Getting a Grip on Hydrogen Bonds

Elizabeth Nagy-Shadman (eanagy-shadman AT pasadena.edu), Pasadena City College
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Summary

The purpose of this brief (~15 minutes) activity is for students to directly observe some of the unique properties of water that are the result of hydrogen bonds, such as capillary action, adhesion, cohesion, and surface tension. Students will compare the behavior of water to that of vegetable oil, a non-polar liquid. I have created an activity handout that can either be given to directly to the students or used as a guide by the instructor. It includes vocabulary terms and a discussion "answer key" that should be edited if given directly to students. There is also an "extension activity" that addresses the concept of immiscible solutions.

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Context

Audience

Introductory level college oceanography course.

Skills and concepts that students must have mastered

Familiarity with chemical composition of water, as well as an understanding of atoms and covalent bonds, would be helpful.

How the activity is situated in the course

This would be a good activity to introduce the concept of hydrogen bonds.

Goals

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

Students will work with a partner or in a small group. Students will write their observations and answer questions related to these observations.

Description and Teaching Materials


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)
  • Pipettes
  • Capillary tubes

**Students will write down their observations during the activity, so they will either need a notebook or paper.

Set-up

  • 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.

Activity

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)

Discussion

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

Assessment (note: Question 6 refers to the extension activity related to immiscible liquids):

  1. 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.
  2. How does water get to the tops of tall trees?
  3. Why does some water cling to the inside of a glass even if you empty it and try to shake out all of the remaining moisture?
  4. How do you think the presence of hydrogen bonds might change the boiling point temperature of water compared to non-polar liquids? What about the melting point?
  5. Describe the difference between a hydrogen bond and a covalent bond. Which bond is weaker? Stronger?
  6. INVESTIGATE AT HOME: If we assume that many types of dirt are oil-based, and we know that oil and water are immiscible liquids, then placing a dirty object (such as a muddy soccer shirt) into water will not do much because the oil and water do not interact. However, the addition of soap changes things. Do a short investigation on how soap "works". Pay particular attention to interactions between polar and non-polar substances.

Assessment Answer Key (Microsoft Word 2007 (.docx) 188kB May19 13)

References and Resources