Activity Overview:

Key concepts:
Water molecules are small and highly polarized. Their polar nature gives water its unique, macroscopic properties. On a microscopic level, water is a network of these polarized molecules linked together by hydrogen bonds.

Students discover that water molecules are polar, and that the polarity makes water a liquid under normal (room temperature and average atmospheric pressure) conditions. In the previous activity, students built their model of a simplified cell, with solutions on both sides of a membrane. In this activity students "turn up the magnification" to look at water molecules and their interrelations in a network of other water molecules, thus expanding their understanding of the properties of water and aquatic solutions.

Learning Objectives:

Students will:

be able to represent, in drawing or text, that on an atomic level, water is a network of polar molecules linked together by hydrogen bonds.

be able to revise their model of an erythrocyte in solution to incorporate their molecular view on water.

Macro to Micro Connection:

Macroscopic properties of water, such as its liquid state under normal conditions, its ability to dissolve polar substances, its high boiling point, and its unique role in living organisms, all depends on the physical characteristics of the water molecules and and the interactions among them.

Conceptual Prologue:

Our bodies are largely water. The liquid portion of the blood, the plasma, is a very complex solution of many organic and inorganic chemicals containing more than 90 percent water. Red blood cells are designed to move about in this watery solution, using its hemoglobin to carry oxygen, and delivering it for burning organic food by the different cells. Without water, nothing would work or move around our bodies! So what is special about the water molecule? How does it behave?

In order to understand water properties, students will build a model that takes an account the structure of a water molecule and the character of water-to-water interactions. In this activity, students will work in a way similar to the way a scientist works; in fact, scientists have developed a large number of "hypothetical" models for water. They know from experience that the right model should explain and successfully predict the physical properties of a system, in our case ­ an erythrocyte immersed in the aquatic solution of changing salinity. Here we invite students to develop a model of such a system based on their knowledge of water molecules and and how the molecules interact with each other. .

We know that the chemical formula of a water molecule is simple (H2O), yet the physical and chemical properties of this chemical compound are very unusual, and in many instances unique. Every water molecule is composed of two hydrogen and one oxygen atom; these atoms are not arranged in a line, but bent in a special way. As a result, part of the molecule is negatively charged and part positively charged. It makes the water molecules highly polar. One region of the water molecule, its oxygen, is negative, while another region, its two hydrogen atoms, is positive.

Polar molecules tend to line up with each other, positive to negative, negative to positive, as do magnets. Polar water molecules create a network that is held together by special weak bonds, called hydrogen bonds. In a hydrogen bond, the slightly positive hydrogen atom of a water molecule is attracted to a slightly negative molecule, usually the oxygen of another water molecule. These bonds give water its special properties. For example, the making of hydrogen bonds is responsible for water being a liquid at normal room temperature, and its ability to dissolve ionic and organic compounds effectively.

The reason a hydrogen atom is somewhat positive within a water molecule is because the pair of electrons that hydrogen and oxygen are sharing in their covalent bond is shifted toward the oxygen atom. As a result, the overall charge on the water molecule gets out of balance: the area around hydrogen becomes slightly positive (we call it a positive pole); the area around oxygen becomes slightly negative (we call it a negative pole). With those two poles, therefore, we call water a polar molecule. Any two polar molecules bond to each other with a hydrogen bond (H-bond). Each water molecule several H-bonds makes with its neighbors, making as a result water a structured network of H-bonded molecules.

Hydrogen bonds are very weak (See Bond Table), and can be easily broken, giving water its flexible, fluid nature.

Water molecules will make as many hydrogen bonds as possible; polar particles of dissolved materials can become part of the hydrogen bonded network, while neutral particles of dissolved materials will "clump" to each other, effectively pushed out of the water network. *

 

Activity Design and Execution:

Major Science Concepts	"Water molecule", charge, polar, hydrogen bond
Assumed Previous Knowledge:	If your students are not familiar with the States of Matter, you might want them to complete this computer modeling activity.
Time:	1-2 50 minute classes
Materials:	String or sashes to go around student waists, with 2 places in the back of the sash for students to "attach" to one another such as tassels or curtain rings. MODEL Molecular Workbench Water: EXPLORING WATER http://xeon.concord.org/webstart/jnlpFiles/water.jnlp
Advanced preparation (if any)	Have Solution: Water available on student computers.

 

STEPS

1: Derive a model of what holds water together. In pairs have students open Molecular Workbench Solutions "Water" activity on the computer and follow the instructions.

MODEL -EXPLORING WATER http://xeon.concord.org/webstart/jnlpFiles/water.jnlp

2. Investigate the proposition: Water tries to make as many hydrogen bonds as possible, forming a network. Students use choreographed science to do this section of the activity.

• Select some students to be observers or "experimenters", and some to be water molecules in the container.
• Explain to students that in this movement exercise their bodies represent the negative oxygen, and their hands represent the two positive hydrogen atoms. The tassels or hooks on the sash represent the attachment possibilities for positive hydrogen atoms in the area (represented by other students's arms.)
• Have each student attach the colored sash loosely around his/her waist. The colored portion should be in the back. Holding arms about 104° apart, each student tries to grab lightly the sash belonging to another student in each hand. Holding onto a sash attachment represents forming a hydrogen bond.
• Have them start in a straight line and then try to make as many hydrogen bonds as possible. (They can jitter about a bit, but ask them to be careful of the hooks or tassels.) They should pull together in a way that represents surface tension.
• The students should shake, bounce, and spin, representing thermal energy [Remember States of Matter], but be careful of the hooks or tassels. After a while, s/he lets his/her grip slip away when it is pulled hard. When this happens, the one with a free hand tries to grab another sash attachment, always keeping the arms straight and about 104° apart. Keeping arms straight is needed because the water molecules cannot get too close.
• Students can then relate what they see. starting with the Observers. This basic water simulation illustrates many key points about water. Hopefully they will notice that:
  - the water molecules remain about the same distance apart, but can slide around.
  -The individuals end up facing every which way (instead of the rigid ordering of crystals).
  -If every kid grabs a sash attachment they will make a compact group. No matter what the starting shape, the liquid will become roughly circular, because students acting as water will want to form the largest number of hydrogen bonds and will pull together. They can relate this to the formation of water drops.
• If a student on the periphery finds no one holding on to his/her sash, nor is he/she holding on to someone else, s/he becomes a gas molecule and starts walking in a straight line and bouncing off the container. Eventually s/he will be able to grab a sash attachment and join the liquid again.

4. Ask students: How were Hydrogen Bonds indicated in both the "dance" model and the computer laboratory model? Ask students to add these to their Erythrocyte in Solution model.

Extensions and Connections

*This ability to "pull" and "push" is useful in shaping proteins. A very large molecule has polar regions that will be "pulled into", or linked into the network, and non-polar, that will be "pushed out" of the network. The result of such "molecular sculpting" is the unique structures of protein molecules in water.

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INDEX