Activity 3: The Structure of Ice and Water

First Draft – May 23, 2003

Copyright © 2003 Charles B. Abrams, Department of Physical Science & Engineering, Truman College, 1145 W. Wilson Ave, Chicago IL 60604. Teachers are encouraged to copy this activity and send comments to cabrams@ccc.edu

Audience: This activity is designed for middle school through college level but has not been tested on any of these groups. Obviously the level of explanation will vary depending on the audience. This handout is designed for chemistry professors who teach future middle school teachers, and middle school science teachers themselves.

Description: Students physically model the behavior of water molecules in ice and liquid water, learning about molecular structures and answering specific questions about water. This activity sets up the format for further activities, e.g. dissolving NaCl (salt) in water, the immiscibility (non-mixing) of water and oil, and the solubility of capsaicin in oil and fat but not water. The last activity explains why the spiciness of hot peppers in your mouth is relieved by yogurt, bread, butter, or anything with fat in it, but not by water, soda (even cold soda), or other water-based drinks. This means that pepper spray can not be easily washed out of your eyes with water, an issue that may play a role in the debate on a pepper spray ban.

Procedure:

1. Clear a large floor space for this activity. You may have to go into a hallway or outside.

2. Ask students to stand up, and announce that they are now all water molecules. Water molecules are made up of one oxygen atom and two hydrogen atoms. Their head will be the oxygen atom, and their hands will be hydrogen atoms. Each student raises their arms and holds them at about a 90° angle apart. The actual angle for H2O is 104.5°, but this is a difficult angle to get right! You do not want students to hold arms apart at a 180° angle. Show the students the pictures from the module website (faculty.ccc.edu/cabrams/pepperspray). Students who have difficulty holding their arms in this position may choose an alternate position with only lower arms raised.

3. The basic rules for the behavior of water molecules, and the chemistry behind these rules, is as follows:

a. Keep your arms at about the same angle at all times. However, you can move your arms a bit closer together and a bit further apart if you want. The water molecule always stays in the same shape, although it does vibrate.

b. Try to point each of your hands towards the back of a different person’s head. You may not be able to have both hands pointing towards someone else’s head at all times. When you point to someone’s head, we will call it a “hydrogen bond”. Each hydrogen atom is often hydrogen bonded to another oxygen atom. Hydrogen bonding is a weak bond between a hydrogen and another atom. It is always directed towards a lone pair of electrons on the atom, which correspond to the “back” of another person’s head, not the front.

c. Only one hand can be pointed to someone else’s head at a time. For our two-dimensional model, we will allow only one hydrogen bond for each oxygen. In a three-dimensional model, two hydrogen bonds occur for each oxygen.

d. Move around slowly and try to get into a position in which your hands are pointing towards two different people’s heads (from the back only). In liquid water, the molecules are moving around slowly, but their movement is constrained by hydrogen bonding. The energy of movement (kinetic energy) of molecules is what we perceive as temperature.

e. Continue to move around, trying to plan your next move, i.e. if you see someone’s head with no one pointing to it, you can stop pointing at one person’s head and move your hand to the other person. Hydrogen bonding is dynamic, and hydrogen bonds are weak so they are relatively easy to break.

4. Once the basic rules for water are established, try to model ice:

a. A group of six students stands in a circle (actually a hexagon) with their left shoulder inside and their right shoulder outside the circle.

b. Raise your hands to the position before (water molecules). Point one hand towards the person in front of you, and one out away from the circle. The ice crystal structure has a hexagonal arrangement of water molecules, although it is three-dimensional and we are representing it in two dimensions, so this is not quite accurate.

c. You should not be touching the other person, just pointing at their back – this may require stretching out the circle. The ice crystal structure requires more space than liquid water – as we will see later when it melts.

d. Do not move from your position, but you can rotate a bit and move your arms a bit closer together and further apart. Even in ice, the water molecules are moving in place. At absolute zero (-273 °C) there is still movement.

e. Other students may join this arrangement, neatly organizing themselves around the first group by following the rules in step 3. In this way you build a regular arrangement of water molecules, which is the definition of an ice crystal. This arrangement should take more space than six students modeling liquid water.

5. Begin to melt the ice:

a. Ask students to start moving in place a bit faster, as you increase the temperature. Have the rest of the class clap to set the pace, indicating slow rate when cold (little movement) and high rate when melting (faster movement) or boiling (much faster movement; some water molecules not hydrogen bonded at all.)

b. At some point the students will be moving in place fast enough that they will not be able to keep pointing towards the same people’s heads. They may temporarily point at nothing, or point towards someone else’s head so long as no one else is pointing there. As the temperature increases, the hydrogen bonding network is not strong enough to hold the water molecules in place, and the crystal structure is destroyed.

c. Continue to move faster and faster, switching where you are pointing more frequently. You will find that you have to move farther apart as you do this. As liquid water is heated, it also expands. Water is most dense (i.e. the water molecules are closest together) at about 4°C (not 0°C!)

d. One or two people might be moving so rapidly that no one can get near them, and an empty space opens up around that person. That is a bubble of water vapor in liquid water.

e. Water vapor can be represented by individual people not pointing to any other person and moving rapidly around the room away from the rest of the group and each other.

f. Begin to slow the pace, all the way back to ice. If all goes well, you should see an expansion as the ice crystallizes.

6. Ask students to take a seat and give them a round of applause!

7. Show the video “Water” by Roy Tasker1,2. The video connects the ideas from the activity to the molecular description of water. Parts of this video are also available on the CD that accompanies Atkins & Jones General Chemistry3

8. I saw the following exercise with a model kit at a national American Chemical Society meeting, at the booth of a molecular model supplier. I can not remember the name of the supplier4. Build the small ice crystal model shown on that web page, and give each student one more water molecule to add to the ice crystal. The more kits you have the better. As the students add water molecules, they build a larger and larger crystal of ice.

Comments:

• The water molecules themselves do not melt! Melting means all the molecules begin moving faster, and the arrangement of the group of water molecules becomes less organized. (If a student says "I'm melting" during this exercise, she is incorrect. The correct phrase must be "We're melting")

• Water is made up of two atoms of hydrogen (your hands) and one atom of oxygen (your head). Everything in the universe is made up of atoms6, and there are only 114 known types of atoms (called elements), all listed in the Periodic Table.

• The images in this activity show the same boy repeated six times as the ice crystal. This is mainly because my 3D graphics software has only one pre-built child model, and it happens to be a boy. While each of us is unique, each water molecule is exactly the same as each other molecule. There are no "boy" or "girl" or "tall" or "short" water molecules! (There are isotopes: hydrogen is replaced by a heavier form of hydrogen or oxygen by a heavier form of oxygen, so there are "heavy" and "light" water molecules ... but this is unnecessary complication here.)

• Atoms are very small – they are the smallest things we can imagine! If we were going to try to model just one drop of water the way we just modeled a very small part of water, we would need 10,000,000,000,000 (ten trillion) times more people than have ever lived on the Earth! (I need to check this calculation. Assume 1 drop = .05 mL)

• Water ice takes up more space than liquid water near zero degrees – so that when water freezes, it expands. Ice that forms in small cracks can break open rocks or pavement.

• The rumbling sound you hear when water starts to boil but has not reached a full boil is the sound of these water vapor bubbles forming and collapsing. Watch an open pot of water boil and you will see these bubbles form and collapse before the water reaches a full boil. (I am not sure if some or all of these visible bubbles are dissolved gases coming out of solution, so this explanation is still tentative.)

• When water freezes, it can no longer hold gasses that were dissolved in it. Ice cubes made in a freezer have air bubbles in the ice. These bubbles of gas were dissolved in the water and came out of the water after the surface of the ice cube froze, so they were stuck inside. Try this: boil water to get rid of the dissolved air, allow it to cool a bit and then make ice cubes with it. Do you get fewer air bubbles in your ice? (I just thought of this and have not tried it yet!)

References:

1. Video on water by Professor Roy Tasker:

www.nepean.uws.edu.au/science/vischem/ - not available 5/23/03!

2. Review of the water video animation in the chemistry classroom:

http://science.uniserve.edu.au/newsletter/vol9/tasker.html (accessed 5/22/03)

3. General chemistry textbook with water animations on CD:

http://www.whfreeman.com/chemicalprinciples/ (accessed 5/22/03)

Atkins, P., Jones, L. Chemical Principles 2e, the Quest for Insight (WH Freeman)

4. Molecular model of an ice crystal:

http://www.webcom.com/darling/kits7.html

5. Image of ice water (and other free science images):

http://data.pg2k.hd.org/_exhibits/natural-science/ice-melting-in-water-with-thermometer-at-zero-degrees-Celsius-AJHD.jpg (accessed 4/24/03)

6. There are two exceptions to the statement that everything in the universe is made of atoms: neutron stars and black holes. These could be considered extremely (x 10!) large elements because they are composed of protons and neutrons. The gravitational force in these objects compresses not only the atoms together but the nuclei together.

Acknowledgements:

A Middle School Teacher Quality Enhancement (MS-TQE) grant supported the development of this module. The author would like to thank Wendy Anderson, Robyn Chaplik, Jackie Murphy, Dan Creely and the staff and other participants at the North Eastern Illinois University (NEIU) Middle Grade Leadership Academy (MGLA) for helpful discussions.
Activity 3 : The Structure of Ice and Water

Questions for reflection and evaluation

1. Describe what happened when you were water molecules in an ice crystal.

2. How did the ice change into liquid water?

3. What happened to the amount of room you needed when you changed from ice to water or from water to ice?

4. Imagine applying pressure on ice – like taking the group of students representing ice and squeezing them into a smaller and smaller room. Would the ice structure remain, even at freezing temperatures? Can you explain why an ice skate leaves a trail of water in its path?

5. Water deep under the ocean is under very high pressure and temperature. The water temperature can be higher than the standard boiling point of water (100°C!) How does the high pressure keep this very hot water in liquid form, and prevent it from turning into water vapor?

6. If we had all the people on Earth with us to model water (5 billion people) what would be the volume of that drop of water? To answer this question, you need to know that there are 55 moles (55 x 6.022 x 1023 molecules) in one liter of water, and that one billion is represented as 109 in exponential notation. For a sense of scale, the volume of a drop of water is about 5 x 10-5 L, and the volume of the small droplets in a cloud or fog is X L.

The next questions are a part of the activity on dissolving salt in water, which will follow this one:

7. Suppose there was something else in the water, e.g. salt, that formed strong hydrogen bonds between it and water. Do you think it would be easier or harder to form ice? Would the temperature have to be lower or higher to form ice?

8. In the same situation as above, would it be easier or harder to cause water molecules to become disconnected from the liquid and become vapor?

For further resources, see

faculty.ccc.edu/cabrams/pepperspray