Chemistry Club1 Edition 2 a/w 27/9/01 11:21 am Page 48
P U P I L S ’
W O R K S H E E T
25: Clock Reactions
This reaction is very dramatic and visually pleasing. It will impress your friends that
you are able to make a chemical reaction take place at the click of your fingers.

eye protection
must be worn
What you will need
10 x 100 cm3 beakers
10 cm3 measuring
cylinders
5 stirring rods
stop clock
solution
solution
solution
solution
A : sodium thiosulphate solution
B : potassium iodide solution
C : starch solution
D : hydrogen peroxide/sulphuric acid solution
eye protection
What you do
1.

Label five 100 cm3 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
solution cm3 (A)
potassium iodide
solution cm3 (B)
starch solution
cm3 (C)
distilled water
cm3
1
1
2
1
16
2
2
2
1
15
3
3
2
1
14
4
4
2
1
13
5
5
2
1
12
2. Stir each beaker with a separate stirring rod.
3. Place a clean 100 cm3 beaker behind each of the five labelled beakers. Add
20 cm3 of solution D to each of the clean 100 cm3 beakers.
4. Using the first 100 cm3 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 100 cm3 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!
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