Handicrafts - 2. The back of the stamp

Handicrafts - 2. The back of the stamp If we don't have glue on hand when sending a letter and pasting a stamp, we just need to apply some water on the back of the stamp, and the stamp will become sticky. If we stick it on the envelope, it will never fall off. It turned out that the back of each stamp was coated with dextrin. Dextrin is a small molecule formed by starch hydrolysis. It is easily soluble in hot water and has good viscosity. However, in terms of chemical structure, dextrin still belongs to polysaccharide. Starch and dextrin can react with iodine. The former shows blue when encountering iodine, while the latter shows purple or red when encountering iodine. � Tools and materials Beaker, balance, measuring cylinder, glass rod, iron frame, iron ring, asbestos net, alcohol lamp, dropper, test tube. � Starch, dextrin, distilled water, iodine water, commemorative stamps (4-5 sheets). � Activity process （1） Reaction of starch, dextrin and iodine 1. Put 0.5g starch and 0.5g white dextrin into two clean beakers respectively, and inject 100ml distilled water into each. Put the two beakers on the asbestos net of the iron frame platform respectively, heat them with an alcohol lamp, and stir them with a glass rod to boil the liquid from time to time, so as to prepare turbid starch solution and colorless and transparent dextrin solution. � 2. After the two solutions are cooled, use a dropper to suck 2ml starch solution and 2ml dextrin solution into two test tubes respectively, and drop 1 ~ 2 drops of iodine water into each test tube to observe the phenomenon. � It was found that the starch solution was blue and the dextrin solution was purple. � The characteristic reaction of starch is that starch turns blue when encountering iodine. Dextrin is the intermediate product of starch hydrolysis, and its relative molecular weight is smaller than that of starch, so it turns purple when encountering iodine. � （2） The substance on the back of the stamp reacts with iodine 1. Put 4 ~ 5 commemorative stamps into a beaker and inject 20 ~ 25ml distilled water to immerse the stamps in water. Put the beaker on the asbestos net and heat it with an alcohol lamp. While heating, stir it repeatedly on the back of the stamp with a glass rod until the solution boils. � 2. After the solution is cooled, use a dropper to suck 2ml of stamp impregnation solution into a test tube, drop 3 ~ 4 drops of iodine water, and observe the phenomenon. � The liquid in the test tube was found to be purple. � The above phenomenon confirms that the colloidal substance on the back of the stamp is dextrin. � Description and extension 1. Dextrin is the intermediate product of starch hydrolysis. The dextrin obtained by preliminary hydrolysis of starch is still large, and it basically shows blue when encountering iodine. The product of continuous hydrolysis of starch shows blue → purple → red in turn when encountering iodine, and finally colorless. � 2. The commemorative stamps used in the experiment must be unused. If there are no commemorative stamps, ordinary stamps can also be used instead, but the effect is worse. � 3. The final product of starch hydrolysis is glucose. In order to complete its hydrolysis, some acids need to be added as catalysts. After the reaction, we can use silver mirror reaction to test whether the starch has been hydrolyzed to glucose. � 4. Because dextrin has better solubility than paste, good viscosity and good finish after drying, dextrin is usually used as the adhesive on the back of stamps instead of paste. Similarly, when making paste in life, we can put a small amount of flour in boiling water, stir it with a glass rod and boil it for a period of time to hydrolyze the starch fully, so as to improve the quality of paste. �