CG2. The greenhouse effect - Teachers’ notes. Vicky Wong. Page 1 of 6

The greenhouse effect – Teachers’ notes Background information Radiation from the sun reaches the Earth. Some of this radiation is absorbed by the Earth and warms it up. A steady state is reached where the amount of radiation coming in is matched by the radiation escaping from the atmosphere, resulting in a fairly stable temperature and climate. Anthropogenic (man made) emissions are changing the composition of the atmosphere, both by adding additional amounts of gases naturally present, such as carbon dioxide and methane and also by the introduction of gases which do not occur naturally, such as CFCs (chlorofluorocarbons). It is worth noting that the sun emits ultraviolet (UV), visible and also infrared (IR) radiation. This IR radiation is mainly absorbed by water vapour in the atmosphere and also contributes to the warming of the Earth. This additional complication has been left out of the activities for students. It is important to differentiate between the natural greenhouse effect, which is essential for life to exist, and the enhanced greenhouse effect. This is the additional effect caused by the anthropogenic emissions and it is what is generally thought to be causing global warming. Some students may ask why some gases in the atmosphere contribute to global warming and others do not. A simple answer is not easy to give, but would be based on the following. When molecules interact with UV radiation the energy they absorb tends to break bonds. This is because the energy of a photon of UV radiation corresponds to transitions between electronic energy levels in a molecule, so when a molecule absorbs UV radiation it is excited to a higher electronic energy state which may result in bond fission. IR radiation is less powerful as the photons contain less energy. Rather than breaking bonds it causes them to vibrate. A molecule will absorb IR radiation if the vibration causes a change in dipole moment. Consequently, nitrogen and oxygen are not IR active, but many of the gases present in the atmosphere in lower concentrations such as water vapour and carbon dioxide are. How Science works • How explanations of many phenomena can be developed uisng scientific theories, models and ideas • The use of contemporary scientific and technological developments and their benefits, drawbacks and risks • Apply scientific information and ideas. Practical demonstration To illustrate how greenhouse gases such as CO2 can have a warming effect, show students The Greenhouse Effect demonstration (demostration 67) in Lister, T, Classic Chemistry Demonstrations, Royal Society of Chemistry, London, 1995. These can be accessed through www.chemistryteachers.org (search for ‘greenhouse effect’).

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67. The greenhouse effect – 1 Topic Environmental chemistry.

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 ▼

Two 1 dm3 plastic fizzy drinks bottles with two-holed rubber bungs to fit.

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Three mercury-in-glass thermometers (0–100 °C).

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One 275 W photoflood light bulb (obtainable from photographic shops) with a plain bulb holder (ie without a shade).

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Clock with second hand.

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Three pieces of lead foil about 3 cm x 2 cm.

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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 Earth’s 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

Folded lead foil 1dm3 plastic pop bottle

Crimp lead foil around thermometer to form a “flag” Thermometer with ‘flag’

Fig. 1 Foil flags Paint this half of the bottle matt black

Fig. 2 Bottle half-painted with matt black

Unpainted bottle containing thermometer with lead ‘flag’

Bottle with rear half painted black containing thermometer with lead ‘flag’

‘Bare’ thermometer with lead ‘flag’

~ 25 cm

Bulb 90°

90°

Note: The ‘flags’are placed so as to be perpendicular to the incident light 90°

Fig. 3 Top view of apparatus

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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’ thermometer’s 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 Earth’s 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

Carbon dioxide Methane Dinitrogen oxide Ozone CFC 11 (CCl3F) CFC 12 (CCl2F2)

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

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.

CG2. The greenhouse effect - Teachers’ notes. Vicky Wong. Page 6 of 6

Answers to Questions Diagram

1. If the greenhouse effect did not exist, the Earth would be cooler by about 33˚C 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

Water vapour Methane

Chlorofluorocarbons and hydrochlorofluorocarbons (CFCs and HCFCs) Oxides of nitrogen (NOx)

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