Chemistry in non-visible ink pen:

 Invisible ink pens are pens that use ink that is invisible to the naked eye or can disappear after a period of time. There are different types of invisible ink pens, depending on the mechanism that makes the ink visible or invisible. Here are some examples:

- Invisible ink pen uses transparent and fluorescent ink. They are only seen under ultraviolet (UV) light. The ink contains a chemical compound called thymolphthalein, which is colorless under acidic conditions and blue under basic conditions. The pen also contains a small amount of sodium hydroxide, which is a base. When using a pen, sodium hydroxide reacts with carbon dioxide in the air and forms sodium carbonate, a neutral substance. This makes thymolphthalein colorless and invisible. To reveal the message, UV light is used to excite thymolphthalein molecules and cause them to emit blue light.

 - Erasable pens use visible ink but can be erased by heat or friction. The ink contains pigments that change color depending on the acidity of the environment. The pen also contains a chemical that acts as an acid or base. When using the pen, the chemical creates an acidic or basic state that makes the pigment visible. When ink is erased with an eraser, the friction creates heat that changes the acidity of the ink and makes it invisible. As the ink cools, the acidity returns, and the ink becomes visible again. - Disappearing pens use ink that is visible but disappears after a while. The ink contains dyes that react with oxygen in the air and lose color. The pen also contains antioxidants that slow down the reaction. When using the pen, the antioxidants are consumed, and the dye begins to react with oxygen and becomes colorless. The rate of discoloration depends on the number of antioxidants and oxygen present³.  There are also other types of invisible ink pens that use different chemicals or physical properties to create hidden messages. For example, some pens use ink that reacts with certain substances, such as iodine, lemon juice, or iron sulfate, to change color⁴. Some pens use ink that has magnetic properties and can be seen by a magnet⁵. Some pens use ink that has thermal properties and can be seen with a thermal camera.