The story

There is no doubt that the gold medals for the London Olympic games are beautiful but how much gold is actually in them?

This is might well be the first of a few Olympic themed postings over the coming months. I wanted to come up with some ideas that were a bit different and unusual so this is my first offering: an  enquiry lesson to use when teaching metal properties or density or just as a way of practising maths and science skills.

It could be done with any age class depending on the activities you choose and how much guidance you give. I have given my outline for how I would teach the lesson but bits and pieces can be taken, missed out and mixed around.

Lesson idea

Starter

Introduce the problem and then get initial ideas. I would then ask the class to work in small groups and discuss the properties of gold. This can then be shared to draw out some common properties of metals. If they mention that gold is heavy it is a great opportunity to discuss what they mean by heavy and why this is not a good way to measure properties of substances, whereas density is. A house sized bit of cotton wool would be heavier than a gold coin but would they really say being heavy was a property of cotton wool? You might like to discuss what we mean by density and how to calculate it but not go into too much detail as later on in the lesson the idea is they work it out for themselves.

Then ask them if comparing any of the properties would be a good way to see if the medal was pure gold. There are a range of possible ways: this image demonstrates a common test. Pure gold is a soft metal so biting it might leave teeth marks (and ruin you gold). A more modern method is to measure the melting point.

However, none are as simple as looking at the medal so a little more investigation is needed.

 

Main

Although it would be nice to get the students to compare a real Olympic medal to a lump of pure gold that’s not going to be possible. So, tell them they are going to investigate a method that could be used to do this. They will be finding out if a copper penny is made of pure copper. How much guidance you want to give on this task depends on the class. Ideally, it could be totally open-ended with students researching and experimenting in groups. The calculating of volume is a great way to practice maths skills (especially when working out the volume of the coin).

For each group you will need:

  • a copper coin (2p is best)
  • a piece of copper (could be foil for an easy volume measurement or a bit of copper piping if you want to be mean stretch them.
  • balances
  • calculators
  • a displacement can (see weblink below if you don’t have any)
  • access to information on density and calculating volume

You might like to direct them to this great video showing the story of Archimedes’ principle and the Crown of Syracuse.

Plenary

The idea (hopefully) is that they will compare the densities of the coin and the pure copper (which is 8.933 g/cm3). Depending on the age of the coin, they will get different densities. You could go round the groups and compare them.

Wikipedia informs me that:

The coin was initially minted from bronze, but since 1992 it has been minted in copper-plated steel except for a few months in 1998 when bronze was used again. As copper-plated steel is less dense than bronze, post-1992 coins have been slightly thicker. The coin weighs 7.12 grams and has a diameter of 25.9 millimetres. The 2p coin is currently 93% mild-steel and 7% copper.

The reason for the change is the fact that copper is getting very pricey, no point in having a 2p coin that is worth more than 2p! This is an interesting discussion point because it is exactly the same reason why Olympic gold medals are not made out of real gold. They are in fact an alloy of 92.5% silver and 1.34% gold, with the remainder copper (a minimum of 6g of gold – the cheapskates)

Weblinks

Information about the medals

What – no displacement cans?

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