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Lemon Juice Christmas Cards - The invisible ink experiment

You will need:

  • Lemon juice, paper, a cotton tip and some pens
  • A spotlight ​


  1. Use a cotton tip to paint your lemon juice onto your Christmas card.
  2. This lemon juice needs to dry so don't use too much!
  3. Turn on a spotlight and allow it to heat up (SAFETY: don't use an open flame like a candle).
  4. Place your paper over the spotlight, allowing the paper to heat up.
  5. Move you paper over the heat source until the lemon juice turns brown in colour.
  6. Stop before the paper begins to burn!

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Why Does This Happen?

This is the classic 'invisible ink' experiment. Lemon juice is acidic and contains sugar. 
The acid from the lemon juice affects the paper, effectively weakening the paper structure. Adding heat to the paper causes the acid affected paper to burn first. Additionally sugar will form caramel when heated, this is present in small quantities within the lemon juice. 

  • Try other fruit juices - which work best? 
  • Also try other weak acids; does this work as well? 
  • What will happen if you add extra sugar to your painting solution?
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