Chapter 2 - Invisible Inks
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Some substances are a different colour when they are cold and when they are hot. We can use
this property to make invisible inks. You can write on paper with the ink and it only becomes
visible when you “develop” it, by heating the paper with an iron or holding it in front of a fire.
Chapter 2 - Invisible Inks
Experiment 2.4
Using a chemical developer for invisible ink
• iodine solution from
experiment 2.5
• conical flask
ª crystallising dish
• filter paper
• dropping pipette
• artists paint brush
• small plate
• spray starch
Collect some starch from a Spray Starch spray in a small container
(for example a mug or a bowl). It is likely that it will be a foam. Let
the foam settle and then pour the solution into the crystallising dish
and write on a filter paper with the paint brush. Let the writing dry.
While the writing is drying carry out Experiment 2.5 to make the
iodine solution which you will need as the developer.
When the writing is dry put 30ml of water into the conical flask.
Add 10 drops of iodine solution using the dropping pipette. Pour
a little of this iodine solution onto a small plate. Drop the filter paper
into the iodine and the writing will magically appear.
What colour is it?
Experiment 2.5
Using a chemical developer for invisible ink
• potassium iodide
• sodium hydrogen
sulphate
• hydrogen peroxide
solution
• test tube
• small clean dry bottle
Put ½ measure of potassium iodide and ¼ measure of sodium
hydrogen sulphate into a clean dry test tube and add about 2cm of
water. Add 10 drops of hydrogen peroxide solution; yellow-brown
iodine will be formed. Add water until the test tube is half full.
Carefully pour this solution into a bottle. Label it “Iodine Solution”.
THIS IS VERY IMPORTANT.
Experiment 2.1
Invisible ink from a lemon
• a lemon
• crystallising dish
• artist’s paint brush
or a cotton-bud
• writing paper
Squeeze a lemon and pour some juice into a crystallising dish. Use
the paint brush or a cotton-bud to write on a piece of white paper.
Unglazed writing paper is best. Let the writing dry.
Get an adult to help you carefully heat the paper by pressing it with
an iron or holding it in front of a fire. Take great care not to let the
paper catch fire.
What colour is the writing?
Experiment 2.2
Other invisible inks
• iron sulphate
• test tube
• crystallising dish
• artist’s paint brush
or a cotton-bud
• writing paper
Dissolve ¼ of a measure of iron sulphate in about 1cm of water in
a test tube. Pour the solution into a crystallising dish and write on
unglazed paper as in Experiment 2.1.
Develop the invisible ink by heating as in Experiment 2.1.
What colour is the writing?
Experiment 2.3
An invisible ink made from two chemicals
• iron sulphate
• test tube
• crystallising dish
• artist’s paint brush
or a cotton-bud
• writing paper
Put ¼ measure of copper sulphate and ¼ measure of ammonium
chloride in a clean dry test tube and add 1cm of water. Shake the
tube gently until the chemicals have dissolved (do not heat the
solution). Pour the solution into the crystallising dish and write on
unglazed paper as in Experiment 2.1.
Develop the invisible ink by heating as in Experiment 2.1.
What colour is the writing?
Later in this instruction booklet you will see how sometimes
when chemicals react together a colourless chemical changes
into a coloured one. This property is used here to develop an
invisible ink.
