Hearing differences between molecules


Students are surprised to find that their sense of hearing can give them scientific information quickly if they know what they are listening for. You can do this demonstration qualitatively, or you can "teach a little physics" by using a resonance tube to measure the actual speed of sound.


2 large balloons
2 pinch clamps
1 pea-whistle
Tanks of gases (CO2, He or H2 at least)

1 metre long tube (3-cm inner diameter)
1 piezo buzzer attached to about 1-m long electrical cord
1 9V battery with a connection harness
1 toggle switch



Blow up a balloon with your lungs, clamp the balloon and attach the pea whistle to the mouth of the balloon. Instruct students to listen to the pitch of the whistle and remember the sound. Release the clamp.

Fill a balloon filled with carbon dioxide gas. Use a pinch clamp to keep the gas in the balloon. Attach a pea-whistle to the mouth of the balloon. Release the clamp and let the gas blow the whistle. You can add a bit of extra pressure by pulling the balloon against your stomach.

Use a wet (to avoid static!) balloon which you fill with hydrogen [Care!] or helium gas and repeat the demonstration.


Connect a piezo buzzer to a 1-metre long lamp cord. Connect a switch and a 9V-battery harness to the other end of the wire. Lower the buzzer into a tube filled with air and closed at one end. Note the distance between positions where the sound grows noticeably louder. This represents one-half wavelength.

The cylinder can be filled with CO2 by holding the tube vertically and add the gas from the top. Repeat the measurement procedure while covering the top of the tube with a paper towel to minimize mixing air with the gas in the tube.

The cylinder can be filled with Hydrogen by holding the tube vertically with the closed end at the top and add the gas from the bottom of the tube. Tape the buzzer to a thin stick and push it up into the tube to repeat the wavelength measurement.

Note that for a 2800 Hz buzzer the H2 resonant positions will be about 23 cm apart, while the CO2 resonant positions are about 5 cm apart.


[These suggestions are NOT intended to be a complete review of all the safety issues involved with this activity. Professional judgement and practices are essential. If you are unsure of the safety precautions that should be taken, seek experienced assistance.]

Hydrogen must be handled with extra care. Moisten the inside of the balloon before filling to avoid any static sparks. Do NOT let the gas vent from the tank into the air rapidly. H2 can auto-ignite under these conditions. Venting the gas from the moist balloon into the whistle poses no hazard.

Be sure there are no sources of flame nearby.

There are no specific hazards associated with small volumes of CO2 or He.

Proper care should be exercised when using any tank of compressed gas.


PART A: The difference in frequency between He (or H2) and CO2 is dramatic. Most students know that they can "talk funny" if they inhale helium, but few know why the trick works. You can explain that the speeds of sound are different in each gas. Since the structure of the whistle determines the wavelength of the sound vibrations, sound with higher speed will have a higher frequency. My point for a chemistry class is that their sense of hearing can give them scientific information about molecules if they know what they are listening for (and it gives a forum to talk about safety and the possibilities of asphyxiation when inhaling an inert gas such as helium).

PART B: There are many sources of student experiments to measure the speed of sound. My version makes it easy to compare speeds of different gases. Of course, the more care taken in loading the cylinder with pure gas, the more accurate your results will be.

In general, speed of sound depends on the temperature, the molecular mass and the number of degrees of freedom of motion exhibited by the gas molecule.

Dr. C.R. Nave of Georgia State University derives the following formulae in the excellent "Hyperphysics" site listed below.

AIR V = [331.5 + 0.6 T Celsius] m/s


GENERAL: V sound = √¯ γ R T / M

R = gas constant = 8.314 J
T = temperature in Kelvins
M = molecular weight
γ = the adiabatic constant, characteristic of the specific gas

Since the adiabatic constant g for a gas is the ratio of its specific heats, the value depends upon the effective number of degrees of freedom in the molecular motion. [ γ = Cp / Cv ]

The frequency of my buzzer is listed on the package as 2800 Hz.

Using v = f * L , we can predict that the distances between resonant points in the cylinder will be:

½ L air= 6.1 cm

½ L H2 = 23 cm

½ L He = 16.5 cm

½ L CO2 = 4.6 cm (@ 0ºC)

Selected Sound Speeds in Gases


Temperature (ºC)

Speed in m/s










Carbon dioxide






Water vapor



[Table from Hyperphysics website]



I first saw The Whistle Demo over 20 years ago in Chem13News (Nov. 1976) listed as a "Quickie" attributed to Denman C. Evans and contributed by C.R. McNeill.

Speeds of sound data are from the excellent web-site called Hyperphysics by Dr. C.R. Nave, of Georgia State University. He also includes a very clear discussion and derivation of the adiabatic constant.

[ http://hyperphysics.phyastr.gsu.edu/hbase/hframe.html ]