REPOST: All the pretty colours…

One of the most spectacular chemistry experiments is flame tests. I think it’s a particularly elegant experiment, and also one that can easily be done in high schools. I remember doing it when I was about 14 or 15, as part of standard grade chemistry. It is very simple; you take some wire, clean it in concentrated acid, dip the wire in acidic solutions of various metal salts and then put the wire in a blue Bunsen flame. It has some of the key components of a good chemistry experiment – acids, Bunsen burners and pretty colours. In a similar manner to fireworks, different metals burn with different coloured flames. I’m sure that my standard grade chemistry teacher had a sense of humour when she suggested to us that we leave sodium until last for it was hard to see. Yes, the bright and vivid orange colour was quite surprising coming after potassium’s pale lilac, strontium’s red and copper’s blue green.

I was thinking about this during a lab session last week when we were looking at exothermic reactions. OK, we were looking at some really great demonstration type experiments including thermite and the potassium dichromate volcano. More about them another time. We were also doing the screaming jelly baby experiment, also known as the jelly baby rocket or rocket to the moon. Simply, this involves taking a boiling tube, clamping it at a 45 degree angle, filling it with some potassium chlorate. The potassium chlorate is melted with a Bunsen burner and the whole thing is done in a fume hood. Once molten, half a jelly baby is dropped into the boiling tube and a rapid and spectacular reaction ensues as the sugar reacts creating flames, sparks, sometimes a roaring noise and lots and lots of smoke (one good reason for the fume hood). We noticed that the Bunsen flame, still set to blue, turned lilac due to the quantity of potassium ions in the smoky atmosphere. It was pretty impressive really, and despite watching 5 or 6 groups of student perform the experiment, never lost its magic.
There are lots of simple, elegant and impressive chemistry demonstrations like this, often used to bribe potential students at university visit days and enchant more disruptive classes at school. They are also the things that people remember most strongly about high school chemistry.

First Published May 19th 2009, on version 2.0!

2 Replies to “REPOST: All the pretty colours…”

  1. Our chemistry teacher let us set fire to little bits of magnesium, which was always fun. He also liked to demonstrate the sodium+water spectacle.
    I promise that I do remember more than this from high school chemistry 😉

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