Summary

In this demonstration, students will observe the teacher carry out two combustion reactions. First the teacher will burn a small sample of propane gas in a beaker. Next the teacher will burn a small sample of methane gas. Students will create particle diagrams in order to support their explanation and model their observations as they improve their understanding of gas density.

Grade Level

High School

NGSS Standards

This demonstration will help prepare your students to meet the performance expectations in the following standards:

• HS-PS3-2: Develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motion of particles (objects) and energy associated with the relative positions of particles (objects).
• Scientific and Engineering Practices:
  • Developing and Using Models
  • Constructing Explanations and Designing Solutions

Objectives

By the end of this demonstration, students should be able to

• Recognize that different samples of gas will have different density values.
• Create particle diagrams that represent the differences in densities of gas samples.
• Understand that the combustion reaction involving propane can be a potential lab hazard, since the gas can collect on the lab table (propane is denser than air).
• Know that some molecules are less dense than air and that gases with different densities pose different lab safety hazards.
• Recognize the importance of lab safety. Gases that cannot be seen are potential lab safety hazards.

Chemistry Topics

This demonstration supports students’ understanding of

• Gases
• Density
• Chemical Reactions
• Combustion
• Lab Safety

Time

Teacher Preparation: 30 minutes

Lesson: 30-45 minutes

Materials

Demonstration 1:

• 1 L beaker
• Small propane tank
• Rubber hose
• Long stick wooden matches/wooden splint
• Safety shield

Demonstration 2:

• Metal can with lid and two holes (approx. 3/8 inch hole)
  • Can must have a lid that can easily pop off
• Small rubber stopper (plug one hole when filling the can with gas/prior to starting demo)
• Matches/wooden splint
• Crucible tongs
• Methane gas
• Safety shield

Safety

• The demonstrations must be practiced by the teacher prior to using it with students in the classroom.
• Always wear safety goggles when handling chemicals in the lab.
• Always use caution around open flames. Keep flames away from flammable substances.
• Always be aware of an open flame. Do not reach over it, tie back hair, and secure loose clothing.
• A working fire extinguisher should be on hand during this demonstration.
• Ensure that the laboratory is well-ventilated.
• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
• Use a safety shield during these demonstrations.
• Maintain a safe distance from students from students during demonstration.
• Exercise extreme caution when working with flammable gases.
  • Methane SDS
  • Propane SDS

Teacher Notes

• This demonstration can be used to teach lab safety at any point in the school year or used specifically during a specific unit. It could potentially fit into a gases unit when studying gas density, or a unit on chemical reactions to demonstrate combustion.
• If used at the beginning of the school year, students can make observations and claims based on their observations. This will foster the growth of scientific reasoning skills.
• If this is used as a demonstration later in the school year then the particle diagrams are a great addition for student understanding.
• The teacher should use a storyline and connect the demonstration to people getting hurt when they improperly light a propane grill. Stories like this can easily be found on the internet. Propane gas can collect in the grill, since propane is denser than oxygen. If the propane/oxygen mole to mole ratio is correct, then an explosion is possible.
• The demonstrations must be practiced by the teacher prior to using it with students in the classroom.
• Using a safety shield during these demonstrations is advised.
• I recommend practicing the first demonstration with a 250 mL beaker. Then try a 500 mL beaker. Do not use a beaker larger than 1 Liter in size.
• The second demonstration requires a metal can with a lid that can easily pop off. Both the lid and the bottom of the container must have pre-drilled holes (see materials list).
• It is an important learning outcome of this demonstration for students to compare/contrast two different gases. I prefer to propane gas for the first demonstration, to show that it is denser than air. I recommend using methane for the second demonstration as the gas sample that is less dense than air.
• However, you could have students collect hydrogen gas via water displacement, and they can use it to compare the densities of different gases.

Extension Ideas:

• If you want to stretch this demo, you can try to liquefy different gases with liquid nitrogen and have students represent the density of a gas versus a liquid using particle diagrams.
• The easiest gas to liquefy in my opinion is butane since butane can be placed in a syringe and compressed to a liquid relatively easily. I have my students do this during the IMF unit.

Demonstration 1 Procedure:

*The demonstration must be practiced by the teacher prior to using it with students in the classroom.

1. Set up a safety shield around the can and demonstration area. Students should not be close to the demonstration.
2. Connect the propane tank to a rubber hose and place it in a 1 Liter beaker.
3. Fill the 1 Liter beaker with propane. Note that it does not take long to fill the beaker (~5-10 seconds).
4. Pause and indicate to the students that the beaker appears empty! It is important for students to understand that is a lab safety hazard since the beaker if filled with flammable propane.
5. Use long stick wood matches, or a wood splint/coffee stirrer to ignite the gas. Another option is to use crucible tongs to hold the match. Propane is denser than air, so most of the propane remains in the beaker.

Step 1

Step 2

Step 3

Step 4

Expected Results:

• The first results photo (far left) shows the propane mixing with oxygen. Notice that the gas is only burning at the top of the beaker.
• The second photo shows how the propane mixes with the oxygen lower in the beakers, since some of the propane has been burned off (second photo from left).
• The third photo demonstrates that oxygen molecules are now mixing with the propane half way into the beaker (second photo from right).
• The final photo (far right) shows that most of the propane has burned, since the less dense oxygen is mixing with the propane at the bottom of the beaker.
• It is helpful to reconnect the results to the story line, so students can fully understand how people can get burned while trying to light a propane grill based on the small-scale combustion of propane that they witnessed. Invite students to imagine how big the flame would be if the whole grill was filled with propane before it was ignited.

Demonstration 2 Procedure:

*The demonstration must be practiced by the teacher prior to using it with students in the classroom.

1. The can must have a lid that can easily pop off. Both the lid and the bottom of the can must be prepared with holes that are approximately 3/8 inch in diameter. The can that I use is can is about 18 inches high and 9 inches in diameter. I suggest starting with a smaller can and practice multiple times.
2. Set up a safety shield around the can and demonstration area. Students should not be close to the demonstration. Students should be at least 10-15 feet away from the demonstration.
3. Using the natural gas source in your lab, fill the can with methane. There is a hole at the top and a hole at the bottom of the can. The hole at the top allows the methane to escape. The hole at the bottom allows the oxygen to enter. Pump the methane gas in from the bottom hole and make sure you can feel the gas leaving through the top hole. This takes less than 15 seconds.
4. Make sure the can is capped with a small rubber stopper. This demonstration can only be done with methane, since methane is less dense than air.
5. When ready, remove the stopper quickly and ignite the methane gas using crucible tongs and a wood splint. If you do not have a wood splint, then use a long wood match.

Expected Results:

• The first photo shows the burning of the methane gas (far left). The methane is quickly leaving the can through the hole at the top since it is less dense than air. Oxygen is entering the can through the hole in the bottom.
• The blue flame cannot be seen easily with the lights on (middle photo). I usually turn the lights on and off when the flame is mostly blue to point out that a blue flame is barely visible when the lights are on.
• The difficulty to see the blue flame is shown with all of the lights on (far right). A combustion reaction is taking place and it is not easily visible.
• When the combustion reaction starts, the flame is large and a lot of methane is leaving the can. The methane is reacting with oxygen outside of the can. As time goes on oxygen enters from the bottom of the can. When there is sufficient oxygen the combustion can take place inside the can and the reaction results in a mini-explosion. When this occurs the top of the can comes off.
• Again, make sure students are not close to the can. Students should be at least 10-15 feet away.
• Make sure the lights are off when the flame is blue so students can see that the gas is still burning.

Particle Diagrams:

• At this point, the students should depict their observations using a particle diagram on a whiteboard. At first, the particle diagrams will be quite simplistic. Below are a couple of examples. As an alternative, students can draw their particle representations on a hand out. I like to encourage students to work together and discuss their observations as part of their understanding.
• The particle diagrams shown below are representative of the second demonstration, methane burning in a can. The can has two holes, one at the top and one at the bottom. The methane is less dense than air so the methane escapes out of the hole in the top. As the methane is burned, air enters the bottom of the can. The focus is on the oxygen in the air sample to simplify the particle diagrams.
• The goal is to get the students to start thinking about what is going on at the particulate level. The photos are taken of whiteboards as students are in progress. There may be a mistake.