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# 15.10: Demonstrations for Chapter 15

Difficulty Level: At Grade Created by: CK-12

## Charles Law with a Balloon and a Bunsen Burner

Brief description of demonstration

A Bunsen burner is lit and adjusted to a cool flame. Students then hold a partially inflated balloon over the flame. The balloon inflates rapidly, which they feel.

Materials

• Bunsen burner
• Balloons, several
• Matches

Procedure

Light the Bunsen burner, and adjust to a cool flame. Inflate the balloons by blowing into them until they reach a diameter of $20$ to $25 \ cm$. Tie the end off. Have the students hold the inflated balloon about $30 \ cm$ over the flame. The balloon will rapidly inflate.

Hazards

The Bunsen burner can cause burns or ignite clothing if the student gets too close. Demonstrate how to hold the balloon before allowing the students to do it. The balloon inflates so rapidly and is such a strange tactile feeling that the students often think it’s alive, which can be startling to some. Be aware of over-reaction, and clear away any breakable items that may come into harms’ way.

Disposal

Throw the used balloons away.

Discussion

For a balloon that is $20 \ cm$ in diameter, the circumference of the balloon will change about $7 \ mm$ for every applied $10^\circ C$ temperature change. This is a relatively small change, but it does happen almost instantaneously, which surprises students.

## Brownian Motion Demonstration

Brief description of demonstration

The Brownian Motion Apparatus consists of a metal chamber with a glass viewing window on top and a lens on one side to allow a light source to shine in the chamber. Smoke from a smoldering rope or an extinguished match is drawn into the chamber through an inlet tube by squeezing the rubber bulb. The chamber is illuminated by light shining through the side lens (laser light is best, if you have a laser pointer). The smoke cell sits on a microscope stage and by focusing through the top viewing window, smoke particles are visible (like shiny stars) against a dark background. The smoke particles jiggle about as they are bombarded by air molecules. There is also some sideways drift due to convection currents. If you have a projecting microscope, the image can be projected on a screen so everyone can see at the same time. Sometimes the smoke particles seem to explode as they rise or sink out of focus. The smoke activity in the cell will die down after a while so a new puff of smoke must be drawn in now and then.

Materials

• Microscope or projecting microscope
• Source of smoke (smoldering rope or an extinguished match)
• Brownian Motion Apparatus

Procedure

1. Place the Brownian Motion Apparatus on the microscope stage.
2. Adjust the side light into the lens.
3. Squeeze the rubber bulb and hold it empty.
4. Light and blow out a match. Hold smoking match near the inlet aperture and release the squeeze bulb.
5. Have the students take turn viewing unless you have a projecting microscope or a TV camera.

Hazards

None.

Disposal

Dispose of cooled matches in waste basket.

Discussion

Make sure the students understand that the smoke particles are much too large to exhibit molecular motion. The movement of the smoke particles is due to collisions from air molecules that do have molecular motion.

## Molecular Motion/Kinetic Energy Demo

Brief description of demonstration

The apparatus is a glass tube with a small amount of mercury in the bottom and some glass chips floating on the mercury. (The apparatus is sometimes referred to as Stoekle’s Molecular Apparatus) At room temperature, the molecular motion of the mercury molecules is not forceful enough to move the glass chips.

When the tube is held over a Bunsen burner, however, the molecular motion of the mercury molecules increases, and the collisions of the mercury molecules with the bottom of the glass chips sends them flying to the top of the tube.

Materials

• Molecular motion demonstration tube
• Utility clamp
• Bunsen burner
• Pot holder (something to lay the hot tube on while cooling)
• Matches

Procedure

Clamp the demonstration tube in a utility clamp so you can hold it without burning your fingers. Show the tube to the students at room temperature. Hold it over a Bunsen burner flame until the glass chips begin flying to the top of the tube. Allow the tube to cool before returning to storage.

Hazards

The tube gets very hot so be careful not to burn fingers. Mercury vapors are hazardous so it is important to be very careful not to break the tube. If you see any cracks in the tube, do not use it. It should be discarded according to disposal rules for mercury. It’s best to store the tube from year to year in bubble wrap.

Discussion

As the mercury molecules leave the liquid due to heating, they collide with the glass chips and knock them high into the tube. When the tube is removed from the heat, tiny droplets of mercury can be seen to condense on the walls of the tube.

## Rate of Diffusion at Various Temperatures Demo

Brief description of demonstration

Drops of food color are added to water at three different temperatures and the time required for complete diffusion is observed.

Materials

• Hot Plate
• Ice
• $3 - 250 \ mL$ graduated cylinders
• 3 – dropper pipets
• Thermometer
• Wall Clock with second hand

Procedure

1. Fill one graduated cylinder with tap water at room temperature, (this water needs to sit in the room for several hours to reach room temperature. (Fill a clean milk carton with tap water the day before the demonstration.
2. Heat tap water to boiling and fill the second graduated cylinder.
3. Make ice water and stir until no more ice melts. Fill the third graduated cylinder with ice water (but no ice cubes).
4. Record the temperature of the water in each cylinder.
5. Allow the water to settle for a couple of minutes.
6. Add two drops of good coloring to cylinder at the same time. Note the time.
7. Record how long it takes for the food coloring to be completely dispersed in each cylinder.

Hazards

Boiling water can be a burn hazard.

Disposal

All solutions can be disposed of down the sink.

Discussion

In which cylinder did the dye spread out the fastest.

In which cylinder did the dye spread out the slowest.

Give reasons for the different times of diffusion of the three cylinders of water.

The same molecules at different temperatures are moving at different speeds. At higher temperatures, the speed of the molecules is greater. Molecular collisions occur with greater frequency and with greater force.

If you wish, you could relate what the student sees in this demo to increased gas pressure at higher temperature and increased effusion rates (escape through a pinhole) at higher temperature.

## Magdeburg Hemispheres Demonstration

Brief description of demonstration

Two five inch diameter hemispheres are placed together and the air is removed from inside the resulting sphere with a vacuum pump. A strong student is asked to pull the two hemispheres apart (unsuccessfully).

Purpose

To demonstrate the powerful force of air pressure.

Materials

• Magdeburg Hemispheres
• Vacuum grease
• Vacuum pump (a hand pump will do)

Procedure

When the valve is opened and air allowed in, the hemispheres come apart easily.

If you have a bell jar large enough to enclose the hemispheres, you can put the hemispheres inside the bell jar and remove the air from the bell jar (surrounding the hemispheres) and the hemispheres will fall apart.

Hazards

An air leak may allow the hemispheres to come apart suddenly.

Discussion

The actual Magdeburg hemispheres were around $20 \ inches$ in diameter and were designed to demonstrate a vacuum pump that von Guericke had invented. When the air was removed from inside the hemispheres, and the valve closed, the hemispheres were held together by the air pressure of the surrounding atmosphere.

It is not known how good a vacuum von Guericke’s pump was able to produce. If all the air were removed from the inside, the hemispheres would have been held together with a force of around 20,000 Newtons, equivalent to lifting a car or a small elephant.

Von Guericke’s demonstration was performed on May 8, 1654. Thirty horses, in two teams of 15, could not separate the hemispheres until the vacuum was released.

Aug 18, 2012

Aug 13, 2014