The Basic Gas l Liquid Water l Gas ReactionsThe following is a sequence of physical simulations that kids could play to illustrate atomic properties. Depending on student age, these activities could also be simulated on the computer using StarLogo and performed in the lab with MBL instrumentation. Student understanding of these three representations and their ability to move between them will be a subject of research.
Activity 1: The basic gas.
Kids are each given 1 to 6 batons and told to walk inside a rectangular area in a straight line at a speed related to the number of batons they carry. Make sure there are 3.5 times as many batons as kids. The kids represent gas molecules in a container. The batons represent energy, so they walk faster if they have more batons and they stand still if they have none. Each baton represents 100 units of energy. [The container is "adiabatic" because it neither adds nor subtracts energy from the gas.]
Mark the floor with tape, dividing it into 20 (or 35) squares using four (five) divisions on the short dimension and five (seven) on the long. Number the squares.
If possible, mount a wide-angle camera video high overhead to see the entire rectangle. Have kids wear white paper hats for contrast. The kids can be told that they are preparing a video to teach younger kids about molecular motion. They will later edit and annotate the videos. This could be used for teaching as well as for evaluating the kids' understanding of the simulation and its significance.
If a kid comes to the edge of the rectangle, s/he should bounce off the wall in any new direction s/he likes. If one comes close enough to grab another kid they "collide". When two kids collide, the person with more batons gives one baton to the other person and they head off in any direction they like. If they have the same number of batons, they flip a coin and the winner gets a batons from the other. [A better alternative to flipping might be to use the stone-scissors-paper game. On the count of three each kid makes the symbol of a stone (fist), scissors (two fingers separated), or paper (hand held flat). The stone wins over the scissors (it breaks it), the scissors over the paper (it cuts it), and the paper over the stone (it covers it). The winner takes a baton from the loser.]
At any time, the teacher can yell, "freeze". This can calm the kids and is a way to take stock of the simulation. Each child can, for instance, report the number of energy points, equal to 100 time the number of batons s/he has as well as which rectangle s/he is standing on. Of course, the total number of batons in the gas will always be a constant each time the teacher freezes the simulation. This should make the idea of conservation of energy (in isolated systems) self-evident.
The counts should be entered into a computer and projected as a histogram so everyone can see it. This will give a distribution around an average. It is possible for one person to have far more than the average, but it is quite unlikely. The computer also computes the temperature and entropy. The temperature in degrees Kelvin is simply the average energy counts per molecule. Subtract 273 to get the temperature in degrees Celsius. Entropy can be either ignored or introduced as the amount of randomness or disorder in that distribution. (It is also proportional the number of different ways you could hand batons to kids and still end up with that distribution.)
The following experiments can be performed on the basic gas. The teacher might suggest the overall question and have the kids work out the details of performing the experiment.
1) How do the molecules spread out? Start all the kids in one corner, in the middle, etc. After a while, freeze the kids and count how many are in each square. There will be a distribution, but after a while, every distribution will look alike. This resulting distribution will also have the highest entropy! This illustrates the second law of thermodynamics: that systems go from order to disorder (increase entropy). The distribution they always get is the most disordered.
2) What happens when you start with different energy distributions? Keeping the number of batons constant, what happens if all molecules start with the same energy? If just a few have lots of energy? If some start with no energy? The kids should realize that in a very short time, they always have approximately the same energy distribution. This again illustrates the second law, that entropy (of an isolated system) always increases. Having many molecules with high energy or zero energy represents a higher degree of order than the final distribution.
3) What happens if you start with one half of the gas hot and the other cold? Give 5-7 batons to half of the kids (representing 600°K) and none to the other half (representing 0°K, or absolute zero). Start the hot kids at one end of the container and the cold ones at the other. Look at the physical and energy distributions after a while. Of course, the same old, maximum entropy distribution will result. The temperature will be the average, or 300°K. Is the final temperature always the average of the starting temperatures? What if the numbers of hot and cold kids are not equal? (The result is always the total number of energy units divided by the number of kids.)
4) What happens if the gas is compressed? The gas density is the average number of people per square. This will increase if the container is shrunk while a simulation is running. But also, count the number of collisions per minute on each wall of the container. The pressure would be the collision rate on a fixed part of the container. This represents the outward force of the gas on the container. What happens to the pressure as the gas is compressed?
5) What happens to the pressure when the number of gas molecules increases? Start with only half of the kids in the container. Measure the pressure by counting hits per minute. Then add the rest of the kids with the same average energy. Of course the pressure should double.
6) What happens when light shines on this gas? Light can be simulated as packages of extra batons (photons) that are tossed into the simulation. One kid catches them and then acts accordingly. Red light would correspond to a package of 20 batons.
7) What happens if the container has temperature, too? Around the edge of the rectangle place baskets every few feet that also contain 1-5 batons. These represent the energy in molecules that make up the container. In a collision with the wall, the same thing happens as kid-kid collisions. If the kid has more batons than the nearest basket, s/he gives one up; if s/he has fewer, s/he takes one; if they are the same, she flips a coin to determine whether to take one or leave one. As before, s/he then takes off in any direction at a speed related to how many batons s/he has. Try different starting temperatures (average batons per basket or person) for the container and the gas. The temperature of the gas and the temperature of its container become equal and somewhere between the starting temperatures.
8) Is the energy of the gas constant when the container is considered? Start with a cold gas and hot container. Kids should see that the energy of the gas is not conserved, but the total energy of the gas plus container is.
9) What happens when one side of the container is heated? Adding extra batons to the baskets on one side can simulate this. After a while, the container is all one temperature and equal to the temperature of the gas.
10) What temperature is something hung in the center of a gas? A collection of some empty baskets can represent an object suspended in the gas. Run the simulation for a while. The object will become the same temperature as the container and the gas. This is the origin of the idea of a "room temperature". The air in a room, its walls, and everything in it tend to the same temperature.
Activity 2: Liquid water
Each kid attaches three short ropes or tassels across the back of his/her belt. Holding his arms about 110° apart, each kid tries to lightly grab a tassel belonging to another kid in each hand. The kids should vibrate, bounce, and spin, representing thermal energy. After a while, s/he lets one tassels slip away when it is pulled hard. When this happens the one with a free hand tries to grab another tassels always keeping the arms straight and about 110° apart. All this takes place inside a container like the one used in the basic gas.
The tassels represent negative parts of a water molecule and the kids hands represent the two positive hydrogen molecules. Holding onto a tassel represents forming a hydrogen bond. Forming a hydrogen bond is like sharing a baton; each is credited with 50 energy units. Keeping arms straight is needed because the water molecules cannot get too close.
If every kid grabs a tassel they will make a compact group. Some on the periphery will not be able to reach a tassel. If a kid on the periphery finds no one holding on to his/her tassel, 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 tassel and join the liquid again.
This basic water simulation illustrates many key points about water. The water molecules remain about the same distance apart, but can slide around. The kids end up facing every which way. Freeze the simulation and measure how many hydrogen bonds are formed and the density of the liquid. There should be almost two bonds per molecule and the density should remain constant, independent of the container, providing it is large enough.
Experiments that can be done on the basic liquid water:
1) What shapes will the liquid take? No matter what starting shape, the liquid blob will become roughly circular, because kids want to form the largest number of hydrogen bonds. What happens when you start with many small drops, each with some average motion?
2) Can you compress a liquid?
3) What happens when a positive ion joins a drop of water? A positive ion could be represented as three kids roped together facing outward with six hands to form hydrogen bonds.
4) What happens when a negative ion joins a drop of water? A negative ion could be represented as a kid with lots of tassels and arms folded so they are not used.
5) What happens when a completely neutral molecule enters a drop of water? This could be represented by a group of kids roped together with no tassels and arms crossed. This is subtle, but the average number of hydrogen bonds will go down as the new molecule forces its way into the drop. Eventually, to make up the missing hydrogen bonds, the water drop will eject the new molecule. This is why it is called hydrophobic.
6) Will liquid fill any container? Turn on gravity by asking all the kids to try to move slowly in one direction. Then create different kinds of barriers. What is the shape of the meniscus for water in a hydrophobic container? Since there are no tassels or arms on the container, the water will tend to bead up in the middle.
7) What happens when water is in a hydrophilic container? This could be represented by students on the edges of the container who have tassels and can grab water molecules. The water will creep up the edges.
8) What happens to a big molecule like soap that is hydrophobic at one end and hydrophilic at the other? The hydrophobic end sticks out of the water.
Activity 3: Gas reactions.
The basic water simulation has not included energy just because counting batons is difficult while also grabbing onto tassels. But without considering energy, one cannot understand evaporation, condensation and many other properties. So next, shift to an energy model and represent the hydrogen bonds another way. Before we do this for water, though we will consider chemical reactions in a gas.
Motivation for this might come from ecology. It is gas-phase reactions that result in smog and the ozone hole. Later, when we add light, we have a good model for stratospheric ozone production. We should, however, start with something simple, like :
Each kid will wear an inner tube (or similarly shaped inflatable toy) around his/her waist. The tubes will have patches of Velcro hooks and loops representing places where reactions can occur. Next to each patch is a pocket for one to five or more batons that represent half of the energy in the bond. Kids should fill each pocket with the required number of batons. The rule is, when they count up their batons to see how fast they should move, they must count the batons in the "bond pockets" only if two kids are stuck together there (making a bond) and ignore them otherwise. These batons represent the energy that can be released when a bond is made. All the batons must be present in the bond pockets of each player to break the bond, because that is the energy needed to break the bond. These idea of energy of bonds come up over and over again, so it is important that students understand these rules.
The temperature of a molecule is the average energy per atom. (This is not strictly true, but close enough.) This includes the energy released when a bond is made. So, creating a bond will heat up a molecule and breaking the bond will cool it. When two kids bond, they will have lots more energy to bounce around. If they like, they can pull apart when they hit another atom or molecule (disassociate) providing the bond pockets on both sides of the bond are filled. After a while, they will cool, giving away all the batons they were carrying and then starting to give away some batons from the bond pockets. When this happens they cannot disassociate because they don't have enough energy.
Continuing on this way, kids could explore all kinds of chemical reactions, explosions, photochemistry, catalysts, polymerization, condensation, evaporation, crystallization, melting, fracture, annealing, diffusion, etc.
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