The Clock Reaction

The Clock Reaction
On SCOPE's BLUE episode, Chris used a few simple chemicals to cause a sudden and
predictable blue colour change. Here’s how you can try it at home!
What you need:
- Starch (we used laundry spray starch)
- Vitamin C tablets (500 mg)
- some measuring spoons, plastic cups and water...
- Iodine (1%), available from a chemist
- Hydrogen peroxide (6%), available from a chemist
- Gloves
What to do:
1. Make a vitamin C solution by grinding up one tablet to a fine powder and
mixing it with ¼ cup of water. Label this cup Vitamin C and don’t worry if
there are floating bits.
2. Label another two cups, one “A” and the other “B” .
3. Into cup A, put 1 teaspoon of the vitamin C, and add some of water.
You’ll need to add put about ½ teaspoon of iodine, but be sure to wear
gloves and put newspaper down, iodine can stain!
4. Part B starts with a little starch and water. Last ingredient in here is a teaspoon of
the peroxide.
5. All that remains is to mix the 2 and wait. Things actually start happening
straight away even though it doesn’t look like it

What’s happening:
When you initially add the iodine to vitamin C, the browny-yellow iodine colour
practically vanishes, thanks to a chemical reaction that turns the iodine into
iodide (colourless).
Then, when you mix the two reaction parts (A and B), the iodide (colourless) is
oxidise back to iodine by the peroxide. This is a slow reaction, and as soon as
more iodine is formed it is reacted away by the vitamin C, back to the
colourless iodide.
But, after a short while, the vitamin C is all reacted, and at that moment there
is now iodine available, and it reacts with the starch present to produce the
sudden inky blue colour. Essentially the timing of the clock is due to the
amount of vitamin C, vary it slightly to change the timing of your “clock”.
Iodine + Vit. C Iodide (colourless) {quick reaction}
Iodide + Peroxide Iodine {slower reaction}
Iodine + Stach Blue colour {Iodine can even be used to test for starch}
Now if you can manage to measure out all your things really accurately, the
reaction will take exactly the same time, every time, but that can be pretty
tricky.
In Chris’s experiments, his three versions took 47 sec, 1.06 sec and 1.01 secs
So not too bad really.
The leftovers can go down a sink, and the cups and spoons are now for
experiments only.