STEM Tuesdays: Invisible Ink

Get ready to become a super spy with today’s STEM activity! We’re going to write some top-secret messages in invisible ink, then use the magic of science to reveal what they say.

Materials & Instructions

Lemon juice
A small bowl
A q-tip
Paper or a paper towel
A hair dryer

STEP 1. Squeeze your lemon or lemon juice into a small bowl.
STEP 2. Use your q-tip to write a message or draw a picture on a piece of paper or paper towel using your lemon juice as ink. Pro tip: don’t use too much lemon juice, or your paper will take a long time to dry.
STEP 3. Let your message dry until all the wet spots have faded. Can you read your secret message?
STEP 4. With a grownup’s help, use your hairdryer to heat the paper. Move the hairdryer back and forth over the paper so that it does not burn; make sure you are using the warm setting instead of the cool setting. After a few minutes, you should see your secret message start to appear!

What Happened?

The carbon compounds in lemon juice break down and oxidize when exposed to heat and air. What does that mean? Well, lemon juice contains carbon compounds or mixtures of carbon and other chemicals. Without heat, those compounds are colorless – that’s why you can’t read your secret message after it dries. When those carbon compounds are exposed to heat, however, they start to break down and release carbon from the mixture. When that carbon is exposed to the air around it or oxidized, it turns brown and your invisible message is revealed. 

FUN FACT!

Invisible ink was used during the Revolutionary War to carry secret messages! Their notes could be brushed over with another chemical or heated over a candle to reveal the words. 

Vocab Words

Oxidize: To combine with oxygen. When our carbon combined with oxygen, it turned brown. 
Chemical compound: A mixture of two or more chemicals. Our lemon juice had chemical compounds in it that contained carbon along with other chemicals. The heat from our hair dryer broke apart that mixture to expose the carbon.
Expose: To make something visible. 
Release: To set something free. We released carbon from its chemical compound mixture when we exposed the lemon juice to heat.
Reveal: To make known.

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