The Carbon Dioxide Spiral Staircase

the author in tie dye lab coat performing spiral staircase demo

There have been many demonstrations and experiments published online and in the Journal of Chemical Education throughout the years dealing with various aspects of gas chemistry, and CO2 sublimation in particular.1 The demonstration where CO2 generated from dry ice or a chemical reaction is used to snuff out a candle in an aquarium or other container is also well known.2 This article describes a dramatic variation on these demonstrations that allows for discussion of such topics as the ideal gas law, densities of different gases, gas density changes with temperature, miscibility, and viscosity. In addition, this demonstration is useful for providing physical evidence for the existence of a colorless, odorless gas. The device described is easily and inexpensively produced and stored. The demonstration is large scale and works well for classrooms and community outreach events. You will find two videos below showing 1) the set up and procedure with the lights on and 2) the demonstration with the lights dimmed.

 

Video 1: CO2 Staircase Demo with Description of the Apparatus, Douglas Mulford's YouTube Channel (accessed 3/9/2021)

 

When candles, which are composed of paraffin wax, combine with oxygen during combustion, they produce carbon dioxide, water, and unburned hydrocarbons. The CO2(g) produced from the combustion of the wax is at a temperature of approximately 275°C and has a density of 0.978 g/L.3 The density of air at room temperature, 25°C, is approximately 1.18 g/L. As the hot CO2 is at a lower density than the surrounding air, it will rise and escape the container. Students can verify this by observing the fact that the candle will continue to burn on its own while sitting in the bottom of the bottle.

Solid CO2, however, sublimes at -79°C and at one atmosphere of pressure the density at this temperature is 2.76 g/L. As this density is much higher than the surrounding air (1.20 g/L) the cold CO2 can therefore be poured and will flow through the apparatus displacing the air at the bottom of each soda bottle. This causes each flame to extinguish in turn.

 

Video 2: CO2 Staircase Done in a Dark Room, Douglas Mulford's YouTube Channel (accessed 3/9/2021)

 

In addition to the above discussions of gas densities and their dependency on temperature,4 the apparatus can be used to discuss several other topics including the low viscosity of gases, miscibility of gases, the ability of gases to flow, gas diffusion, and ideal gas law calculations. The advice to crawl along the bottom of a burning room can also be explained with this demonstration based on the evidence that the hot gases rise and escape the bottles. Similarly, hot gases from a fire, which would burn your lungs, will be found at the top of the room. Also, the hot carbon dioxide produced in the fire will displace oxygen at the top of the room pushing it down.

This demonstration also allows for an interesting discussion on how chemists can investigate and provide evidence for things that cannot be seen with the human eye. Carbon dioxide is a colorless, odorless gas and cannot be directly observed.5 However, this experiment demonstrates that the gas must exist to be able to explain the observations.6  

Concepts: 

density, gases, gas laws, miscibility, sublimation, viscosity

Time required: 

Classtime: 5-7 minutes if only doing the demonstration or 20-30 minutes if students perform the gas law calculations.

Prep time: One time of an hour to build apparatus then ~10 minutes each use.

Materials: 

Building the apparatus:

  • 6 3/8” x 12” dowels
  • 1 1” x 36” dowel
  • 2 1”x4” 2-foot-long flat boards or similar for base
  • 6 empty plastic soda bottles
  • Clear silicon caulk or similar
  • Knife/scissors for cutting bottles.
  • 5-gallon water jug or similar container
  • Wood glue
  • 1” and 3/8” drill bits

Performing the Demonstration

  • 6 tea light candles
  • ~2 pounds dry ice
  • Butane lighter
Procedure: 

Performing the demonstration

Begin by preparing the CO2 by placing the dry ice in a large container such as a 5-gallon plastic water cooler jug or an aquarium. If you have access to pellet form dry ice you can place this directly into the jug. If you have access only to block dry ice you can carefully cut a door into the side of the 5-gallon jug to introduce larger pieces.  Cut the door on three sides to create a hinge so that it will stay mostly closed while the dry ice sublimates.  Allow the container to sit for approximately 10 minutes to fill with cold CO2 gas.

Light each of the tea light candles. Note that it is easier if you start at the top and light the candles through the COexit holes.  Be careful lighting the last candle.  A long commercial butane lighter, such as often used for lighting campfires, is most convenient.

Carefully pour the CO2 into the top bottle. The candles will extinguish one by one from the top down. Hold the jug with CO2 carefully due to the fact that while pouring, the dry ice inside can shift, sliding forward. If you wish to repeat the demonstration quickly, allow the dry ice to sublimate a few minutes and blow into each of the soda bottles to displace the pooled CO2

Safety

Wear gloves, goggles, and a non-flammable lab coat whenever dealing with hazardous materials and flames.  Dry ice can cause burns—handle with appropriate gloves and never place in a sealed container.  Excessive dry ice in an enclosed space can represent an unlikely, but real, asphyxiation hazard. Keep flammables away from open flames.  Be careful to prevent burns when lighting the candles and use only tea candle sized candles to prevent the dowels from burning.  

Questions: 

The following questions are included in a Student Document that can be found in the Supporting Information. A Teacher Key is available there as well. Log into your ChemEd X account to ensure that you find all the available resources.

Let’s take a moment to explore some gas law calculations and how they can relate temperature to the density of gases. Recall that density is measured in mass per volume. For liquids, this is usually in g/mL but gases, being much less dense, are usually reported in g/L.

1. When candles, which are composed of paraffin wax, combine with oxygen during combustion, they produce carbon dioxide (CO2), water, and unburned hydrocarbons. Looking at the CO2, when it is released from the candle flame it is at about 275°C. Use PV=nRT to figure out what volume 1.0 mole of CO2 at this temperature would occupy at 1.0 atmosphere of pressure.

2. We really want to know the density of CO2 at this temperature. Remember that densities of gases are reported in g/L so what is the mass, in g, of 1.0 mol of CO2?

3. Time to bring it all together. What is the density of CO2 gas at 275°C in g/L?

4. Solid CO2, dry ice, sublimes at -79°C. What is the density of CO2 at this temperature? Hint: calculate the same way as in Q1-3 above but at the new temperature.

5. The air you breathe is a mixture of mostly nitrogen and oxygen along with several other gases.  If the average molar mass of air is 28.9647 g/mol, calculate the density of air at a room temperature of 25°C.

6. So, what does all this mean? Compare the density of CO2 at 275°C compared to air at 25°C. If we put a candle in the bottom of a 2-L soda bottle that has had the top cut off, should the candle keep burning or will it run out of oxygen?

7. Now let’s compare room temperature air with the cold CO2 from dry ice. If we add the cold CO2 to the bottle with the candle in the bottom what would you predict to happen?

Preparation: 

Building the Demonstration Apparatus 

  1. Create a stable base using 1x4s or similar in an x-pattern approximately 2 feet on a side.  You can place one board on top of the other and add small blocks on the bottom ends of the top board to create a very stable structure. Drill a 1” hole in the center of the base to place your vertical dowel in. 
  2. Drill six 3/8” holes into a 36” x 1” wooden dowel with each hole 90° around and 3.75” down from the first hole. Note that a slightly larger apparatus can be made by using a 1.25” dowel, 8 holes, and 60° spacing around the dowel.  Affix with glue each of six 12” by 3/8” wood dowels to support the soda bottle candle holders.
  3. Cut the top off of six 2-liter soda bottles and hang them from the dowels by cutting two 3/8” holes near the top of each soda bottle. Note that the bottles need to have straight sides to allow for good contact with the neighboring bottles—not all soda bottles have this shape. 
  4. Cut a rectangular entry and exit hole for CO2 flow in all bottles. Entry holes should be approximately 2” from the top of the bottle to prevent CO2 from escaping and should be at least 1” wide and 1.5” tall. Ensure that the exit holes are placed above the top of the candle flame to allow for CO2 to pool before flowing to the next level.  Exit and entrance holes must be carefully aligned. This is most easily done by hanging the bottles first and marking where the holes need to be based on your bottle arrangement. Note that the topmost bottle will only have an exit hole and the last bottle will only have an entrance hole. 
  5. Use a sealant such as clear silicon caulk to join the entry/exit holes between bottles to prevent significant amounts of CO2 from escaping. 
  6. Place one tea light candle in each bottle.
Attribution: 

The author wishes to thank Emory and Pepperdine Universities for funding and support. The author also wishes to thank Rebecca Pulk who helped in the initial design and construction while an undergraduate at Pepperdine University.

References

1. For example, see:

2. See for example:  (accessed 3/2021) and (accessed 3/2021).

3. These values are obtained from simple PV=nRT calculations and students can be asked to produce these numbers to increase the interactivity of the demonstration. See the supporting information documents for an example.

4. Ties into Next Generation Science Standards (NGSS) MS-PS1-4 “Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed”

5. Ties into NGSS 5-PS1-1 “Develop a model to describe that matter is make of particles too small to be seen” and 2-PS1-1 “Plan and conduct an investigation to describe and classify different kinds of materials by their observable properties.”

6. In addition to the above NGSS standards, this experiment can relate to the Disciplinary Core Ideas of:

PS1A: Structure and Properties of Matter, PS3A: Definitions of Energy, PS3B: Conservation of Energy and Energy Transfer, PS3C: Relationship Between Energy and Forces, PS3D: Energy in Chemical Processes and Everyday Life ( (accessed 3/2021))

Safety

General Safety

For Laboratory Work: Please refer to the ACS .  

For Demonstrations: Please refer to the ACS Division of Chemical Education .

Other Safety resources

: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies