Unit #1

Activity 18
Rainstorm in a Bag

Activity Overview

The cycle of boiling, cooling, and condensing can be observed in a ziplock bag, using a liquid with the appropriate boiling point.

Students put some butane in a ziplock bag, observe it boil and then inflate the bag. Then students place some dry ice on the bag and observe the cycle of condensation and boiling that occurs.

Learning Objectives

Students will:

• Describe how temperature and phase changes cause the water cycle.

Conceptual Prologue

Macro-Micro Connection

The clouds one encounters when ballooning are part of the Earth's natural water cycle. Water from lakes and oceans is heated and evaporates, rises into the atomosphere, cools and condenses into liquid drops we commonly call clouds and is transported many miles on the winds. Eventually, the drops get big enough to fall back to Earth, completing the cycle. As one rides the winds in a balloon, floating by clouds, one may feel a part of this natural cycle.

Science Concepts

Butane or some other refrigerant has a boiling point that is between the freezing point of dry ice and room temperature. This means that dry ice can cause this gas to condense, but room temperature will cause it to boil. If you put some liquid butane (from a lighter refill container) in a ziplock bag it will boil, expanding in volume to fill the bag. If dry ice is placed on the bag, the butane will condense into a liquid where the dry ice is in contact with the bag. The liquid butane will then drip away from the dry ice and boil.

This lab demonstrates many different science concepts:

• States of matter: Gasses occupy much larger volumes than liquids.
• Energy transfer: Energy is transferred through direct contact where the dry ice touches the bag. The butane loses energy and condenses while the dry ice gains energy and sublimes.
• van der Waals forces: As the butane molecules transfer their kinetic energy [energy of motion] to the dry ice, the attractive force between the butane molecules pulls them together into a liquid state, while the additional energy transferred to the dry ice allows those molecules to overcome the attractive forces holding them together as a solid.
• Temperature is the average kinetic energy of atoms: As the dry ice cools the butane, the volume of gas in the bag decreases due to the condensation of the butane into liquid (taking up much less space) and due to the decrease in temperature of the gas inside the bag, slowing down the molecules and decreasing the pressure inside the bag.
• Pressure: As the butane boils it inflates the bag because the increase in the number of gas molecules causes the pressure to increase inside the bag. When the bag expands the presure decreases due to the larger volume available for the molecules.

 

Naive Conceptions

• Condensation is due to something coming through a container.

Condensation is due to a gas changing state into a liquid. Many students think that the water that forms on the outside of a glass during a humid summer day is due to the water leaking through the glass. Water is not leaking through the glass. Water molecules in the air collide with the cold glass, transfer some of their kinetic energy to the glass, in doing so, slow down and begin to clump together. Eventually, a liquid drop is formed. In this activity, some students will think that the liquid condensing under the dry ice inside the bag is "melted dry ice that leaked through the bag". The liquid condensing inside the bag is liquid butane forming in the same way as water on a cold glass as described previously.

Activity Design and Execution

Major Science Concepts:	• Phase change
Assumed Previous Knowledge:	• That gasses occupy more space than liquids. • That molecules are in continual motion. • That temperature is related to kinetic energy [energy of motion] of molecules. • That there are attractive forces between molecules. • That substances change phase by overcoming the attractive forces between molecules or succumbing to those forces. An increase in temperature/kinetic energy [energy of motion] will increase the frequency and intensity of collisions between molecules aiding in their breaking free of the intermolecular forces. If there is a decrease in temperature/kinetic energy, the intensity and frequency of collisions decreases and the molecules are able to clump together.
Time:	• Approximately 50 minutes
Materials:	For each group: • 1 quart sized ziplock bags. • Butane fluid from a lighter refill canister. (The teacher should dispense this. It is helpful to place a piece of glass tubing inside a short collar of rubber tubing to form a nozzle on the butane canister.) • Small pieces of dry ice. • Tongs to handle the dry ice.
Advanced Preparation: (if any)	• If you have a good cooler, you can pick up the dry ice at many local ice merchants a day or two in advance if necessary.

Investigative Question: How does the water cycle work?

1. Warn students that during this activity they will be using two dangerous substances: dry ice and butane.
2. Dry ice is dangerous because it is so cold. They must not touch it with their hands because it can cause instant frostbite. The dry ice is to be only handled with tongs.
3. The butane is flamable, so all sources of flame must be extinguished completely during this experiment.
4. Have students remove all air from a ziplock bag and seal all but a small portion of it.
5. Squirt a small amount of liquid butane from the canister into the students’ bags and have them complete the seal immediately.
6. Have them closely observe the butane as it boils, and tell them to pinch it with their fingers. (It will feel cold because it is absorbing energy from their fingers in order to boil.)
7. Then have them use tongs to place the dry ice on the bag and observe the cycle of condensation and boiling that occurs. If they hold the bag vertically in the air and press the dry ice against the side, then they can collect a good portion of the butane in its liquid form in the corner of the bag.
8. Discuss with them what happened. Ask them to write, including as much detail as possible, about everything that is happening at the atomic level during the cycle they observed. Include motion of atoms, energy transfer, van der Waals attractions, and phase changes for both the butane and the dry ice.

Assessment

Have students write several things in their notebooks:

1. Life on our planet exists due to water. The way water travels around our plant and is recylced is called the water cycle. Explain on an atomic level the water cycle. Be sure to describe how the water evaporates, becomes clouds, and then rains. Why does this happen? You should be able to explain all of this using the model of how atoms move, have energy, and interact.
2. Why do you think ice is slippery?