 minutes. The final mass is then taken and a percent change in mass is calculated. The students will determine that because of water potential and the semi-permeable membrane, water, as indicated by a mass change, is diffusing through the membrane. For example, a dialysis tubing unit filled with distilled water that is immersed in a beaker of 1M glucose solution will lose mass over time as water moves out of the tubing unit and into the glucose solution. This happens because the water in the tubing has higher water potential than the water in solution surrounding it.
This simple activity is good practice for students not only in experimental design but also with mathematical calculations. This also provides them with a tangible example of water potential in action.
Activity: Viewing Stomata / Stomata Impression
You can easily have students create an impression of the underside of a leaf in order to more easily view the guard cells of stomates with a microscope. Stomates are the plant structure that allows for the movement of water out of a plant and the intake of carbon dioxide. The guard cells of stomates open and close in response to light, circadian rhythm, abiotic factors, biotic factors, and hormones. Terrestrial plants will have stomates located on the underside of their leaves.
This activity is a simple hands-on activity that will help garner interest in the students for plant life. Most students will not have thought about a plant having active structures on their leaves. Looking at stomates is also important because they are represent the end of the unbroken chain of water that is present within a plant. This is a common and simple activity that uses clear nail polish, tape, microscopes, and glass slides. You can find numerous iterations of the activity on the internet. I will provide the basics in the next paragraph.
You begin by asking students to select and bring a leaf from three different plants to class. This will provide students with the chance to compare stomate density among plants. On the underside of the leaf, using clear nail polish, create a thick layer of nail polish in an area about equivalent to a square centimeter. Allow the nail polish to dry completely and use a piece of clear scotch tape to remove the impression from the leaf. You do this by applying the sticky side of the tape to the area with nail polish. Peal the tape off and the nail polish impression will come with the tape. The waxy layer on the leaf surface helps prevent the nail polish from adhering to the leaf. Next lay this tape and impression on a clean slide and use a microscope at 400X to view the impression. Have students select an area of the impression that has visible stomates and count them. They can repeat the procedure with different leaf types and compare the stomate densities among them. To determine the number of stomates per mm
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multiply the number of stomates found within a viewing area by eight. This will provide the stomate density per mm
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