What it shows
Lemon juice dries clear on paper, but gentle heat from a clothes iron turns the written lines brown before the rest of the paper changes. The acid in the juice weakens the paper fibres, so they scorch at a lower temperature than clean paper.
The science
Paper is made of cellulose. Citric acid in lemon juice breaks some of the long cellulose chains where it dries, leaving sugars and weakened fibres. When heated, these damaged parts react with oxygen and turn brown well before the undamaged paper would. Milk and vinegar work for the same reason, because they also leave organic residues that char first.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Juice of one lemon in a small bowl
- A cotton bud or a matchstick without its head
- White paper, 3 sheets
- A clothes iron on a low setting (adult only)
- An old towel to iron on
- Milk or diluted vinegar for comparison, optional
Safety
For this project: An adult must operate the iron and keep it on a low setting; the child never touches the hot plate. No candle or open flame is used.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Squeeze the lemon and dip the cotton bud in the juice.
Write a short message on the paper; use plenty of juice but do not soak through.
Let the sheet dry completely; the writing should be invisible or very faint.
Spread the towel on a table and ask an adult to set the iron to the lowest setting.
The adult passes the iron slowly over the sheet for ten to twenty seconds, lifting it to check.
Watch the writing turn light brown; stop as soon as it is readable.
Repeat with a message written in milk and another in diluted vinegar on the other sheets, and compare how fast each appears.
Record which liquid gave the clearest message at the lowest heat.
What you should see
The lemon juice message appears in brown on an otherwise white sheet; milk works too but needs slightly longer, and the plain paper around the writing stays white.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the writing go brown first?
The acid weakens the paper fibres there, so they scorch at a lower temperature than clean paper.
2Why does the lemon juice dry invisible?
It is nearly colourless and soaks into the paper.
3Is the browning a chemical change?
Yes, the weakened fibres and sugars react with oxygen to form new brown substances.
4Why not use a candle to reveal it?
An open flame can set the paper alight; a low iron gives gentle, even heat.
Take it further
Dilute the lemon juice with equal water and then with three parts water, and find the weakest juice that still gives a readable message.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Science Projects for Class 6 to 8
Working models matched to middle-school science topics
Science projects
Safe Chemistry Projects for School
Experiments and models with kitchen materials and proper safety