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