Conceptual learning and baobab trees
Friday 18 October 2024
In the new guide (page 17) it states that conceptual learning leads towards adapting previous understandings, and identifying and dispelling misconceptions. One of the real benefits of including options in all the previous IB chemistry syllabuses since 1996 is that they related the underlying chemistry theory to practical and often very modern uses. This served to stimulate students' interest and made the subject relevant, even to those who were not going on to study a science-based subject at university. Hopefully, many teachers still integrate practical uses of chemistry into their teaching as they cover the assessed theoretical content in the new syllabus, even if they will not be formally assessed. One such area is carbon dating which has a variety of practical uses. When explaining carbon dating I used to tell students that it could only be carried out on dead organic material. I have recently discovered that this is not completely true.
An African baobab tree
Carbon-14, a radioactive isotope of carbon, is a weak beta emitter which is formed in the upper atmosphere by the action of cosmic radiation on nitrogen. It breaks down by natural radioactive decay to form 14N and has a half-life of 5730 years.
In living systems, which are continuously absorbing carbon from the atmosphere, the ratio of 14C to12C is constant. However, once the living system dies, carbon exchange ceases to take place and the ratio of 14C to12C gradually decreases as the 14C decays. In the late 1940's Willard Libby at the University for Chicago realised this could be used to date how long it was since the living material died. This technique, which now uses an accelerator mass spectrometer (AMS), can determine the 14C to12C ratio to a high degree of accuracy. Although there are some recognised limitations to the method, it can be used to determine the age of once living material up to about 50,000 years since death. Carbon dating has a huge number of uses. For example, in archaeology to date artefacts made from wood or leather and in climatology to understand how the Earth's climate has changed over time.
Madagascar is renowned for its baobab trees, some of which are thought to be hundreds of years old. On a visit to the d'Antsokey Arboretum in southern Madagascar I asked how the trees are dated, as unlike most trees they have a hollow centre and the normal method of counting the rings (dendrochronology) cannot be used reliably. I was surprised (and a little sceptical) when our guide informed me that it was done by carbon dating, even though the trees are obviously still alive and thriving. However, when I looked it up it is indeed the case. In the past few years several research articles have been written on dating baobab trees using carbon dating1,2. The technique involves taking tiny bore samples through the core of the tree and using acid-base-acid treatment to remove soluble and mobile organic components, such as lignin, to leave the resulting cellulose of the dead material present. This is then combusted, using a closed-tube combustion method, to give carbon dioxide, followed by reduction to graphite using an iron catalyst. The graphite is then analysed by AMS.
To me this is a nice example of how chemistry techniques are continually developing and why we need to keep up with these developments to inform our teaching.