25: Clock Reactions

What you do

1. 
   Label five 600cm³ beakers with numbers 1-5 and place them in a row on the display table. Place the following combinations of solutions in each of the beakers. Use a separate measuring cylinder for each solution.

   beaker	sodium thiosulphate	potassium iodide	starch solution	distilled water
   1	10	20	10	160
   2	20	20	10	150
   3	30	20	10	140
   4	40	20	10	130
   5	50	20	10	120

2. Stir each beaker with a separate stirring rod.
3. Place a clean 250cm³ beaker behind each of the five large beakers. Add 200cm³ of solution D to each of the clean 250cm³ beakers.
4. Using the first 250cm³ beaker pour solution D carefully into beaker 1. Note the time required for the colour to appear.
5. Repeat step four using each of the four remaining 250cm³ beakers containing solution D.

Now that you have a good idea about how much sodium thiosulphate solution and water you need to change colour in a known time, you can start to develop your trick. You could amaze your friends by your ability to change the colour of the solutions in the beaker on cue. (For example when you click your fingers or when some music is played, or when your Headteacher brings a visitor around to your stand at the open evening.) Have fun!