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Timing
About 30 min.
Level
Any.
Description
The 'greenhouse effect' in the Earth's atmosphere is caused by a number of gases that behave in a similar way to glass in a greenhouse. In the demonstration, three thermometers are clamped close to a photoflood bulb and their temperatures monitored regularly. One is clamped in the air, one is enclosed in a plastic pop bottle, and one enclosed in a pop bottle one half of which has been painted with matt black paint. The final steady temperatures obtained are in the order bare thermometer (lowest), thermometer in unpainted bottle, thermometer in painted bottle (highest).
Apparatus
w w w w w w Two 1 dm3 plastic fizzy drinks bottles with two-holed rubber bungs to fit. Three mercury-in-glass thermometers (0100 C). One 275 W photoflood light bulb (obtainable from photographic shops) with a plain bulb holder (ie without a shade). Clock with second hand. Three pieces of lead foil about 3 cm x 2 cm. A little matt black paint such as blackboard paint.
Method
Before the demonstration
Check that all three thermometers give the same reading in the same surroundings. Clean and dry the bottles. Cut three identical pieces of lead foil and fold them round the bulbs of the thermometers to form flags (Fig. 1). These absorb the light energy and radiate it as heat, simulating the Earths surface. Ensure that the thermometers will still fit through the openings in the bottles when the lead flags are fitted. Paint half of one of the bottles with matt black paint as shown in Fig. 2. Fit two of the thermometers through the bungs ensuring that it is possible to read their scales from room temperature upwards. Place the bungs holding the thermometers into the two pop bottles.
The demonstration
Stand the photoflood bulb in its holder on the bench. Clamp the three thermometers (two of them inside their bottles) so that they are about 25 cm from the bulb. The actual distance is not critical, but it is important that all three distances are the same.
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Thermometers
Bottle with rear half painted black containing thermometer with lead flag
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A previously prepared paper template on which the positions of the apparatus are marked will help when setting this up in front of a class. The bulbs of the thermometers should be at the same level as the photoflood bulb and the lead flags should be perpendicular to the incident light (Fig. 3). Allow the thermometers to adjust to room temperature and take a reading of each. Switch on the photoflood bulb, start the clock and take a reading of each thermometer every minute for about 15 minutes. The temperatures of each will rise and gradually level off to a steady reading. Typically the bare thermometers reading will rise by 5 C, the one in the clear bottle by 8 C and the one in the half-blackened bottle by 13 C.
Teaching tips
Get members of the class to take the readings and enter them on a pre-prepared table on the blackboard or OHP. The class could prepare suitable graph axes before the experiment and plot the temperatures against time as they are recorded.
Theory
In a greenhouse, visible light passes through the glass (which is, of course, transparent to visible light) and is absorbed by dark coloured surfaces inside. These heat up and re-radiate energy, but at longer wavelengths in the infrared region of the spectrum. This is absorbed by glass and so the greenhouse warms up. The greenhouse effect in the Earths atmosphere is caused by a number of gases that behave in a similar way to glass, ie they are transparent to visible light, but absorb in the infrared. Some of these are listed in the table. It can be seen that carbon dioxide is the most important greenhouse gas because of its relatively high concentration in the atmosphere rather than its intrinsic greenhouse efficiency. Gas Relative greenhouse efficiency per molecule 1 30 160 2 000 21 000 25 000 Concentration in the atmosphere / ppm 350 1.7 0.31 0.06 0.000 26 0.000 24 Relative efficiency x concentration / ppm 350 51 49.6 120 5.46 6
Carbon dioxide Methane Dinitrogen oxide Ozone CFC 11 (CCl3F) CFC 12 (CCl2F2)
This experiment demonstrates the greenhouse effect caused by the plastic of the bottle. The teacher can explain that gases have the same effect. It also shows the effect of a black surface absorbing and re-radiating energy. The following articles give useful background for the teacher or post-16 students on the greenhouse effect: I. Campbell. What on Earth is the greenhouse effect? Chem. Rev., 1991, 1 (2), 2. I. Campbell. The chemical basis of global warming. Chem. Rev., 1992, 1 (4), 26.
Extensions
Try a thermometer in a glass bottle for comparison with a plastic bottle. Try sunlight (when available!) instead of the photoflood bulb. See also demonstration 68.
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Further details
This would be an ideal experiment for computer interfacing if thermocouple thermometers were used together with suitable interfacing boxes and software. The graphs could then be plotted on-line on a monitor and hard copies printed for distribution to the class. The book by Robert Edwards, Interfacing chemistry experiments. London: RSC, 1993 gives some helpful advice about interfacing.
Safety
Wear eye protection. The two-holed stoppers are used for the thermometer to prevent pressure build-up inside the bottles caused by the rise in temperature. It is the responsibility of teachers doing this demonstration to carry out an appropriate risk assessment.
Answers to Questions
Diagram
1. If the greenhouse effect did not exist, the Earth would be cooler by about 33C on average. 2. An increase in the concentration of greenhouse gases could be a concern because it might increase the strength of the greenhouse effect/mean that less radiation is radiated into space/mean that more radiation is absorbed and re-radiated back to Earth. This could cause the Earth to warm up. 3. A few possible answers are suggested below. There are several other sources. Students are probably less likely to know about sources of oxides of nitrogen. Greenhouse gas Carbon dioxide Possible sources Burning fossil fuels eg transport, electricity generation, gas central heating Burning rainforest Respiration Evaporation of oceans Rotting vegetation Cows digesting grass Volcanoes Rice cultivation Refrigerants/refrigerators Aerosols Agriculture Vehicle exhausts