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Experiment 10.6
A clock reaction
• potassium iodide
• sodium thiosulphate
• hydrogen peroxide
solution
• sodium hydrogen
sulphate solution
• starch solution
• 2 test tubes
• conical flask
• white paper
The reactions of iodine with starch and of iodine with sodium
thiosulphate in the experiments above can be combined in a very
clever chemical reaction that you can use to amuse and amaze your
friends. Like all good magicians try out this experiment on your own
before showing it to someone else.
It is very important that you use the quantities given here if this
experiment is to work.
Dissolve 1 measure of potassium iodide and ¼ measure of sodium
thiosulphate in water in a test tube and add water to fill the test tube.
This is solution A.
In a second test tube mix 2cm hydrogen peroxide solution and 2cm
sodium hydrogen sulphate solution and add water to fill the test
tube. This is solution B.
Measure 2cm of solution A in a test tube, put it into the conical flask
and add 4 drops of starch solution (either spray starch or potato
starch). Add 2cm of solution B from its test tube. The solution in the
flask will be clear.
Stand the conical flask on a sheet of white paper and wait.
After about 30 seconds the solution suddenly turns blue.
This is called a clock reaction.
What is happening here is that iodine is being produced as in
Experiment 10.1. This is immediately reacting with the sodium
thiosulphate as in Experiment 10.4. Eventually all the sodium
thiosulphate is used up and some iodine remains in the conical
flask. This then reacts with the starch as in Experiment 10.2 to give
the blue colour.
If you warm the tubes of solutions A and B, by holding them in hot
water, before mixing the blue colour will appear in a shorter time.
If this experiment does not work it is most likely that you have too
much sodium thiosulphate (or too little potassium iodide) in solution
A. Make new solutions and try again.
Experiment 11.1
Heating sugar
• sugar
• small evaporating spoon
Put 1 measure of sugar in the small evaporating spoon and heat it
over the burner flame until nothing further happens. Put the HOT
SPOON in the tin lid to cool. Describe what happens. The black
residue left in the spoon is carbon. This is because sugar contains
a lot of carbon.
In Experiments 8.7 and 8.8 you heated tartaric and citric acids.
These are similar to sugar but contain less carbon. Was there any
carbon residue left when they were heated?
Experiment 11.2
Heating a mixture of sugar & sodium hydrogen carbonate
• sugar
• sodium hydrogen
carbonate (sodium
bicarbonate)
• small evaporating spoon
Thoroughly mix 1 measure of sugar with 1 measure of sodium
hydrogen carbonate (sodium bicarbonate) on a sheet of paper. Put
the mixture into the small evaporating spoon and heat until nothing
further happens. Put the spoon on the tin lid to cool. Describe what
happens.
The sugar is forming carbon as in Experiment 11.1. At the same time
the sodium hydrogen carbonate is forming carbon dioxide gas. This is
blowing up the carbon to give the very light residue which you have.
Experiment 11.3
Using sugar to produce copper from copper oxide
• sugar
• sodium hydrogen
carbonate (sodium
bicarbonate)
• copper oxide
• sodium hydrogen
sulphate solution
• small evaporating spoon
• beaker
• conical flask
Repeat Experiment 11.2 but use 2 measures of sugar and 1 measure
of sodium hydrogen carbonate (sodium bicarbonate) this time.
Also mix in 1 measure of copper oxide. Very thoroughly mix the
3 substances together and put the black powder in the small
evaporating spoon. Heat the spoon until nothing further happens.
Put the spoon on the tin lid to cool.
When the spoon is cold, knock the residue of it onto a piece of paper
and crush it to a fine black powder. Put this into the conical flask and
add about ½ a test tubeful of your sodium hydrogen sulphate
solution. Boil the solution for 2 to 3 minutes. Make a holder for the
conical flask from a piece of folded paper as in Experiment 5.4.
Pour the liquid into the beaker and carefully look at the residue left in
the conical flask through the bottom of the flask. In amongst the black
solid will be shiny particles of copper metal that have been produced
from the copper oxide.
Chapter 10 - Some iodine chemistry
Chapter 11 - Some sugar chemistry
