When you go shopping in the mall, pick out the goods you need and pay for them, and watch the cashier print out the shopping receipts for you, have you ever wondered how these receipts are printed out?

What we usually call printing is to transfer ink to a specific area of paper in a specific way, so as to get the desired text or graphics. The chemicals that make up the paper do not absorb any color of light, so when light is reflected from the surface of the paper into our eyes, we will see that the paper is white. Pigments or dyes in ink can absorb some or even all colors of visible light. So when the ink is applied to the surface of the paper, the surface of the white paper is colored. The printers we use in our home or office are mainly inkjet printers and laser printers. Inkjet printers spray tiny droplets of ink onto paper, while laser printers use electrostatic attraction to absorb carbon powders onto light drums before transferring them to paper. However, the printing of shopping receipts is not done in this way. It uses a special kind of thermal paper.

Compared with ordinary paper, POS thermal paper has a thin coating on its surface, which contains some special chemicals called leuco dyes. The latent dye itself is colorless, so the newly purchased heat-sensitive paper looks as white as ordinary paper. However, once they meet the right conditions, they react chemically, and the new substances absorb visible light, so we see the color. For example, crystal violet lactone and many other substances, although colorless, once encounter acid substances will become purple. That is to say, when we print with POS thermal paper, the ink is not stored in the printer, but has been covered on the paper.

However, although leuco dyes such as crystal violet lactone can easily react with acids, they are solids at room temperature, and molecules are firmly bound to their positions. If the acid in contact is solid, it will be safe for a long time at room temperature despite close contact. Therefore, we can grind these dark dyes, which are solid at room temperature, with the solid of another acidic substance, into fine powders, mix them and apply them on the surface of the paper, and then we can get the heat-sensitive paper. At room temperature, POS thermal paper does not seem to be different from ordinary paper. Once the temperature rises, the leuco dyes and acidic substances melt into liquids one after another, and the molecules that can move freely meet immediately react, so the white paper quickly shows color. That's why heat-sensitive paper gets its name - only when the temperature is high enough to discolor it.