A Flame Tube, also known as a Ruben’s Tube, is a classic physics experiment that provides a spectacular visual demonstration of sound waves.1 To make a Ruben’s Tube, a bunch of tiny holes are drilled in a line about 1 cm apart along one side of a steel pipe (Figure 1). A flexible membrane is stretched over one end of the pipe, while the other end is sealed off, except that a flammable gas (I use propane, C3H8) is pumped through the closed end. Finally, a speaker is set up next to the flexible membrane. When the gas is ignited, flames form all along the tiny holes on the side of the tube. When the speaker is turned on, sound waves travel through the propane inside the tube. Because sound is a compression wave, the sound waves from the speaker make the gas molecules in the tube bunch up in certain regions and thin out in others. The places in the tube where the gas molecules bunch up causes areas of high pressure – and therefore large, yellow flames! The places in the tube where gas molecules thin out result in areas of low gas pressure – and therefore small flames – or no flames at all.
Figure 1: A waveform of flames emitted from a Ruben’s Tube. Notice that a rubber glove, acting as a flexible membrane, is stretched over the open end of the tube on the left. Propane gas is pumped into the far end of the tube on the right. A speaker is set up next to the flexible membrane. Sound waves emitted from the speaker set up compression waves in the tube, which produce the waveform observed.
I have used a Ruben’s Tube in my classes to teach students about properties of waves for quite some time. In addition, there happens to be a few chemistry concepts that can be explored when using the Ruben’s Tube, and I’d like to share some of these with you. In the video below you can see my Ruben’s tube in action, and also view a few ways that chemistry experiments connect to its operation and use. Below the video I discuss some of these chemical topics in a bit more detail.
Video 1: Flame Tube Experiments2
Chemistry experiments with the Ruben’s Tube:
The combustion of propane (C3H8) is certainly important in the operation of the Ruben’s Tube. When propane exits the tiny holes in the top of the tube, it reacts with O2 in the air. The chemical reaction that describes this reaction is:
C3H8 (g) + 5 O2 (g) → 3 CO2 (g) + 4 H2O (g) Eq. 1
If there is plenty of oxygen around to completely burn all of the propane flowing out of the tube, a beautiful blue flame results. When this is the case, complete combustion is occurring. Blue flames are commonly observed when carbon containing compounds are combusted. I happen to find it particularly fascinating – as seen in the video – to notice that water collects around the holes shortly after the gas is first ignited. Being a chemist, I shouldn’t be surprised to see water form as a result of combustion. Nevertheless, seems a bit remarkable to me to observe water collecting in such close proximity to a flame. My students enjoy it when I point this out to them, too.
As the tube fills with more propane, the rate of gas flow out of the tube increases. This increases the rate of combustion, and the flames to burn a bit hotter. As a result, the water that collects around the holes evaporates:
H2O (l) → H2O (g) Eq. 2
Furthermore, when the gas flow increases, the concentration of oxygen in the air isn’t high enough to completely combust all of the propane. In this case, incomplete combustion occurs along with complete combustion. During incomplete combustion carbon monoxide and soot, instead of CO2, are formed. Two chemical equations that can be used to describe the incomplete combustion of propane include:
2 C3H8 (g) + 7 O2 (g) → 6 CO (g) + 8 H2O (g) Eq. 3
C3H8 (g) + 2 O2 (g) → 3 C (s) + 4 H2O (g) Eq.4
The shift from a blue colored flame to a yellow colored flame indicates that some incomplete combustion is occurring. The yellow color results from the fact that soot emits yellow light (from incandescence) when heated to the high temperatures within the flame.
Interestingly, the Ruben’s Tube can be used to measure the molar mass of propane. Here’s how this works: The velocity, v, of sound in a gas can be calculated as:3,4
Eq. 5
Where R = 8.314 J mol-1 K-1, T is the Kelvin temperature, g is the heat capacity ratio of the gas (Cv/Cp), and M is the molar mass (units of kg mol-1). For propane, g = 1.29 at 300 K.5 Rearranging Equation 5 yields:
Eq. 6
The velocity of a wave is simply the product of its frequency, f, and wavelength, l:
v = f l Eq. 7
In the video above (Video 1) you can see that four measurements of frequency and corresponding wavelength were made (Table 1). These measurements allowed for determinations of both the speed of sound in propane (Equation 7) and the molar mass of propane (Equation 6).
Table 1: Speed of sound in propane and molar mass of propane calculated from measurements in Video 1. T = 300 K and g = 1.29 were used in the calculation the molar mass.
|
Trial |
Wavelength / m |
Frequency / s-1 |
Velocity / m s-1 |
Molar mass / g mol-1 |
|
1 |
0.60 |
430 |
258 |
48 |
|
2 |
1.04 |
252 |
262 |
47 |
|
3 |
0.82 |
324 |
266 |
45 |
|
4 |
0.70 |
374 |
262 |
47 |
The average molar mass of 47 g mol-1 calculated in these trials matches fairly well with the known molar mass of propane (44 g mol-1). The difference between the measured value and the known value could be due to the propane not being pure.6 It could also be that the gas in the Ruben’s Tube was lower than 300 K. For example, g = 1.27 at 295 K.5 Using these values for g and T in Equation 7 and the measurements obtained in the trials, one obtains an average of 45 g mol-1 for propane.
Conclusion:
The Ruben’s Tube is a fascinating experiment that is conducted in many physics classes, but it also provides avenues for exploring chemical themes. One thing we have tried (with mixed success) is building a Ruben’s tube out of copper in the hopes of generating green flames. I’d be very happy to hear if anyone has any suggestions on how to get a fully functional, green-flamed Ruben’s Tube to work!
Acknowledgements:
A HUGE thank you to Joshua Frederick and John Bush who built the Ruben’s Tube for me as part of their small research project in my General Chemistry II class.7
References:
1. Gee, K. L. The Ruben’s Tube Proc. Mtgs. Acoust. 2009, 8, 025003. https://asa.scitation.org/doi/pdf/10.1121/1.3636076
2. Tommy Technetium, Flame Tube Experiments (Ruben’s Tube)
3. Varberg, T. D.; Pearlman, B. W.; Wyse, I. A.; Gleason, S. P.; Kellett, D. H. P.; Moffett, K. L. Determining the Speed of Sound and Heat Capacity Ratios of Gases by Acoustic Interferometry, J. Chem. Educ. 2017, 94 (12), 1995-1998. https://pubs.acs.org/doi/10.1021/acs.jchemed.7b00526
4. Molek, K. S.; Reyes, K. A.; Burnette, B. A.; Stepherson, J. R. Measuring the Speed of Sound through Gases Using Nitrocellulose, J. Chem. Educ. 2015, 92 (4), 762-766. https://pubs.acs.org/doi/10.1021/ed400653t
5. Trusler, J. P. M. Equation of State for Gaseous Propane Determined from the Speed of Sound, International Journal of Thermophysics, 1997, 18(3), 635-654.
6. Safety Data Sheet for Blue Rhino Propane https://www.ferrellgas.com/media/66083/sds_propane_12_18.pdf
7. https://www.chemedx.org/blog/small-research-projects-chemistry-classroom
Safety
Safety: Video Demonstration
Safety: Video Demonstration
Demonstration videos presented here are not meant as tools to teach chemical demonstration techniques. They are meant as a tool for classroom use. The demonstrations may present safety hazards or show phenomena that are difficult for an entire class to observe in a live demonstration.
Those performing the demonstrations shown in this video have been trained and adhere to best safety practices.
Anyone thinking about performing a chemistry demonstration should first read and then adhere to the ACS Safety Guidelines for Chemical Demonstrations (2016) These guidelines are also available at ChemEd X.
General Safety
General Safety
For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).
For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.
Other Safety resources
RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies
NGSS
Analyzing data in 9–12 builds on K–8 and progresses to introducing more detailed statistical analysis, the comparison of data sets for consistency, and the use of models to generate and analyze data.
Analyzing data in 9–12 builds on K–8 and progresses to introducing more detailed statistical analysis, the comparison of data sets for consistency, and the use of models to generate and analyze data. Analyze data using tools, technologies, and/or models (e.g., computational, mathematical) in order to make valid and reliable scientific claims or determine an optimal design solution.
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models.
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models. Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.
Mathematical and computational thinking at the 9–12 level builds on K–8 and progresses to using algebraic thinking and analysis, a range of linear and nonlinear functions including trigonometric functions, exponentials and logarithms, and computational tools for statistical analysis to analyze, represent, and model data. Simple computational simulations are created and used based on mathematical models of basic assumptions. Use mathematical representations of phenomena to support claims.
Mathematical and computational thinking at the 9–12 level builds on K–8 and progresses to using algebraic thinking and analysis, a range of linear and nonlinear functions including trigonometric functions, exponentials and logarithms, and computational tools for statistical analysis to analyze, represent, and model data. Simple computational simulations are created and used based on mathematical models of basic assumptions. Use mathematical representations of phenomena to support claims.
Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
*More information about all DCI for HS-PS1 can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org.
Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
Assessment is limited to chemical reactions involving main group elements and combustion reactions.
Examples of chemical reactions could include the reaction of sodium and chlorine, of carbon and oxygen, or of carbon and hydrogen.
Students who demonstrate understanding can use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling in various media.
*More information about all DCI for HS-PS4 can be found at https://www.nextgenscience.org/topic-arrangement/hswaves-and-electromagnetic-radiation.
Students who demonstrate understanding can use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling in various media.
Assessment is limited to algebraic relationships and describing those relationships qualitatively.
Examples of data could include electromagnetic radiation traveling in a vacuum and glass, sound waves traveling through air and water, and seismic waves traveling through the Earth.