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.
Phase Change of Water and the Energy Involved
This is a demonstration to be done in front of the class. The main demonstration will show the rapid condensation of water vapor to liquid and the resulting contraction of volume. This requires a 1000ml Erlenmeyer flask with a neck spout, a 2’ vinyl hose of 3/8” ID, and a rubber stopper for the flask. Add about 200ml of water to the flask, attach the hose to the neck and insert the rubber stopper on top of the flask. Place the flask onto a hot plate and heat until boiling. The other end of hose must be submerged in a second vessel of cold water. As the water boils the resulting water vapor will expand, the only path available will be through the hose resulting in bubbling of the second container. Allow the water boil for a minute and then turn off the heat and remove the flask from the heat, place it on the lab bench. As the flask and water cools it will draw cold water back up the hose from the second vessel. As soon as the cold water reaches the hot flask it will rapidly cool the hot flask causing the water vapor to condense immediately. This will in turn suck the remaining cold water up the hose in an explosive manner.
While doing this demonstration students will discuss in pairs what is happening with energy, phase change, and molecular movement. You will want them to identify where energy is being added and what that energy is doing to the liquid water. They will be able to observe that the boiling water is the phase change of water to a gas that requires more volume. They may first mistake the bubbles in the second cold vessel as boiling. This is not boiling, it is merely the gas escaping the hot flask through the only path allowed. You will want them to identify what is happening with energy when the cold water reaches the hot flask. At this point energy is being absorbed by the cold water. This energy is coming from the water vapor, causing this water vapor to condense to a liquid. You may want to add red food coloring to the hot flask and blue to the cold.
Super Cooling Water
It is possible to cool liquid water below freezing and let it remain as a liquid. This phenomenon is possible with readily available 1L bottles of spring water. Lay an unopened bottle of water in the freezer for 45 minutes. Remove the bottle and gently take off the cap. Be careful not to shake the bottle as you may cause it to phase change to ice. After you have removed the cap, pour the water onto a piece of ice and it will instantly phase change to ice. You may have to experiment with different freeze times depending on the temperature and type of freezer that you have.