When I write on some papers, a copy of the writing appears on the underlining page. How does this happen?
This is called No Carbon Required (NCR) paper and is used to make original copies of handwritten or typed documents without the use of electronics (copy machines).
The process for preparing the paper is called microencapsulation.
WOW! Now that is a mouthful, but is really a cool! process.
micro–This prefix tells you that something is very small. In fact, it is teeny tiny-invisible–microscopic.
encapsulate —This means that something is is placed inside a capsule–some medication is encapsulated, but the capsules are not microscopic (invisible to the naked eye).
The microscopic capsules on the paper is filled with invisible ink.
Ink is a mixture of a dye and a solvent. The ink is invisible because the dye molecules are colorless and transparent to visible light. This means, like transparent tape, light passes through the dye molecules without being absorbed or reflected. The solvent would also have to be colorless and transparent to visible light.
I know–How can one see invisible ink?
The answer is that when the transparent (invisible ink) is mixed with a special chemical, the a chemical developer covers the top of the second sheet of paper. When you write on the top sheet of paper, the pressure of your pen or pencil breaks some of the capsules and the invisible ink is released. The ink reacts with the chemical developer on the the second sheet producing a colored ink.
Microencapsulation is also the process for preparing scratch-and-sniff stickers. The stickers differ from NCR paper in that the stickers don’t need a chemical developer. When you scratch a sticker, you break break individual capsules releasing the perfume contents.