Kinetic Molecular Theory

Introduction

The kinetic molecular theory may be summarized in one simple phrase—molecules in motion. The following demonstrations provide visual evidence for this important concept. Visualizing molecules in motion helps students understand and compare the kinetic and potential energy of molecules in the solid, liquid and gas phases.

This set of three demonstrations includes:

1. Instant Hot Air Balloon

What forces allow a hot air balloon to rise and stay aloft? Build and launch a mini hot air balloon to discover how temperature affects the motion of molecules and the flight of a balloon.

2. Pink Panther Breath

Investigate the diffusion of gas molecules into a liquid as pink swirls appear when ammonia gas dissolves in a phenolphthalein solution!

3. Molecular Motion Demonstrator

How does the movement of molecules in solids, liquids and gases vary? Use a model to compare the arrangement of molecules and their relative motion in different states of matter. Relate these properties to the kinetic molecular theory.

Concepts

• Conservation of mass
• Density
• Gas laws
• Kinetic molecular theory
• Scientific method
• Gas-liquid miscibility
• Gas diffusion
• Molecular motion
• Solids
• liquids and gases

Materials Included In Kit

Additional Materials Required

Experiment Overview

Instant Hot Air Balloon 

Hot air balloons provide a wonderful example of applied science. The gas laws, in conjunction with the concepts of density and Archimedes’ principle are brought to bear on one of mankind’s oldest preoccupations—flight! Hot air balloons have found their way into science museums and exhibit halls around the world, and many classroom activities have recently been developed for constructing workable, small-scale models, usually made out of tissue paper or lightweight plastic. The following describes the simplest imaginable hot air balloon model—easy to build and quick to launch! 

Pink Panther Breath 

Pink swirls appear as ammonia gas diffuses across a filter paper membrane and dissolves in phenolphthalein solution. Molecules in motion! 

Molecular Motion Demonstrator 

The molecular motion demonstrator can be used to illustrate and help students visualize what solids, liquids and gases look like at the level of atoms, molecules or ions. It can also demonstrate random molecular motion, molecular distances, kinetic theory and concepts related to the results of altering the molecular structure of specific substances.

Materials

Safety Precautions

A “hot” hot plate looks exactly like a cold one! To avoid burns, place a HOT sign next to the hot apparatus before the demonstration and also afterwards to warn students and other teachers that the hot plate is indeed hot.Ammonium hydroxide liquids and vapors are extremely irritating—especially to eyes. Dispense in a hood and ensure an eye wash is accessible. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all laboratory safety guidelines.  Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron whenever working with heat, chemicals or glassware in the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. If there is any ammonia left in the gas-collecting jar allow it to evaporate in a hood. The solution containing water and dissolved ammonia should be disposed of according to Flinn Suggested Disposal Method #10. The BBs can be removed from the tray and stored. The Molecular Motion Demonstrator apparatus can be reused many times.

Prelab Preparation

Instant Hot Air Balloon

1. Have the provided Mylar balloons filled with helium before beginning this demonstration. Helium may be found at party supply stores or gas supply companies.
2. Preheat a hot plate on high before beginning this demonstration.

Pink Panther Breath

1. Obtain a 400-mL beaker and 300 mL of DI water.
2. Using a Beral-type pipet, add 4–5 drops of phenolphthalein solution to the DI water and stir.
3. Transfer the solution to one of the 240-mL gas-collecting bottles so it is completely full.
4. Obtain a piece of 7.5 cm filter paper to cover the opening of the filled bottle. Note: The paper should hang approximately 0.5 cm over each edge of the bottle opening. Trim the filter paper with scissors if necessary.
5. Using a clean Beral-type pipet, add 3–5 drops of concentrated ammonia solution to the second gas-collecting bottle. Keep bottle covered with a watchglass until it is time to perform the demonstration.

Molecular Motion Demonstrator

Practice using the device on an overhead projector before demonstrating with it to the class. With a little practice you will become adept at shaking the apparatus to illustrate key ideas while keeping the BBs in focus on the overhead projector. Use the demonstrator with various lessons during different times of the year to illustrate the nature of matter and molecular motion.

Procedure

Instant Hot Air Balloon

1. Cut off a small portion of the neck of the Mylar balloon, and squeeze out a small portion of the helium (about 5%) to allow some noticeable slack in the balloon. Note: Do not remove too much helium; more helium can always be squeezed out, but more cannot always be put back in! See the Tips section for an alternative method of removing some helium from the balloon.
2. After removing a small amount of helium, twist the neck of the balloon, and use the plastic twist-tie to close it off.
3. Cut off the top of the Styrofoam cup so that 5 cm of the bottom of the cup remains. The bottom portion will be used as a gondola.
4. Use a pencil tip to poke three evenly spaced holes around the outside of the cup (see Figure 1).
5. Attach a 6" piece of string to each hole in the cup and tie overhand knots. Tie the ends of the string to the neck of the balloon using overhand knots as well (see Figure 2).

6. Add a small amount of weight (pieces of masking tape, paper clips or paper towel) to the cup until the entire balloon assembly just barely sinks.
7. Show the class that the balloon has lost some of its helium, and that it no longer has enough buoyancy to lift the attached gondola. Have the students brainstorm different ways the balloon could be made to float again (add more helium, lighten the load, flood the room with a denser gas...). Have the students answer Question 1 of the Instant Hot Air Balloon Worksheet at this point.
8. Hold the balloon over the hot plate but not too close that it might cause melting. 
9. Have students record all observations in the Instant Hot Air Balloon Worksheet. The balloon will noticeably expand as it is heated. When it appears to be inflated to capacity, release the balloon and observe as it rises to the ceiling. This is especially impressive in an auditorium with a high ceiling. Then, as the balloon cools back to room temperature, observe as it descends back down to the floor.
10. Have students answer the remaining questions on the Instant Hot Air Balloon Worksheet.

Pink Panther Breath

1. Place the filter paper on the top of the bottle containing the phenolphthalein solution. Note: It is crucial that the bottle is completely full so no air bubbles are trapped between the water and the paper.
2. Uncover the bottle with the ammonia solution.
3. Holding the filter paper on the bottle containing the phenolphthalein solution, invert the bottle and place it over the bottle containing the ammonia solution.
4. The ammonia gas will diffuse up into the bottle containing water and phenolphthalein, resulting in pink streaks in the water. To better view the color changes due to gas diffusion, place the bottles on an overhead projector or light box and place a white sheet of paper behind the bottle. As the light is shown from below the pink color is accentuated.
5. Have students record observations and answer the questions on the Pink Panther Breath Worksheet.

Molecular Motion Demonstrator

Solid/Liquid/Gas

1. To represent the three states of matter, fill the demonstration tray as follows:

Gas ⅓ full
Liquid ⅔ full
Solid nearly full

2. Gently shake the tray slightly above the glass on an overhead projector. Find the focal plane so that the BBs are clearly visible on the screen. Note: The particles in the solid state should be closely packed so that only rotational and vibrational motions of the particles are possible.
3. Have students answer Questions 1, 2 and 3 on the Molecular Motion Demonstrator Worksheet.

Heating and Cooling Solids

In the preparation of metals, metallurgists often change the nature of metals by heating and cooling the metals in different ways. Different treatments result in different final arrangements and give metals different final properties. Annealing, hardening and tempering of metals are examples of different treatments. These processes can be illustrated with the BB arrangements in the demonstration tray.

1. Annealing results when iron is heated red hot and then slowly cooled. Annealing results in the iron atoms arranging themselves into more perfect crystals. This process makes the metal easier to bend. To visualize the annealing process, fill the tray about⅔ full with BBs and slowly rotate the tray in a circular fashion to simulate the slow, red-hot heating process followed by slow cooling. Slowly tilt the tray to about a 10° angle and continue to use short little shakes to get the BBs to line up in a nice crystalline lattice. The BBs should be perfectly lined up all in rows and columns. This arrangement of iron atoms makes it easy to bend.
2. Hardening results when metal is heated and then allowed to cool quickly. When cooled quickly, the atoms do not have a chance to line up in a neat crystalline pattern as they do when they are annealed. Instead, they line up in a disorganized pattern resulting in a metal that is hard and brittle. Knife blades and drills are made of hardened metal. Simulate this process by slowly rotating the tray again until the BBs are all spread out. Then quickly stop the movement while tipping the tray quickly at a 10° angle. The BBs should form a disorganized array in the tray and should be very different than the annealed structure.
3. Tempering of metal is accomplished by hardening the metal (step 2) followed by gentle warming and slow cooling of the metal. This process results in a metal that has properties intermediate between those annealed (easily bent) and those hardened (nearly unbendable). Tempered metal is often used to make things like springs. The demonstration to illustrate the tempering process starts with a repeat of step 2 (hardening). After showing a hardened pattern of BBs, gently shake the tray until slightly more than half the BBs line up (to show slow warming) and then stop (to show cooling). The result will be a pattern that resembles a mixture of steps 1 and 2.
4. Have students record all observations and answer the questions in the Molecular Motion Demonstrator Worksheet.

Student Worksheet PDF

11946_Student1.pdf

Teacher Tips

• In Instant Hot Air Balloon, to release some helium without cutting the balloon, insert a rigid plastic tube (such as the stick from a small Mylar balloon) into the neck of the balloon. When the end of the tube extends beyond the sleeve inside the balloon, helium will escape. When enough helium has been released, withdraw the tube. No need to tie off the neck of the balloon—the pressure inside seals off the sleeve.

• The amount of helium expelled, the amount of weight added, and the duration of the heating all depend on the size of the balloon and the output of the heat source. None of these factors, however, is all that critical, so long as the balloon has sufficient helium left in it and sufficient room to expand, and providing the balloon is not overly weighted down. It is best to “play around” with these variables until the desired effect is obtained. Once it is, the balloon should last several days with little adjustment needed.
• If a balloon with dark lettering or images is used, the dark side needs to be kept on top, away from the heat source. If not, these dark areas will absorb the heat and possibly melt holes through the Mylar.
• If using a Meker burner with a well-spread out flame to heat the balloon, hold the balloon at least 50 cm above the burner. Rule of thumb—if it is too hot to place your hand there, then it is also too hot to place the balloon.
• Without physical damage, the Molecular Motion Demonstrator can be reused many times.
• It is imperative to practice the techniques in this demonstration to achieve the desired visual effects.
• With your imagination, the tray and BBs can be used to illustrate other principles of molecular motion.

Answers to Questions

Instant Hot Air Balloon Worksheet

1. Describe several possible ways the mini hot air balloon could be altered to make it float.
   

   Heat the balloon, lighten the load of the gondola, flood the room with a denser gas, etc.

2. According to the kinetic molecular theory, temperature is related to the average kinetic energy of particles. Describe the effect of heating the balloon on the kinetic energy of the gas particles and the overall volume of the balloon.
   

   Heating increases the kinetic energy of the gas particles and increases the overall volume of the balloon.

3. Why does the balloon model in this demonstration float?
   

   The balloon is being struck by more gas particles per unit time on its underside than on its top. This amounts to a slight buoyant force acting upward on the balloon. This buoyant force acts upward on all objects surrounded by air. However, it is usually negligible compared to the weight of the object. For very low density objects, however, this buoyant force can be substantial and can even overcome the object’s weight—as it does in the case of the hot air balloon model.

4. How is the balloon in this demonstration similar to a hot air balloon? How is it different?
   

   The model made in this activity is not a perfect replica of a down-scaled hot air balloon. Real hot air balloons are filled with hot air rather than hot helium, but given the limited temperature increase, even the lightweight balloon is too heavy to be buoyed up by such a small volume of hot air, so helium is necessary. Also, hot air balloons are open systems—not closed, but again, the use of helium necessitates that the balloon be closed. In addition, hot air balloons carry their heat source on board and can stay aloft as long as their fuel lasts. The model shows a similar effect in that as the temperature of a gas sample rises, its density decreases and its buoyancy increases.

Pink Panther Breath Worksheet

1. What gas molecules are present in the “empty” bottle at the bottom of the demonstration setup?
   

   Ammonia gas molecules.

2. Describe the evidence from this demonstration that these invisible gas molecules are always moving.
   

   Pink streaks formed in the solution in the upper container as the “invisible” gas molecules moved from the lower container and across the filter paper.

3. What is the term(s) given to the effect that occurred during this demonstration?

Gas diffusion, pH indicator change, etc.

Molecular Motion Demonstrator Worksheet

1. What do the BBs in this model represent?
   

   Each BB represents a particle of matter—an atom or a molecule.

2. Describe the nature of each state of matter as represented by the BBs.
   

   Gas—The BBs moved freely and very rapidly when shaken.

   Liquid—The BBs still had random “free” movement but not as much movement occurred as was seen in the gas example.

   Solid—The BBs were packed close together and occupied fixed positions. The molecules still moved but could only rotate and vibrate.

3. Describe how are the different states of matter are alike and how they are different.
   

   A solid is matter that has a definite shape and volume. A solid does not depend on the shape or volume of a container. A liquid is a form of matter that flows, has a fixed volume, and takes the shape of its container. A gas is matter that takes the shape and volume of its container.

4. What occurs when a solid metal is heated and then cooled slowly?
   

   Annealing results when certain metals are heated red hot and then slowly cooled. Annealing results in the iron atoms arranging themselves into more perfect crystals.

5. What occurs when a solid metal is heated and then cooled rapidly?
   

   Hardening results when metal is heated and then allowed to cool quickly. When cooled quickly, the atoms do not have a chance to line up in a neat crystalline pattern as they do when they are annealed. Instead, they line up in a disorganized pattern resulting in a metal that is hard and brittle.

6. Describe the process of tempering.
   

   Tempering of metal is accomplished by hardening the metal followed by gentle warming and slow cooling of the metal. This process results in a metal that has properties intermediate between those annealed (easily bent) and those hardened (nearly unbendable).

Discussion

Instant Hot Air Balloon

The model made in this activity is not a perfect replica of a down-scaled hot air balloon. A classroom discussion, however, concerning the similarities and differences between the two systems would likely enhance the understanding of both. Real hot air balloons are filled with hot air rather than helium, but given the limited temperature increase in the lab model, even the lightweight balloon is too heavy to be buoyed up by such a small volume of hot air, so helium is necessary. Also, hot air balloons are open systems—not closed, but again, the use of helium necessitates that the balloon be closed. In addition, hot air balloons carry their heat source on board and can stay aloft as long as their fuel lasts. The model’s small scale again makes this unfeasible. In effect, the helium balloon model is best thought of as a cross between a blimp (dirigible) and a hot air balloon. Whatever the case, the demonstration is effective in conveying the main idea behind hot air balloons—that as the temperature of a gas sample rises, its density decreases and its buoyancy increases.

It is especially effective to show students the demonstration first, and then have them explain it while showing it to them a second time. While heating the balloon ask, for example, “What is happening now to the mass of the balloon?” This usually elicits three different responses. Those who confuse mass with how big an object is (volume) will say the mass is increasing. This misconception is worth discussing. Those who anticipate that the balloon is about to float away and must therefore be getting lighter will argue that the mass is decreasing. This misconception also warrants discussion. Those who really understand the concept of mass will know that the mass must be staying constant (with nothing entering or leaving the system, how could it be otherwise?). With mass remaining constant and volume quite visibly increasing, the density must clearly decrease, and when the density drops below that of air, the balloon is able to lift off. Once aloft and away from the heat source, the balloon begins to cool. As it does, its volume decreases, its density increases, and the balloon sinks back down to the floor.

Students may ask if the balloon could be kept from cooling off by heating up the entire room. This might keep the balloon fully inflated, but it would also decrease the density of the air in the room and would thus prevent the balloon from ever taking off at all. It is important that students understand it is not the hot air (or hot helium) that lifts the balloon; it is actually the surrounding air. We tend to think of gas particles as being in true random motion, completely uninfluenced by gravity. Yet these particles do have mass—albeit small—and are therefore slightly attracted to the Earth. (Indeed, if they were not, we would have no atmosphere at all surrounding our planet.) Since they are attracted to the Earth, the particles are invariably more concentrated closer to the Earth’s surface. This is quite evident when one compares air at sea level with air at the top of a high mountain, but it also holds true over smaller distances—the air is slightly more concentrated beneath a given object than it is above it. Like all objects, the balloon is thus being struck by more gas particles per unit time on its underside than on its top. This amounts to a slight buoyant force acting upward on the balloon. This buoyant force acts upward on all objects surrounded by air, including ourselves, but it is usually negligible compared to the weight of the object. For very low-density objects, however, this buoyant force can be substantial, and can even overcome the object’s weight—as it does in the case of the hot air balloon.

Pink Panther Breath

Gas diffusion refers to the mixing of different gases throughout an enclosed container due to the random molecular motion of gas particles. As ammonia molecules enter the gaseous phase from the concentrated ammonia solution, they mix with the existing oxygen and nitrogen molecules in the air and pass through the pores in the filter paper. The ammonia gas molecules then dissolve in the water. The resulting chemical (acid–base) reaction with phenolphthalein indicator causes a color change from colorless to pink.

There is a dual equilibrium which appears to exist when ammonia gas dissolves in water. The fact that ammonia gas is easily driven off when the concentrated ammonia solution is heated suggests a simple solubility equilibrium (Equation 1) to give hydrated ammonia molecules, NH₃(aq).

The indicator color change, however, reveals that the resulting aqueous ammonia solution is basic. The acid–base reaction of ammonia with water is shown in Equation 2.

The solubility of ammonia gas in water is extremely high—approximately 400 liters of ammonia gas will dissolve in one liter of water at room temperature! As with any gas, the solubility of ammonia gas in water decreases as the temperature increases.

Molecular Motion Demonstrator

The material things around us are all various types of matter. Matter is anything that takes up space and has mass. Matter can exist in three different physical states: solid, liquid and gas. A solid is matter that has a definite shape and volume. A solid does not depend on the shape or volume of a container. A liquid is a form of matter that flows, has a fixed volume and takes the shape of its container. A gas is matter that takes the shape and volume of its container.

The word kinetic means motion. The kinetic theory states that the tiny particles in all forms of matter are in constant motion. These particles may be atoms, ions, or molecules of gases, liquids, or solids. In the Molecular Motion Demonstrator the various states of matter will be visualized using small metallic spheres (BBs) to represent the particles in matter. The BBs can be placed in motion by gently or vigorously shaking the motion demonstrator from side to side. The arrangement and activity of the BBs in the apparatus can be used to represent the different states of matter as well as to observe the changes that occur on the atomic or molecular level during changes of state (e.g., melting, boiling, freezing).

References

A video of the Instant Hot Air Balloon activity, presented by Bob Becker, is available in Kinetic Molecular Theory and PTV, part of the Flinn Scientific—Teaching Chemistry eLearning Video Series.

A video of the Pink Panther Breath—Macroscale activity, presented by John Little, is available in Properties of Ammonia, part of the Flinn Scientific—Teaching Chemistry eLearning Video Series.