The greenhouse effect 2

Yasmin Jayathirtha

A few months ago in this column (Teacher Plus, April 2015), I had explored the idea of modelling the greenhouse effect. The ‘earth in a bottle’ acts as a very simple model for the earth and its atmosphere. The last experiments showed the effect of dark surfaces in absorbing and emitting radiation and acted as a model for the earth absorbing visible light and re-emitting it as infrared radiation.

This experiment shows the effect of the greenhouse gases, actually the most famous one, carbon dioxide.

Apparatus
Thermometers mercury (0-100ºC) – 3
1 dm³ bottles (plastic/glass) with stoppers – 2
Pieces of lead foil (2 cm by 3 cm) – 3
Measuring cylinder 250 cm³ – 1
Fruit salt sachets – 3

Experiment
Check that the three thermometers show the same reading in air. Fix the lead foil on to the thermometer bulbs. Fix two of the thermometers into the stoppers. Add about 250 cm³ of water to the bottles and fix the stoppers. Set the bottles and the ‘bare’ thermometer in sunlight. Monitor the temperature for about 15 minutes, then add the contents of one sachet of fruit salt to one of the bottles and re-stopper it immediately. Continue to monitor the temperature for about an hour.

The fruit salt reacts with the water to form carbon dioxide. One bottle now contains an atmosphere richer (by a lot) in carbon dioxide than the other one. A set of sample readings I obtained is shown below:

These results are rather interesting. It takes a while for the temperature to rise. In one of my earlier experiments, the ‘bare’ thermometer showed a much larger rise than those in the bottles! What was I to think of that? I realized that the bulb of the thermometer, encased in the foil, was resting on the concrete of the surface and absorbing heat from it. I moved the thermometer so that the bulb was in the air, and the temperature promptly dropped. The students may find that the rise is not too large. After all, they learn in their classes that about 100 ppm rise in the concentration of carbon dioxide is expected to raise the temperature by 2ºC. Here we appear to have the atmosphere full of carbon dioxide and the change is just about that much. The discussion can centre around two points: first is the effect of the water in the bottle. Carbon dioxide is fairly soluble in water and the water in the bottle is dissolving some of the CO₂ generated. This in fact is what the oceans do. We can repeat the experiments, adding more and more fruit salt till the water is saturated and measure the temperatures again.

The other is the set up of the experiment. In ‘the earth in a bottle’, the light is falling obliquely on to the bulb. On the real earth, it is falling more directly on to the surface. In fact, in the experimental setup given in the book, Classic Chemistry Demonstrations, the earth is a disc of lead foil at the bottom of a beaker and is illuminated from the top. Now, this can be a discussion, both on how to set up experiments and the differential heating effects found in the temperate regions, depending on the angle at which the light falls. The fall in temperature after time is related to the carbon dioxide diffusing out of the bottle and in this experiment, to the lessening of sunlight. If we leave the bottle in fairly constant light, the temperature should gradually fall till it reaches that in the other bottle.

Notes
Fruit salts are an easy way to generate carbon dioxide, but there are others. One site on the Internet uses fizzy drinks, allowing one can to go flat. We could check the temperatures using soda + acid to generate the CO₂, putting the same volume of water in the control bottle. This will not have ‘the ocean’. We can compare bottles with ‘oceans’ and ‘no oceans’.

The author works with Centre for Learning, Bengaluru. She can be reached at yasmin.cfl@gmail.com.