Whistling Gases

Introduction

A whistle is a “wind” instrument that only makes one note. Or does it? Students will be perplexed as you blow the whistle with various gases to produce a symphony of sounds.

Concepts

• Pitch
• Mass/density of gases
• Speed of sound

Materials

Safety Precautions

Methane gas (natural gas) is flammable. Perform the demonstration in an operating fume hood or well-ventilated laboratory if methane or any flammable gas is used. Do not inhale any of the gases used in the demonstration. Wear safety glasses. Please consult current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

For safety, disperse all balloon gases in an operating hood. The whistles, clamps and extra balloons can be stored for future demonstrations.

Procedure

1. Attach a buret clamp to each of the three ring stands. Place the ring stands in a row, spaced about a foot apart.
2. Attach a whistle to each buret clamp. Rotate the clamp so that the whistle hole faces up (see Figure 1).
3. Blow up one balloon with your lungs and attach a pinch clamp to seal the balloon opening.
4. Blow up two more balloons with different gases and seal each with a pinch clamp.
5. Attach the balloon openings to the mouths of the whistles (see Figure 2).
6. Open the pinch clamp of the air-filled balloon to produce a constant pitch, then clamp the balloon. Ask the class to remember that pitch.
7. Repeat step 6 for a second balloon. Have the class decide whether the note is a higher or lower pitch than that produced by the air-filled balloon.
8. Repeat the procedure for the final balloon. Have class list the balloons in order of the frequencies produced, from lowest to highest. Reproduce the notes, if needed.
9. If using the worksheet, tell the students the name of the gas in each balloon.

Student Worksheet PDF

12538_Student1.pdf

Teacher Tips

• This kit contains enough materials to perform the demonstration seven times: Three whistles, three pinch clamps and 21 11" latex balloons.
• Practice the demonstration until you are comfortable manipulating the balloons and whistles to produce clear tones. It helps to twist the balloon and then attach the pinch clamp.
• Besides using bottled gases, other sources of gases can be used. Chunks of dry ice can be added to a balloon. As the carbon dioxide vaporizes, the balloon inflates. The lab natural gas fixture can be used to fill a balloon. Helium is normally available from party supply stores. Propane burners can also be used, as well as butane burners.
• Use a sound meter to determine wavelength for quantitative measurements.
• A student worksheet is included as an optional assessment tool for the instructor.

Answers to Questions

1. Was the note produced by the second balloon a higher or lower pitch than that of the first balloon?
   

   Higher if the second gas is less dense than the first and lower if the second gas is heavier.

2. List the order of the balloons from lowest pitch to highest.
   

   Answers will vary, but should be in order of highest molar mass of gas to the lowest.

3. The speed of a sound wave in a gas is equal to its wavelength times its frequency (υ = λ × f). For any whistle, the wavelength of the sound produced is constant. Based on the identity of the gases and the notes produced, what can you conclude about the speed of sound in various gases?
   

   If the wavelength is constant, λ = k, then the velocity of sound in a gas is directly proportional to the frequency of the sound produced. Therefore, the speed of sound increases as the density of the gas decreases.

Discussion

The frequency of the notes produced by the gases is a function of their molecular weights, or densities. The speed of a sound wave in a gas (υ) is equal to its wavelength (λ) times its frequency (f).

Each whistle is manufactured with a specific shape so that it produces a small range of wavelengths of sound, independent of the gas moving through it. Thus the frequency or pitch of the note produced depends only on the speed of the sound in the gas being used.

Sound travels faster in lighter gases. Lighter molecules move faster than heavier molecules at a given temperature and the speed of sound in a gas is directly proportional to the speed of the molecule. Sound travels faster in helium (965 m/s @ 0 ºC) than air (331 m/s @ 0 ºC) and carbon dioxide (259 m/s @ 0 ºC). The lighter the gas, the greater the velocity of sound moving throughit. Therefore, the whistle produces higher frequencies when helium is “blown” through it than when air is blown through it and produces a much higher band of frequencies than when carbon dioxide is used (see Figure 3).

A sound meter can be used to measure the wavelength of the sounds produced by the whistle. The frequencies produced by each gas can then be calculated by dividing the speed of sound in the particular gas by this wavelength.

In this demonstration, a good difference in pitch can be found between helium and air or carbon dioxide. The difference between air and carbon dioxide is less noticeable. Methane is between air and helium.