Where We Are in Place and Time- Analysing Evidence

Students in grade 5 have been busy investigating ancient civilisations and plotting major events on a timeline.  Through research, students discovered that the Egyptian pyramids were built about 4,500 years ago, farming in the fertile crescent began around 12,000 years ago, and pottery has been around at least 20,000 years.  This led students to wonder:

 “If the first written languages didn’t arise until 5,000 years ago, how do we know the age of things older than that?”

This was the perfect time to dive deeper into our 3rd line of inquiry: Processes involved in collecting, analysing and validating evidence.

Students were led through a series of activities to explore how scientist date artefacts.

The first step involved students recalling what they learned from a previous inquiry into properties of matter. Students recalled some basic information about the periodic chart and the structure of atoms. 

In the first activity, students learned about carbon-14 dating.  As a class, we watched a segment of the documentary, Hunting the Elements (link here, 22:30) in which scientists explain how they use the radioactive isotope of carbon-14 to help find the age of fossils and artefacts.  Students then broke into groups to watch a brain pop video and define some key terms. 

Students watch videos and define key vocabulary

After getting a better understanding of how carbon-14 dating works, students set out to explore the concept of a half-life. Students used M&Ms to help with this.  On an M&M, there is a small ‘M’ on one side of the candy.  This was used to show a carbon-14 atom. After starting with a known number of M&Ms (Carbon-14 atoms) students shook them up and dumped them onto a plate, some of the M&Ms would appear with ‘M’ side up others with the ‘M’ side down.  If the ‘M’ was not showing, this would indicate that the radioactive carbon-14 atom had decayed and turned into something else.  

Here is Rosa explaining how Carbon-14 dating works.

Students carried this out for 5 rounds. Afterwards, we found the average of all the groups.  We knew that about half should decay each round, but also knew that not every group would have exactly half each round.  Here we explored the idea of sample size- that with enough trials we would move closer and closer to the statistical result of exactly half. 

Exploring half-life with M&Ms

Firmly secure in their understanding of half-life, students began creating a graph of carbon-14’s half-life.  The graph can then be used to find the age of a fossil or artefact.  The only information needed to construct the graph is the starting number of carbon-14 atoms  in the artefact (at age 0), the remaining number of carbon-14 atom in the artefact, and the half-life of carbon-14 (5,700 years). 

In this example, the sample contained 48 Carbon-14 atoms (M side up) at the start. After a certain amount of time, some have decayed and no longer have an M. 27 atoms have decayed and only 21 Carbon-14 atoms remain.

Using this Carbon-14 half-life graph, students can plot the data along the curve. If 21 Carbon-14 atoms are remaining, that means the sample is 7,500 years old.

With all this knowledge, students were ready to take the next step and apply these skills to actually dating a fossil. 

Very simply, students were given a fossil of a bone and asked to find out how old it was.

An elaborate story was told of finding a small pyramid in the park, exploring it, uncovering an ancient burial ground inside, removing the bones, and bringing them to school for students to analyse.  More or less, nobody believed this story and quickly realised that the 'bones' were actually baked dough with grains of rice inside.  In this case, the rice represented carbon-14 atoms.  

To find out the age of the bone, students meticulously picked through the sample to find out how many Carbon-14 atoms were remaining. By finding out how many were remaining, they were able to determine how many had decayed from the original sample. They were then able to plot this information on a half-life graph and determine the age of the fossil.

Here are the young archaeologists hard at work.