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How Greenhouse Gases Absorb Heat

Earth's atmosphere is composed of a mixture of gases: 78% nitrogen, 21% oxygen, >1% argon and trace amounts of other gases, including carbon dioxide. some gases absorb and re-radiate infrared energy that we sense as heat. these heat-absorbing gases are often referred to as greenhouse gases. human activities have been adding carbon dioxide and other greenhouse gases to the atmosphere. how will earth's atmosphere respond to this increase in the amount of greenhouse gases? Scientists create physical models or experiments to compare how systems respond to changed conditions.
In this experiment students will observe two model atmospheres: one with normal atmospheric composition and another with an elevated concentration of carbon dioxide. These two contained atmospheres will be exposed to light energy in a sunny window or from a lamp.

Objective

Students will:

  • understand that greenhouse gases in the atmosphere absorb and hold heat
     

Materials Per Group

  • Student Worksheet
  • 15 ml of Bromothymol Blue (BTB), an acid and carbon dioxide indicator
  • 1 small beaker or jar
  • 2 large jars with lids
  • 2 pieces of black construction paper of equal size to place inside the jars
  • 2 thermometers to place inside the jars
  • 1 Erlenmeyer flask (250-500 ml)
  • 1 one hole stopper for above flask
  • 1 straight piece of glass tubing
  • 1 50 cm piece of flexible tubing (aquarium air tubing works fine)
  • 100 ml of vinegar
  • 4 heaping teaspoons of baking soda
  • Watches or classroom clock to time readings
  • Lamp with 100 watt bulb or sunny window sill

Preparation (one day prior to class experiment)

Make a sufficient solution of BTB and water by filing a bottle 9/10ths full with tap water and adding BTB concentrate until the solution is a deep, transparent blue. While the exact concentration is not critical, the solution should be tested. Pour 15 ml BTB (1/2 oz) solution into a small clear cup. Using a straw, bubble one lungful of breath through the small cup of solution. A greenish color is good. If it stays blue or only slightly bluish green, it is too concentrated and can be remedied by pouring out some of the solution and adding more water. If it turns yellow, it is too dilute and more BTB should be added. Continue to test until correct. The solution may also be titrated using vinegar to establish the correct concentration. [NOTE: If using BTB in powdered form, prepare the concentrate by putting 1 gram of BTB powder into a 1 liter container. Add 16 ml of 0.1 molar sodium hydroxide (NaOH) and dissolve the BTB crystals. Then add 1 liter of water to make 1 liter of concentrate.] Each group of students will need about 15 ml (1/2 oz) of prepared BTB solution.

Procedure

Prior Knowledge
  1. Explain to students that air is a mixture of many different gases, including some greenhouse gases that absorb infrared energy.
  2. Ask students if they know any greenhouse gases and their sources. (Answers may include: Water Vapor; naturally present from evaporation and transpiration. Carbon dioxide; burning fossil fuels, burning forests. Methane; rice agriculture, digestive systems of cattle, decaying organic matter. Nitrous oxide; agriculture through the use of nitrogen based fertilizers, livestock waste). Also ask: What human activities have been changing the concentration of these gases in our atmosphere? (Answers: see above.) Tell students that over the past 200 years, the concentration of these gases increased from approximately 278 ppm (parts per million) in 1800 to 385 ppm in 2008.)
  3. Distribute student worksheets and materials to each team.
Wrap-Up
  1. Gather student teams together and have them share their findings.

Student Worksheet Note
When students place the flexible tubing into the BTB solution and observe the color of the liquid as the gas bubbles through the indicator solution, the color should change from blue to a yellow. This color change indicates the presence of carbonic acid, which forms when the carbon dioxide bubbles through the water.

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