Message written with invisible ink.

Invisible Ink

Chemical Reactions 4 minutes read

Welcome, secret agents and codebreakers! Imagine living in a world where your most important message could be hidden in plain sight – completely invisible until the right trick reveals it. That’s exactly how invisible ink has been used throughout history, from ancient times to modern spy stories. Kings, rebels, and even real-world spies have relied on these clever techniques to secretly pass on confidential information, hidden clues, and covert instructions.

In this activity, you will step into their shoes and learn how to write, hide, and reveal secret messages of your own. Along the way, you will also uncover the science behind why these special inks stay invisible until you know the key to revealing them.

Science Involved

Different inks work in different ways. Here are some common invisible inks and the science behind them:

  • Lemon Juice / Vinegar: These acids weaken paper fibers. When the paper is heated, those fibers burn more quickly, so the writing darkens and becomes visible.

  • Milk: Milk contains proteins and fats that turn brown when heated. Your message slowly appears as the milk burns.

  • Fruit Juice (like apple or grape): The natural sugars in fruit juices caramelize when heated, just like when sugar browns in cooking, revealing the writing.

  • Baking Soda Solution: Baking soda solution is basic in nature. Like acids, it weakens the paper fibers, and the message can be revealed by heating the paper. The message also shows up clearly when brushed with an indicator – phenolphthalein (pink in bases) or turmeric solution (reddish-brown in bases).

  • Starch Solution: Starch reacts with iodine. When iodine is brushed over starch writing, it turns a dark blue-black color.

  • Phenolphthalein Solution: A message written with it will turn bright pink when exposed to a base such as baking soda solution.

  • UV Ink (from a UV marker): Invisible under normal light, it glows brightly under ultraviolet (UV) light.

Activity

Requirements

White paper, cotton swabs or paintbrush, lemon juice or vinegar, baking soda, milk, apple or green grape juice, water, turmeric solution in alcohol, phenolphthalein solution, iodine solution, clothes iron (heat source) glass of water, spoons, and small bowls or cups.

Safety Instructions

You will use a heat source to reveal many of your messages. Always ask an adult for help when working with a hot iron. Never leave heat sources unattended and handle them carefully to avoid burns. Keep water nearby just in case.

Getting Ready

Baking Soda Solution

  1. Mix 1 teaspoon of baking soda in 2 tablespoons of water.

  2. Stir well until the baking soda dissolves.

Turmeric Solution

  1. Mix 1 teaspoon of turmeric powder in 2 tablespoons of alcohol (such as rubbing alcohol or spirit).

  2. Stir well and let it settle.

  3. Use the clear yellow liquid for testing.

Starch Solution

  1. Mix 1 teaspoon of cornstarch (or any starch powder) in 2-3 tablespoons of warm water (not boiling).

  2. Stir well until you get a cloudy liquid.

If it becomes thick and jelly-like, just add a little more water and stir again. You want it thin enough to dip a brush or swab and write with.

Iodine Solution

  1. Add a few drops of tincture of iodine to 2 tablespoons of water to make a light brown solution.

Procedure

  1. Choose one invisible ink and dip a cotton swab or paintbrush into it. Write a secret message on a sheet of white paper. Let it dry until the writing is no longer visible.

  2. Use the correct method to reveal it:

    1. Heat (for lemon juice, vinegar, milk, fruit juice).

    2. Apply an indicator (for baking soda, starch, phenolphthalein).

    3. Shine UV light (for UV ink).

  3. Record your observations: Was the message easy to reveal? Was it clear or faint? Did it take a long time or happen quickly?

  4. Repeat the process with other inks. Compare your results.

  5. At the end, decide which invisible ink you would choose for your own top-secret spy work, and explain why.

Observations

Invisible Ink Message is Invisible Easy to Reveal Clear or Faint Time Taken
Lemon Juice        
Vinegar        
Milk        
Fruit Juice        
Baking soda solution        
Starch solution        
Phenolphthalein solution        
UV marker ink        

Reflect and Discus

  1. Which invisible ink gave the clearest and quickest results? Which one was the hardest to reveal?

  2. If you were a spy in the past, which invisible ink would you have chosen? Would your choice be different today?

  3. Can you think of modern-day uses of “invisible ink” or hidden messages outside of spying?