In this demo, students witness three types of gases and observe their relative densities to air. It is the second part of a larger gas unit featured in Chemistry Solutions.
High or middle school
By the end of this lesson, students should be able to
- Understand that not all gases have the same density.
This lesson supports students’ understanding of
Teacher Preparation: 1 hour
Lesson: 1 class period
- Dish soap
- Graduated cylinder (10 mL)
To generate H2
- 250-mL vacuum flask
- Plastic thistle tube or funnel
- Mossy zinc
- 6-M HCl
To generate natural gas
- Gas jets in the lab
To generate CO2
- Dry ice
- Erlenmeyer flask
For releasing bubbles
- Delivery tube
- Glass tube in stopper
- 2-L soda bottle cut in half (top half only)
- Ring stand with ring and clamps
- Always wear safety goggles when handling chemicals in the lab.
- When working with acids and bases, if any solution gets on your skin immediately rinse the area with water.
- Make certain there are no open flames in the lab, some of the gases being produced are highly flammable.
- If you choose to ignite the gases, make sure students are about 10 feet from where the bubbles will be combusted.
- Wear gloves when handling dry ice. It is extremely cold and can cause frostbite if directly touched with skin. Only maneuver the dry ice with proper handling equipment.
The first lesson in the gas properties sequence, “Three Station Gas Lab,” is critical because gases are very different from liquids and solids. They are rarefied, that is, the molecules of gases are very far apart in relation to the molecule size. The difference is about a thousand-fold! What this means is that the gas density depends on pressure and temperature. At higher temperatures, gas density is lower because the gas expands. At higher pressures, gas density is higher because the molecules are closer together. So whenever density is expressed, the pressure and temperature also have to be stated.
In “Chillin’ and Heatin’,” students work with a sealed, variable volume syringe at two temperatures. For the purposes of this investigation, the actual temperatures are not important; they need to recognize only that one temperature is higher than the other. They should note that as temperature increases, the volume of the gas also increases, because the average velocities of the air molecules increase. Students need to understand that the pressure didn’t change, it is room pressure. They also need to know that the amount of air inside was constant. Thus, to measure density, all gases measured must be at the same temperature, or the volume (and so the density) would not be controlled.
In “The Steak Bottle and the Dime,” students use their hands to warm a chilled bottle that has a dime acting as a kind of valve. I use a dime because it exactly fits over an A-1 Steak Sauce bottle and the dime itself has a low mass. That means as air molecules attain a higher average speed, some will escape the bottle and cause the dime to move as they do so. Pressure is constant (room pressure) so as the molecules speed up, fewer can fit in the constant volume bottle.
In “Distortion of Marshmallows,” students discover that as they increase the volume of a sealed syringe, air molecules in the marshmallow inside the syringe are able to occupy a larger volume. The total volume of air in the syringe, and the internal marshmallow, increases as pressure goes down. This cycle of expansion is followed by a compression, during which the marshmallow shrinks because the air molecules are confined to a smaller space at higher pressure. (Ultimately the marshmallow fails because of the attraction of the molecules of the marshmallow for each other and the irreversible escape of the last of the gas inside the marshmallow.) This means that during a density measurement, the pressure on all measured gases must be constant, or the densities will be unreliable.
After discussing the meaning of these three activities, it’s time for an exciting exploration of gas densities. I like to do this qualitatively at first, by watching H2, natural, and CO2 gas bubbles in air.
Prepare the soap solution
- Mix 200 mL of water with 6 mL of Dawn detergent mixed in; add a few drops of glycerine for bubble longevity.
- Rather than collecting the H2 gas in gas-collecting bottles, you can use this modified set up from Flinn (Figure 1). The water in the soda bottle should be soapy water.
- Once the apparatus is set up, pour 10 mL of 6-M HCl through the thistle tube. Allow some H2 gas to run through the tubing before hooking it up to the soda bottle apparatus.
- You should see bubbles form and rise rapidly. They are soap on the outside, but H2 inside.
- I light the bubbles to show flammability—the “gee whiz” component. Make sure students are at least 10 feet away and wearing safety goggles.
Generating natural gas
- Using the same Flinn modified set up, connect the delivery tube to a gas outlet in the lab.
- You should see bubbles form and rise slower than the H2 bubbles. They are soap on the outside and natural gas inside.
- You can also light these bubbles on fire. Make sure students are at least 10 feet away and wearing safety goggles.
- Place dry ice in a soapy water solution.
- Bubbles will form, but rather than float, they will stay low.
Once you bubble the three gases through the soap solution, students should make a chart to compare the relative densities of the gases to air. H2, the least dense gas, rises through the air the fastest. Natural gas, depending on the gas type, size of bubbles, and density of soap solution, either rises slowly or hovers in the air. CO2 falls, so it is the most dense.
The demonstration gives a good relative density of three gases, but should raise questions about whether it is possible to measure those densities. By the end of class, students should understand that the densities, if measurable, are small, that pressure and temperature need to be constant, and that they’ll be doing the measurement the next day.