Greenhouse gas demonstration

The effect of carbon dioxide on the atmosphere has been studied experimentally since 1856, when Ms Eunice Foote compared the warming of two flasks, one filled with “carbonic acid gas”, when they were simply placed in direct sunlight (1). Now, thanks to Warwick Cathro at 350.org Canberra, here is a neat way to reproduce this classic demonstration, ideal for a winter science fair, as we discovered.

The following is an excerpt from Warwick's notes, available in full here.

In recent years a number of “greenhouse gas demonstrations” have been featured on YouTube (2). Each of these experiments involves the warming of two flasks (sometimes partly filled with water) using a heat lamp, and the measurement of their temperatures using digital thermometers. Carbon dioxide is first introduced into one flask, for example by using a substance containing sodium bicarbonate. The presence of CO2 causes that flask to warm noticeably more than the control flask, due to the absorption of infrared radiation by the CO2 molecules.

During 2019 a group of Canberra citizens and teachers with affiliations to 350 Canberra and the Australian Association for Environmental Education sought to reproduce these experiments. After several trial runs we settled on the following apparatus conditions.

We purchased two “DeltaTrak” digital thermometers from Instrument Choice, calibrated them using the recommended ice bath method, and checked them in warm water (at about 50 C) to ensure that they agreed within 0.1 C

To avoid any bias caused by the absorption of energy when sodium bicarbonate is added to water, we omitted water and instead used a few pellets of dry ice in the active flask and waited for the pellets to vaporise and for the two flasks to reach the same temperature


To avoid any bias caused by a pressure difference between the two flasks, a small hole was drilled in the stopper of each flask (in addition to the hole that accommodates the thermometer). This allows the carbon dioxide in the active flask to escape slowly, but it ensures that both flasks remain at atmospheric pressure


To avoid any bias caused by the positioning or angle of the heat lamp, with the potential for different amounts of infrared radiation to impact on each flask, we placed the two flasks at opposite ends of a clear storage container mounted on a music turntable, and rotated the turntable while the 275 watt heat lamp was switched on.

The apparatus:





Here is a typical data table and plot using this set up:

WHY IS CARBON DIOXIDE A GREENHOUSE GAS?

A greenhouse gas is a substance that contributes to the warming of the atmosphere by absorbing infrared radiation. There are several such substances, including methane (CH4) and water vapour (H2O) but carbon dioxide (CO2) is particularly important because it persists in the atmosphere for hundreds of years.

CO2 molecules strongly absorb infrared radiation (3) at a wavelength of 4260 nanometres (4). But why does it do this?

When molecules that consist of two or more atoms absorb energy, one thing they do is vibrate more rapidly. The laws governing atomic physics (quantum mechanics) prescribe that when a molecule vibrates faster, it must jump from one energy level to a higher energy level. It cannot absorb any arbitrary amount of energy - it has to absorb energy of a particular wavelength.

Now the CO2 molecule comprises three atoms joined together like this:









When it absorbs energy, this molecule can respond in different ways, including stretching or bending with more energy. It can stretch symmetrically (both bonds getting slightly longer at the same time, followed by getting shorter at the same time) or asymmetrically, (one bond getting longer while the other gets shorter, and then reversing).

As it happens, it's this asymmetric stretch (5) which absorbs energy at a wavelength of 4260 nanometres and thus creates the greenhouse properties of CO2.

Another term for infrared energy is radiant heat. Thus, CO2 is an absorber of radiant heat. The more of it there is, the more radiant heat is absorbed or trapped after it is reflected back from the earth. This additional absorbed energy is shared with the neighbouring molecules through molecular collisions. These molecules become more energetic and faster moving. This is another way of saying that they get hotter.

To sum up: it's the asymmetric stretch of CO2 that is a key source of atmospheric warming, and the more CO2 there is, the more pronounced that effect becomes. Since about 1800 (and much more rapidly since about 1970) the atmospheric concentration of CO2 has been rising because of the burning of fossil fuels, to meet the expanding energy needs of an expanding human population. Result: global warming and climate change.


Footnotes:

1 Eunice Foote. Circumstances affecting the heat of the sun’s rays. The American Journal of Science and Arts, November 1856, page 383.
https://archive.org/stream/mobot31753002152491#page/383/mode/2up

2 Including the Alka Seltzer experiment, the Dry ice experiment and the Bicarb with vinegar experiment.

3 Infrared energy ranges in wavelength from 700 to one million nanometres

4 A nanometre is a billionth of a metre

5 This is due to fluctuations in the “dipole moment” of the molecule as it vibrates, as explained in this lecture (start viewing at 17 minutes). Think of the “dipole moment as an arrow pointing from the negatively charged end of the molecule towards the positively charged end.