Xiao Kang poured half a cup of water into a plate, added 10 drops of iodine and mixed it evenly with the water. Then cut a lemon and squeeze the lemon juice into another clean glass.

Xiao Kang found a piece of white paper and cut it into a size that could fit on the plate. Dip a small brush in lemon juice and write or draw on paper. Dry the paper with writing (or pictures) on it. Finally, the paper that had been written on and dried was put into a plate with iodine solution. As a result, the parts of the paper that were written with lemon appeared colorless, and the rest of the paper turned blue-purple. It's like a calligraphy and painting work with the handwriting in white. Can you tell why?

Principle:

The starch contained in the paper will react chemically with iodine to form iodine starch molecules, which appear dark blue-purple; lemon juice is rich in vitamin C. When vitamin When C reacts chemically with iodine, it will appear colorless. Therefore, the color of parts written or drawn with lemon juice will not change.