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 stays on the paper as it dries and slowly weakens the cellulose fibres there, and the sugars in the juice brown easily too. When heated, these treated parts react with oxygen and turn brown well before the untouched paper would. Milk works because its proteins and sugars brown when heated, and diluted vinegar works mainly through its acid, as lemon juice does.
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 toothpick
- White paper, 3 sheets
- A dry clothes iron with the steam turned off (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, keep it away from the table edge and unplug it afterwards; 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.
Step 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 dry iron to a low setting.
The adult passes the iron slowly over the sheet for ten to twenty seconds, lifting it to check; if nothing shows, the adult may raise the setting one step, never to the hottest.
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.
01Why does the writing go brown first?
The acid weakens the paper fibres there, so they scorch at a lower temperature than clean paper.
02Why does the lemon juice dry invisible?
It is nearly colourless and soaks into the paper.
03Is the browning a chemical change?
Yes, the weakened fibres and sugars react with oxygen to form new brown substances.
04Why 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.
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