IS THE EARTH WARMING?
A TEST OF THE EFFECT OF INCREASED CARBON DIOXIDE ON THE
THERMAL PROPERTIES OF AIR
David J. Davies, Department
Ernest W. Blakeney, Department of Chemistry
Centenary College of Louisiana
Shreveport, LA 71134
Level: Senior high (and junior high for Part B)
The purpose of this demonstration/experiment is to show the environmental and economic effects of increased carbon dioxide (CO2) content in the atmosphere. It has been claimed that the Earth's climate is warming due to changes in the relative concentration of gases in the atmosphere. The gases involved are:
Of these, CO2 is building up in the atmosphere at a much
greater rate. Several factors contribute to this build up. The use of fossil
fuels in internal combustion engines and in the generation of electricity
has greatly increased the amount of CO2 released to the air. Green plants
remove CO2 from the air during photosynthesis and turn it into O2. Deforestation
of large areas, especially the rain forests in tropical regions of the world,
has decreased the amount of CO2 removed from the atmosphere by plants.
The question must be asked as to how the increased amounts of these gases in the atmosphere could contribute to the hypothesized climate warming or "Greenhouse" effect. One possibility is that the changes in gas composition of the atmosphere increase the heat capacity of the atmosphere. The gases of the atmosphere are heated by the sun during the day. At night, some of this heat is radiated away from the Earth into space. The heat capacity of a substance can be regarded as the relative tendency to retain heat. Some materials lose heat to surroundings quickly, while others tend to retain heat longer. If the changes in the composition of the atmosphere allow its heat capacity to increase, less heat is lost to space during the night. The long-term effect of this decreased loss of heat would be a gradual warming of the atmosphere.
Scientists have documented that the concentrations of these "Greenhouse" gases are increasing in the atmosphere today; for example, today the concentration of CO2 is approximately 350 parts per million (ppm), or 0.035%, versus the pre-Industrial Revolution concentration of 200-280 ppm (0.020 - 0.028%). We must determine the effect of the future increases in concentrations of these gases on air temperature. Could doubling the concentration of carbon dioxide, as is expected to occur in the next few decades, significantly warm the atmosphere?
In this experiment/demonstration air with increased concentrations of CO2 is produced and the rate of heat loss of that air is compared to today's "normal" air.
Equipment and Materials
You will treat the air in the large glass vessel as a model
of the atmosphere. The vessel will be covered and heated by the "sun"
(the heat lamp) until the temperature rises to 15 degrees C above the ambient
(surrounding) temperature. The heat lamp is then turned off ("nighttime"),
and the air allowed to cool naturally. Temperature readings are taken at
definite time intervals and plotted on graph paper. This will show the rate
of heat loss from "normal" air. Then, the CO2 concentration of
the air will be increased and the experiment repeated. The results will
demonstrate the effects of increased CO2 on the rate of heat loss from air.
You will then evaluate claims that increasing carbon dioxide will significantly
increase the temperature of the atmosphere, and thus lead to climate change,
in your lifetime.
PART A: DETERMINING CO2 CONCENTRATIONS
When Alka-Seltzer dissolves in water, a gas is released. This gas is CO2, so Alka-Seltzer tablets serve as a convenient source of this gas. The determination of the amount of CO2 released is an interesting laboratory procedure in itself, and we recommend that high school students be involved in this portion of the experiment .
The first step is to determine the volume of the glass vessel. If a gallon jar is used, use the conversion factor
1 gallon = 3.785 liters.
If some other sized vessel, such as an aquarium, is used its volume can be measured by filling with water using a pint jar as a measuring vessel. The relationship of
1 gallon = 8 pints
can be combined with the conversion factor above to calculate
the volume of the vessel in liters. The remainder of this procedure will
be described assuming you use a gallon jar.
The next step is to calculate the mass of the air in the vessel. The true density of air is dependent on the amount of moisture present in the air (the humidity) and on the barometric pressure. There are tables and equations which allow a precise computation of the density of air under any set of conditions. These tables can be found in the CRC Handbook of Chemistry and Physics. This calculation is rather complicated; thus for the purposes of this demonstration/experiment, the density of dry air at a pressure of 760 mm of Hg will be assumed. This density is:
Since there are 1000 milliliters in 1 liter, the gallon jar contains
3.785 liters X (1000 milliliters/liter) = 3785 milliliters.
The mass of air in the jar can be calculated from the density:
3785 milliliters X (0.0012 gm/milliliter) = 4.542 gm
At this point, assume you wish to determine the rate of heat loss from an atmosphere in which the CO2 concentration has doubled, e.g. which contains 700 ppm CO2. The air in the gallon jar should contain:
4.542 grams X (700/1,000,000) = 0.0032 gm CO2
Thus, we will need to add an additional 0.0016 gm of CO2
to the air in the vessel.
The CO2 will be obtained from the solution of Alka-Seltzer in water. We have performed the following simple test to determine how much CO2 is released from an Alka-Seltzer tablet:
A typical determination gave the following values:
|Weight of vessel plus water||122.37 gm|
|Weight of Alka-Seltzer tablet||3.35 gm|
|Total weight||125.72 gm|
|Weight of vessel, water, and Alka-Seltzer||125.09 gm|
|Weight of CO2 released||0.63 gm|
This information can now be used to determine
the amount of Alka-Seltzer necessary to give the 0.0016 gm of CO2 which
must be added to the air in the jar to give a concentration of 700 ppm CO2:
(3.25 gm Alka-Seltzer/0.63 gm CO2) X 0.0016 gm CO2
= 0.0083 gm Alka-Seltzer
This is equal to only 0.25% of one Alka-Seltzer tablet
(a quarter of 1%); this is a very small amount.
PART B: EFFECT OF CO2 CONCENTRATION ON HEAT LOSS OF AIR
I. Measurement of heat loss for today's "normal" air:
II. Measurement of heat loss from tomorrow's air containing increased CO2:
Interpretation of your results:
Using the outcome of your experiment, answer the following questions.
|Return to Activity-Age Table|