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Invisible Inks

Making an invisible ink is the first step to becoming a secret agent scientist! 

In this experiment you will be using some lemon juice. Lemon juice (and the juice of most fruits) contain carbon compounds. These compounds can be colourless at room temperature but when heated the compounds break down, releasing carbon. When this carbon comes into contact with the air, oxidization occurs and turn the solution brown. No longer invisible and ready for your science agent friend to read. 

You will need: 

  • Juice of a lemon

  • White paper

  • Paintbrush

  • Hairdryer.

Experiment
  1. First prepare the lemon juice by chopping and squeezing a lemon. A grown up would be helpful here. 

  2. Next use the paint brush to write your message with the lemon juice on some plain white paper. 

  3. Leave it to dry out completely. 

  4. Finally use the hair dryer to heat the ink from underneath. Secret message revealed! 

What is happening here? 

The chemical compounds within the lemon attach onto the fibres of the white paper. As you heated the paper the carbon was released into the air breaking the compounds down. This process is called oxidization and caused a colour change. Have you ever seen this happen without heat? What happens if you chop an apple and leave it out too long? 

Science Challenge

Try writing your message in other liquids: 

  • Apple juice

  • Vinegar

  • Milk

  • Water