Background Information for Teachers
Developing middle school students’ knowledge of water’s chemical properties, including its molecular structure, is not the main focus of this publication. However, middle school students can be introduced to the properties of water with the expectation that they may not master the related concepts until high school chemistry. Referring to atoms and molecules, the National Science Education Standards says:
. . . use of such terminology is premature for these students and can distract from the understanding that can be gained from focusing on the observation and description of macroscopic features of substances and of physical and chemical reactions. At this level, elements and compounds can be defined operationally from their chemical characteristics, but few students can comprehend the idea of atomic and molecular particles."
With that in mind, the following simulation of the water molecule polarity can be conducted without using the terms "atom" or "molecule." After participating in the simulation, students should gain these understandings: there are two hydrogens for every one oxygen; these three are bonded to each other and constitute a "unit" (water molecule); these units align themselves in a specific pattern; and these units are not actually bonded to each other the way the Hs are to the O.
For the activity, give two-thirds of your students hang tags labeled "H, slightly +", and one-third hang tags labeled "O, slightly –". If the number of students is not evenly divisible by three, include yourself or an aide, or make an extra student your assistant and observer. Instruct students to form "units" of three students. All students have to be in a group, and no group can consist solely of all Hs or all Os. Have the Hs flank the single O in each group. Ask them to link arms (to model the atomic bonds between the atoms holding the molecule together). Finally, instruct the "units" to come close to each other, paying attention to the positive and negative signs on their hang tags.
Ask the students what happens when like charges come near each other, similar to the north ends of two bar magnets? Here, the Os need to consider their slightly negative charge and the Hs need to consider their slightly positive charge. They should end up aligning backs of student Os to a student H of another unit by standing near each other, but not actually connected or bonded to each other. Voila! You have a life-sized model depicting the polarity of water, the covalent bonds of H and O, and hydrogen bonding. You can have your students draw a picture on the board of what each unit consists of and how the units align with each other.
For eighth graders, or students cognitively prepared, you may want to now introduce the terms "atom," "molecule" and "bond" by connecting them to the simulation. You can extend the model by asking students to model the three states of matter. Students will stand fairly still to model the solid state, move moderately while compacting a bit to model the liquid phase (recall, water expands in the frozen state, due to H bonding), and increase their kinetic energy while increasing the distance between water molecules to model the gas state.
Caution students that water is unique in its ability to expand when changing state from liquid to solid, that is, when freezing. For most compounds it's the opposite; volume decreases when changing state from liquid to solid. You may consider a demonstration involving the melting of chocolate as an example. The solid volume can be expressed in cubic cm, for example, 10cc, and the liquid volume for that same solid 10cc sample in ml. Since 1ml = 1cc, an easy conversion and comparison is made. It may take a considerable volume of chocolate to be able to observe a measurable difference in the two volumes between the two states.
Briefly describes how and why living things use water. Provides an experiment one can do to illustrate the ratio of water to solids in a living thing. MSP
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This module provides an introduction to the chemical properties of water. The dipole across the molecule, hydrogen bonding, surface tension, and solvation are all introduced. MSP full record
This downloadable article raises some interesting questions regarding water and living things and may give you additional insights and perspective as you begin your unit on the water cycle. Having found no clear reason why water should be the only liquid in which life could form and survive, scientists continue to search for the reason. Here we follow the first steps along the challenging path to this answer, trying to understand (i) what is unusual about water; (ii) why water has anomalies; (iii) which are the full implications of these unusual properties; and (iv) if these anomalies are exclusive properties of water. Since the change of local liquid structure could be relevant in biological processes, the possibility of a wide class of liquids with this property could help in understanding if water is essential for life.
Aquaporins are membrane channels that control the flow of water across a cell membrane. Water's polarity is a problem when one needs water to pass through a nonpolar lipid bilayer, and yet all cells require water to survive! This Nobel Prize in Chemistry award-winning animation clearly illustrates the process. Sharing this with your students will build a bridge from water's universal chemical and physical properties to living things and how these properties are accommodated by and manifested in cells—a major cog in the larger water cycle on earth. Click on Download a detailed movie of water permeation.
LEARN was created to increase middle school teachers’ knowledge of and interest in the atmospheric sciences. The module was created by teachers. The seven sections include introductions to the atmosphere, climate, ozone, stratospheric ozone and tropospheric ozone, the greenhouse effect, and global climate change. Each section provides background information, general learning concepts, and a list of classroom activities. Choose Introduction to the Atmosphere, and scroll down to Activity 4, The Water Cycle, at the bottom of the page. MSP full record
The U. S. Geological Survey provides maps, reports, and information to help others meet their needs to manage, develop, and protect America's water, energy, mineral, and land resources. Some of the 17 topics included here are flood forecasting, reservoir management, water allocation and bioremediation. Perusing this site will give you insight into how this country conceives of and executes water management programs. You might decide to have your students use one or two topics for their own research and information. MSP full record
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Copyright
July 2007 — The Ohio State University. This material is based upon work
supported by the National Science Foundation under Grant No. 0424671. Any
opinions, findings, and conclusions or recommendations expressed in this
material are those of the author(s) and do not necessarily reflect the views of
the National Science Foundation.
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This work is licensed under a
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