Water Cycle Simulation, With Energy

This physical simulation lets a group act out the water cycle -- an important earth system -- and see how humans are part of this system. Students will take on the role of water molecules, walking and running between different areas, and physically interpreting phase changes from ice to water, and water to water vapor. Humans depend on water for many reasons, not the least of which is to keep ourselves cool by sweating. To understand how sweat keeps us cool, it is important to understand how energy is involved in the water cycle, specifically in the processes of evaporation and condensation.

As a simulation, this activity presents a simplified look at the water cycle -- no simulation can reflect every level of detail. Indeed, with students actively running back and forth, some of the more subtle concepts will doubtless need to be revisited. We suggest using the simulation activity first, then having a discussion to debrief. In the discussion, you can ask students for their ideas about how energy was involved in the water cycle, and have a more theoretical discussion. While the water cycle is often taught without a focus on energy, the role of energy transfer is critical to understanding evaporative cooling and is an important chemistry concept. The final step in this Instructable provides ideas for extending the simulation depending on the student group and time available.

Water Cycle Simulation, With Energy, was developed as part of the Science of Sweat Workshop, including investigations of how sweat, and more generally evaporating water, can cool things down, and the impacts that extreme heat can have on our bodies. Designed for out-of-school-time educators, the workshop is easily adaptable to other educational settings. Topics include evaporative cooling, the water cycle (including the role of energy), urban heat islands, and staying cool while outdoors. These topics are relevant to spending time outdoors in the heat, they introduce environmental justice and equity issues, and they provide a hands-on introduction to widely taught science concepts.

• Playing field, outdoor space, gym, or large open room
• Play or field cones, or other easily visible markers
• 2 small shopping bags to hold energy cards (for the cloud captain and Sun)
• 5 energy cards per student (print from the Energy Card PDF, ideally on light red, heavyweight paper)
• Easel pad (or large paper), and pens, for making signs for each area
• Folding table
• Blue Tape to hold signs

Ideally, run the simulation outdoors in a playing field or, if necessary, indoors in a gym or large open room. (Wind can be an issue if the cards are going to blow off the table.)

First set up the area. (The instructions are written for the basic simulation, but the final step gives directions for making the simulation more complex, by adding areas for trees and groundwater, ponds, etc.) The illustration shows the locations for the two main areas: Clouds, and Ground. These are defined by two lines of cones. The table for the Human is set up on one side of the Ground area.

Use the folding table to set up the Person. Draw a large thermometer cartoon on easel paper, as shown in the illustration, and tape it to the top of the table. Put some energy cards on the thermometer to bring it up to the “good” zone.

The Sun Captain (you, or whoever is leading the simulation) should have the supply of energy cards in a shopping bag.

Assign a Cloud Captain -- ideally another teacher who has been briefed on their role -- and give them an empty shopping bag. The Cloud Captain will be directing activity in the Cloud area.

The students will be playing the role of individual water molecules. Have all the water molecules start in the ground area. The Sun should distribute 3 energy cards to every water molecule.

As the Sun, you (or whoever is leading the simulation) should introduce the simulation. What follows is the script we have used (with stage directions and notes in parentheses):

We know that sweating cools you down -- but how does that really work? Sweat is mostly water, and as it evaporates it cools you. But how can evaporating water make you cooler?

To understand this, we are going to look at the water cycle -- and we are going to do it by becoming water molecules. We are going to take a molecule-eye-view of the water cycle. As a water molecule, you will be traveling from here -- the ground -- to the Clouds, and you will be changing phases, between ice, liquid, and water vapor.

You will need to listen to directions -- I am the Sun, I provide energy to the earth, and I will be giving you energy (show the cards) as well as directions. Let me also introduce the Cloud Captain who will give you directions when you get to the cloud.

Let’s do a practice run all together, to see how the whole water cycle works. Water can be in three states: Solid, which we call ice, Liquid, which we call water, and Gas, which we call water vapor. They are all made up of water molecules -- H2O -- that’s you. Let’s see how it works!

We are starting out as liquid water. You are all starting with 3 energy cards. You can see from the instructions on the cards that you are liquid water. When you are liquid water -- now or any time in the game -- you should walk around, always keeping close to other liquid water molecules. Stay within arm’s length - but without bumping into each other. Just walk around randomly. (It can help to have people keep their arms crossed.) In liquid form, water molecules are free to move around, but they also stick close to other water molecules. (Chemistry Note: in liquid form, water molecules form, and reform, loose bonds. Students can change who they are next to but need to stay within arm's length of someone.)

(After the water molecules have had a minute or so to get used to their role, continue.) Now, let’s pretend it is winter. The air is getting colder. You are going to lose some of your energy to the air. Everyone give the Cloud Captain one of your energy cards. You are cooling down, giving off energy. When you lose energy you slow down. When you have 2 energy cards you are still liquid -- keep walking, but very slowly. When we talk about temperature we are talking about the amount of energy the molecules have. The more energy, the faster the molecules move, the less energy, the slower they move.

Now let’s say the air gets even colder. We are about to cool down even more. Everyone give the Cloud Captain one more of your energy cards.

When you have one energy card you are cold enough to freeze. When water freezes, the molecules stop moving freely and link together into a crystal structure: ice! Everyone should stand at arm's length from another water molecule. You have frozen into ice. (Chemistry Note: because of the spacing between molecules in the crystal structure, ice takes up more space than liquid water. The individual molecules can still jiggle about a bit, but everyone should stay in place.)

Winter is over. Let’s warm things up. As the Sun, I will give you each an energy card. When you get a second energy card, you melt. You leave the solid crystal form and become liquid again. When you have two energy cards you should walk around as liquid water -- but remember to stay close to the other water molecules. (One by one, give everyone an energy card until all the ice melts.)

What happens when things get even warmer? As the Sun, I am going to give you more energy cards. When you have three energy cards you should move around faster — but you are still liquid water -- stay close to the other water molecules. (Give out more energy cards so most of the molecules have three.)

Ok, let’s heat things up even more. When the Sun gives you a fourth energy card, something new happens -- you will have enough energy to evaporate! When you evaporate you will go from being liquid to being a gas. You are free. You leave the other water molecules and fly. In fact, you will fly into the air… and all the way to the Cloud area over there. When you are in the Cloud area, fly around. Unlike liquid water, you don’t need to stay within arm’s length from other water molecules -- spread out. (As the Sun, give out energy cards until all the water molecules have evaporated and moved to the Cloud area.)

Now we have a lot of water vapor -- all of you. You are high up in the air, and it is cold up here! You will start to lose some heat to the air. The Cloud Captain is going to take some of your energy cards. (The cloud captain should start taking energy cards, one at a time, from water molecules.)

When you only have three energy cards left, you are cold enough to condense back to liquid water. Look for another water molecule with two or three cards. When you find someone, start acting like liquid water: walk around next to each other staying within arm’s length. Together you are a small droplet — the sort that makes up clouds. Stick together as you walk around. Look for other water molecules with two or three cards to join you. When you have three or more of you, you have a big enough water drop to start falling out of the cloud. “Fall” down to the ground as a raindrop! (Have the cloud captain collect one energy card from every molecule until they have all condensed and rained back to the ground.)

(Chemistry Note: water vapor itself is a clear gas. When we see clouds, fog, or steam, what we see are tiny droplets of liquid water that have condensed out of the water vapor. The droplets are small enough to easily be carried by air currents. You can sometimes observe this with a rapidly boiling tea kettle: right next to the whistle you may see a ¼ inch gap before you see any white steam. The gap is “pure” water vapor before many droplets have started to condense. In the simulation, raindrops fall when they have collected three molecules -- an actual raindrop would contain billions of molecules!)

Now, you have all been through the basic water cycle -- evaporation, and up to the atmosphere as water vapor. Cooling and condensing into liquid water, forming drops, and raining back to earth. This cycle has been going on for millions of years, recycling water molecules over and over again, over the whole earth!

Let's make it a bit more complicated. Let’s add a person. This table is our person. What happens if our person drinks some water? (Pick out about four water molecules and have them mill around on the far side of the table.)
Now the sun is also shining on the person, heating them up. As the Sun, I am going to add some heat energy to the person. (Put some energy cards down on the thermometer, raising the person’s temperature into the hot zone.)

OK, the person is getting hot. The Hypothalamus, a part of the brain that controls our sweat glands, is going to send some water to the skin as sweat. (Pick two water molecules and bring them to the near side of the table.) So now we have sweat on the skin -- but to cool this person these water molecules will have to evaporate. But wait! To evaporate, you need four energy cards. But you only have two. So take two more energy cards from the person (they should each pick two energy cards off the thermometer). You have just absorbed some heat from the person. The person is cooler, and you are warmer. Now you have four energy cards and can evaporate! Fly back up into the air and go to the cloud area. (Repeat this until there are no more water molecules left at the table.)

Oh no. (Put more energy cards down on the thermometer, raising the person’s temperature back to the hot zone.) The sun is still shining, the person is absorbing all this energy, but we don’t have enough water to sweat! They are getting to the heat exhaustion zone. We better get this person a drink! (Pick another group of water molecules and have them move to the far side of the table.)

Now, let’s try to keep going, as a cycle. It is going to get a little crazy -- the water cycle is cycling all the time, all over the earth! So let’s go!

(At this point, the group will no longer be all together. As the Sun, keep heating up the person. Have the water molecules line up to take heat from the person and evaporate. Also, as the Sun, give energy cards to some molecules who are in the ground area but not near the person, so evaporation can happen from multiple places. Meanwhile, the Cloud Captain should keep collecting energy cards fast enough to keep rain falling, but keep some water in the clouds. Try to keep things moving quickly. You can add drama by adding enough heat to the person to put them into the hot (heat exhaustion) zone. Try for a happy level of chaos. Continue until everyone has been through a few cycles and/or pandemonium sets in!)

The water cycle is made more complex when we consider all the different paths a water molecule can take. For example, a water molecule might get so cold in a cloud that it freezes, then falls as snow, and then sits on the earth for some time. Trees and other plants also use water and they sweat (transpire) it back into the atmosphere. Water can also flow underground as groundwater, sometimes staying underground for long periods of time. Below are some additions to consider adding to the basic simulation. Have students practice the simple version first, then add in complexity when they are experts!

You can add a pond and a tree to the simulation. Mark these areas out with cones and signs. When rain hits the ground it sinks in. Then the water can travel as groundwater either to the pond (stream, reservoir), or be picked up by a tree root. Water that goes to the Pond can evaporate directly if the Sun gives them an energy card or the water can become a drink for the person and then evaporate as sweat. Water that is absorbed by the tree can take energy from the tree and evaporate through “tree sweat” (transpiration).

For a more complex simulation to work you will need additional captains:

• A Ground Captain to direct raindrops hitting the ground: sending some to the Pond, some to the Tree.
• A Hypothalamus to direct sweating: they should stop the sweating if the person gets into the cold zone, or direct more sweat if the person starts heating up.
• A Tree Spirit: like the Hypothalamus, the Tree Spirit should monitor evaporation from the tree. Just as with the person, as the water evaporates it absorbs energy and cools the tree. The Sun will need to heat up the tree by giving it energy cards.

You can also simulate snow. Have the cloud captain take away enough energy cards so that water molecules in the cloud only have one energy card each. They should follow the instructions on the cards, starting to freeze and form crystals by staying at arm’s length from each other. Small crystals are light enough to be blown around in the cloud, slowly gathering more molecules and building up bigger crystals. When three or more molecules have frozen together, they are heavy enough (in the simulation) to fall to the ground as snow. On the ground, they should stay frozen in place, until warmed up by the sun.

Energy Card Text (See PDF in step one for a printable version):

Energy Card

Instructions:

4 energy cards = water vapor

• When you get your fourth energy card you are ready to evaporate and become water vapor -- a gas!
• Walk around quickly, you don’t have to stay near other water molecules.
• If you are on the ground: fly up to the clouds!

3 energy cards = liquid water

• If you were water vapor: condense, find other molecules with two or three energy cards.
• You are liquid water. You’re warm. Walk around fast, but stay within arm’s length of other water molecules.

2 energy cards = liquid water

• If you were ice, then melt.
• As a liquid, walk around slowly, but staying within arm’s length of other water molecules.

1 energy card = you are cold enough to freeze

• If you are liquid: Freeze - find other water molecules with one energy card. Stay at arm’s length from them. You are now solid ice!
  
• If you are in the cloud: blow around the cloud together as a snow crystal. When you collect three or more molecules, fall to the ground as snow!

This work is made possible by support from STAR, a Biogen Foundation Initiative. The team at Lesley supporting this initiative includes faculty and staff in the Lesley STEAM Learning Lab, Science in Education, the Center for Mathematics Achievement, and other related Lesley University departments and programs.