Getting a Grip on Hydrogen Bonds
Skills and concepts that students must have mastered
How the activity is situated in the course
Content/concepts goals for this activity
Students will learn that:
- water molecules are polar, that is each H2O molecule has a slight + and - charge. This is because the structure of a water molecule results in one end of the molecule having an excess negative charge and the other end having an excess negative charge,
- a special kind of bond, called a hydrogen bond, exists between water molecules where the + side of one molecule is attracted to the – end of another molecule,
- Hydrogen bonds thus help to hold water molecules together. Each water molecule can form a maximum of four hydrogen bonds with other water molecules, and
- as a result of the presence of hydrogen bonds, water has some unique properties when compared to most other liquids. These include cohesion, resistance to temperature change, and expansion during freezing. (Note that this activity only focuses on cohesive forces and related phenomena, but this is a good launching activity to then cover the unique heat capacity, density, etc. of water).
Higher order thinking skills goals for this activity
Application: Students will use the concepts learned during the activity to explain "new" situations, such as how water reaches the tops of trees and why drops of water cling to a glass that is emptied.
Analysis: Students will compare and contrast the behavior of water and oil during the activity.
Other skills goals for this activity
Description and Teaching Materials
- Water (1 L)
- Vegetable oil (one bottle)
- Sheets of wax paper
- Food coloring
- Paper towels
- 1 large beaker (1000 mL)
- permanent markers
- Graduated cylinders (maybe do this as a demo for the class to illustrate meniscus?)
For the following items it would be ideal to have one per student, but items can be shared:
- Small beakers (50mL)
- Small plastic cups (such as dosage cups that come with OTC medicines)
- Capillary tubes
**Students will write down their observations during the activity, so they will either need a notebook or paper.
- Fill one of the 1000mL beakers with water. Use the food coloring to color the water so that they can be easily seen during the activity.
- Students should be in pairs (or groups of four if materials need to be shared).
- Each group will have two pipettes, two small beakers, two small plastic cups, two capillary tubes, a sheet of wax paper, and a paper towel.
At this point the instructor can lead the activity using the instruction sheet, or can give copies of the instruction sheet for the students to follow on their own. ***IMPORTANT NOTE: If instructors decide to give this sheet to the students, they will want to EDIT IT BEFOREHAND because it includes an answer key for the discussion.
Hydrogen Bonds Instruction Sheet (Microsoft Word 2007 (.docx) 128kB May19 13)
Following the activity the instructor can now introduce the concepts of polarity, hydrogen bonds, cohesion, adhesion, and surface tension. See instruction sheet.
Teaching Notes and Tips
The results become complicated if students use the same pipettes or cups for oil and water, so be sure they use separate equipment for each liquid. Also, you may want to press the wax paper sheets ahead of time (for example under a pile of books) so that they don't curl.
Assessment (note: Question 6 refers to the extension activity related to immiscible liquids):
- Using the concept of hydrogen bonds, explain...
- a. ...why beads of water and oil are shaped differently and why water and oil behave differently when filled to the top of a cup.
- b. ...how and why absorption of water and oil into a paper towel or capillary tube is different.
Assessment Answer Key (Microsoft Word 2007 (.docx) 188kB May19 13)
References and Resources
- Trujillo and Thurman, Essentials of Oceanography, 10th Edition, Pearson Prentice Hall, New York, © 2011.