Climate and Earth's Energy Balance
Part D: Greenhouse Gas Lab
In the previous lab, you read about greenhouse gases and used computer models to investigate the effect of greenhouse gases on temperature. As you recall, greenhouse gases, which include water vapor, carbon dioxide, methane, nitrous oxide, and other man-made gases are relatively transparent to incoming shortwave (including visible) solar radiation however they absorb outgoing long-wave radiation emitted from Earth and the atmosphere, hence their name "greenhouse" gases. In this next hands-on lab activity, you will test the greenhouse potential of two easily acquired greenhouse gas samples: water vapor, carbon dioxide.
Note: this lab takes about 45 minutes to complete.
Materials:Note the list of materials described below is for one lab team.
- 3-4 (depending on variables to be tested) - Clear plastic water bottles with hole drilled into cap. The bottles should all be the same type and size (approximately 1-liter). The bottles should be transparent plastic (remove any labels), black bases are acceptable. The bottles need to have tightly fitting screw-on tops. Seltzer water bottles are a good type of bottle. (Recommend: 1 bottle for every variable).
- 4 - Thermometers (analogue, digital or digital recording; one for each bottle). Inexpensive digital probes, such as the ones pictured here, can be purchased online. Vernier and Pasco, as well as other companies, sell excellent probes for this type of investigation.
- A clock or watch showing minutes and seconds.
- 150 ml (2/3rd cup) of Vinegar
- 250 ml (1 cup) of Baking Soda
- 3 - Sponge pieces of equal dimensions; saturated with water for one bottle, left dry for the other two. Kitchen sponges can be cut into 3 pieces to fit the bottles.
Indoors: Light source (clamp lamp or goose neck) and bulb (standard incandescent or directed spot; one setup for each bottle).
Outdoors: Sheltered area with direct sunlight for duration of lab. A means for shading the bottles for the second half of the lab. A cardboard piece folded in half and placed tent-like over the bottles works well. Or you can plan to move the bottles into the classroom for the cooling phase.
Note:1-2 Alka-seltzer tablets, dissolved in water, can be used instead of the vinegar and baking soda. If using this method, be sure to add the equal volume of water to the bottle with water vapor and sponge. Another method for acquiring CO2, is to sublimate dry ice, or use a CO2 canister, such as for filling a bicycle tire. In both of these cases, let the CO2 warm to the same temperature as the other bottles before beginning the experiment.
- Assemble all materials and select a sheltered site (out of the wind) to work if working out-of-doors.
- Pre-drill holes in the caps of the bottles. Make sure you have enough bottles and caps for the entire class.
- Insert temperature probes and seal the holes with modeling clay, hot glue, or silicone sealant.
- Cut sponge pieces to size (should cover half the of bottle bottom) and insert in bottles. Leave the lids off the bottles and let the sponge pieces dry for the dry air and CO2 bottles.
- Prepare the gases for the bottles before class using the instructions below. Seal the bottles after preparing them.
- Preview the lab methods with the students so that you are ready to go at the start of the class period.
Preparation of Bottles:For the bottle with air: Be sure that air is dry use a warm hair dryer to heat the air, for a few minutes then just tighten the cap. Let the bottle cool before the lab.
For the bottle with saturated air: Pour small amount of water in the bottle to saturate the piece of saturated sponge in the bottom of the bottle. Make certain the sponge is wet but there is no extra water in the bottle. Seal the bottle.
For the bottle with carbon dioxide and for pouring the gas into the bottle: Carbon dioxide can be easily made with baking soda and vinegar. Vinegar (acetic acid) CH3COOH, and baking soda (sodium bicarbonate) NaHCO3 produces an acid-base reaction when they come in contact with one another. The fizzing and bubbling indicates that a gas (CO2 ) is being produced.
Pour 30 ml (about 1 ounce) of vinegar into an extra plastic bottle or tall beaker. Spoon in ½ tsp of baking soda. Allow the reaction to bubble and fizz without disturbing it. When the fizzing is over, carefully pour the CO2 into the bottle. [Adding more vinegar and baking soda will just make the reaction bubble excessively, and the CO2 will tend to bubble over the beaker and you won't be able to get it into the bottle.] BE CERTAIN NOT TO POUR ANY LIQUID INTO THE BOTTLE! Repeat this process two more times. Put the cap with the probe on the bottle. Another way to produce CO2 is to dissolve an alka-sletzer table in the bottle. If using this method, you will need to put an equal amount of water in the bottle with water.
Note: CO2 gas is more dense than air. It will stay in the beaker, forcing out the air. Although you can't see it, you can pour CO2 gas out of the beaker just like you would pour a liquid. By way of teacher demonstration, a match can be lit and placed down into the gas. The match will be extinguished showing that the oxygen in the air has now been forced out, replaced by the carbon dioxide. Students can also feel the CO2 being poured out of the beaker because it's cold (similar to cold carbon dioxide gas coming out of a fire extinguisher). As the reaction with baking soda and vinegar is "endothermic," meaning that energy (as well as CO2) leaves the products during the reaction cold, care should be taken not to introduce any of the liquid into the bottle as it will continue to keep the temperature of the liquid depressed.
Lab activity instructions:
- If possible, divide up members of your class into several different groups (one for each bottle containing a different gas). Each group of 2-3 students will have one bottle into which one of the gases (regular air, water saturated air, CO2 ) has been placed. (For the saturated air, carbon dioxide, see above.)
- For each bottle, prepare a data chart with three columns: time, temperature, and notes. You will need to collect data for 25 minutes, once every minute. Be sure to locate a stopwatch or other clock showing minutes.
- Record the starting "room" temperature by holding the control temperature probe in the air for 1 minute. Record this temperature. NOTE: Do not set the probe on the desk. If you do, you'll be recording the desk temperature. Also, don't hold the tip of the probe because then you'll be taking your temperature.
- Place all the bottles at a designated distance from the light source (Recommend: 10-25 cm (4-10 inches) away from light source, if using an incandescent light source. If working outdoors, place the bottles all in direct sunlight be aware of shadows.)
- Either plug in the lamp and turn it on, or move the bottles into the sunlight. Immediately start collecting and recording temperature on a data chart. Continue to do so every minute for 15 minutes. After 15 minutes, turn the light off, or move the bottles into the shade, and continue recording the temperature for an additional 10 minutes. Safety Note: Be careful around the hot lamp.
- Plot the data you collected in step 5. Plot temperature on the (Y) axis and time on the (X) axis. Label your axes. If sharing data with the class, agree on a scale before plotting the data. If you have access to Excel or another graphing program, you can chart the graph electronically. Share the data / graphs so that each team has all of the data sets.
- Describe the general trends that you see in the temperature over time.
- Did one gas warm more quickly than the others? Was the increase in temperature gradual or were there changes in the slope of the line?
- Which gas had the greatest change in temperature while heating?
- How did the cooling of the gases compare to the warming? Which gas appeared to hold the heat the longest of the three that you tested?
- Recall that temperature is a measure of kinetic energy of molecules. Explain, in terms of kinetic energy, why the bottles remained warm after the light source was turned off or the bottles were shaded.
- How does the composition of the gases in the bottles differ from the composition of gases naturally found in the atmosphere?
- If you increased the concentration of CO2 in the bottle, how might this affect the temperature trend in the lab?
- How do greenhouse gases affect the Earth's radiation balance?
Stop and Think
Analyze your data. Consider the following questions: