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ENTERTAINING EXPERIENCES AT HOME
Sawdust sugar. Chemical experiments
Entertaining experiences at home / Chemistry experiments for children Carbohydrates got their name by mistake. It happened in the middle of the last century. Then it was believed that the molecule of any sugary substance corresponds to the formula Cm(n2ABOUT)n. All then known carbohydrates fit this measure, and the formula for glucose C6Н12О6 written as C6(n2ABOUT)6. But later, sugars were discovered that turned out to be an exception to the rule. So, a clear representative of rhamnose carbohydrates (it also gives Molisch reaction) has the formula C6Н12О5. And although the inaccuracy in the name of a whole class of compounds was obvious, the term "carbohydrates" has already become so familiar that they did not change it. However, today many chemists prefer a different name - "sugar". We will try to obtain one of the sugars from sawdust by hydrolysis, that is, by decomposition with water. This is a very common chemical process. Sawdust and other wood waste contain carbohydrate fiber (cellulose). Glucose is prepared from it at hydrolysis plants, which can then be used in different ways; most often it is fermented, turning it into alcohol, the starting product for many chemical syntheses. A large and independent branch of the chemical industry is called the hydrolysis industry. Before reproducing the process of wood hydrolysis, let's try to understand what its essence is, and for this it will be more convenient to start not with sawdust, but with cucumbers and splinter. Wash a fresh cucumber, grate it and squeeze out the juice. The juice can be filtered, but it is not necessary. Prepare copper hydroxide Cu(OH) in a test tube2. To do this, add 2-3 drops of copper sulfate solution to 0,5-1 ml of sodium hydroxide solution. Add an equal volume of cucumber juice to the resulting precipitate and shake the test tube. The precipitate will dissolve and a blue solution will be obtained. Such a reaction is typical for polyhydric alcohols, i.e., for alcohols that contain several hydroxyl groups. Now heat to a boil (or put in boiling water) a test tube with the resulting blue solution. It will first turn yellow, then turn orange, and after cooling, a red precipitate of copper oxide Cu2O. This reaction is characteristic of another class of organic compounds - aldehydes. This means that in cucumber juice there is a substance that is an aldehyde and an alcohol at the same time. This substance is glucose, which in structure is an aldehyde alcohol. Thanks to her, the cucumber has a sweetish taste. You probably guess that this experiment does not have to be done with cucumber juice. It also works well with other sweet juices - grape, carrot, apple, pear. You can also take cucumber toilet water, which is sold in perfume shops. And, of course, just glucose tablets. Now the second preliminary experience; saccharification of the splinter. Prepare a solution of sulfuric acid: add one volume of concentrated sulfuric acid to one volume of water (never pour water into acid!). Put a splinter into a test tube with a solution and heat the solution to a boil. At the same time, the splinter will be charred, but this will not interfere with the experience. After heating, remove the splinter, lower it into another test tube with 1-2 ml of water and boil. Both tubes now contain glucose. You can check this by adding two or three drops of copper sulfate to the solutions, and then caustic soda - a familiar blue color will appear. If this solution is boiled, as we expected, a red precipitate of copper oxide Cu2O. So, glucose is found. The fact that our splinter is sugared is the result of the hydrolysis of cellulose (and its share in wood accounts for about 50%). As in the hydrolysis of starch, sulfuric acid is not consumed in this process, it plays the role of a catalyst. Finally, we come to the main experience that was promised in the title: making sugar from sawdust. Pour 2-3 tablespoons of sawdust into a porcelain cup and moisten them with water. Add a little more water and an equal amount of the previously prepared sulfuric acid solution (1: 1), mix the liquid slurry well. Close the lid and place in the gas stove oven (or in the Russian oven) for about an hour, maybe a little less. Then take out the cup, add water to the top and stir. Filter the solution and neutralize the filtrate by adding crushed chalk or lime water until no more carbon dioxide bubbles are released. The end of neutralization can also be judged by testing the liquid with a litmus test or one of the homemade indicators. It is not necessary to drip the indicator directly into the reaction mass. You should take a sample, literally 2-3 drops, and place it on a glass plate or in a small test tube. Pour the contents of the cup into a milk bottle, shake the liquid and let stand for several hours. Calcium sulfate, formed during the neutralization of the acid, will settle to the bottom, and a glucose solution will remain on top. Carefully pour it into a clean cup (preferably over a glass rod) and filter. The last operation remained - evaporation of water in a water bath. After it, light yellow glucose crystals remain at the bottom. They can be tasted, but only - the product is not pure enough. So, we have completed four operations: sawdust pulping with a solution of sulfuric acid, acid neutralization, filtration and evaporation. This is how glucose is obtained at hydrolysis plants, only, of course, not in porcelain cups ... And we can reproduce another industrial process without much difficulty: we turn one sugar into two others. When stored for a long time, homemade jam is often candied. This is because the sugar crystallizes out of the syrup. With jam, which is sold in the store, such a misfortune happens much less often. The fact is that in canneries, in addition to beet or cane sugar, sucrose C12H22O11, other sugary substances are also used, for example, invert sugar. What is sugar inversion and what it leads to, you will learn from the following experience. Pour into a test tube or glass 10-20 g of a weak sugar solution and add a few drops of dilute hydrochloric acid. After that, heat the solution in a boiling water bath for ten to fifteen minutes, and then neutralize the acid, preferably with magnesium carbonate MgCO3. Pharmacies sell the so-called white magnesia, a substance of a slightly more complex composition; she fits too. In extreme cases, you can take NaHCO drinking soda3, but then table salt will remain in the solution, which somehow does not harmonize with sugar ... Once the carbon dioxide bubbles have stopped, let the liquid settle. Just in case, check with an indicator whether the acid is completely neutralized. Drain the settled liquid and taste it: it will seem less sweet than the original solution (for comparison, leave a little of the original sugar solution). In the finished solution, there was practically no sucrose left, but two new substances appeared - glucose and fructose. This process is called sugar inversion, and the resulting mixture is called invert sugar. And here's what's curious: outwardly, there is nothing to detect a reaction. And the color, and volume, and the reaction of the environment remain the same. No gases or precipitation are emitted. Nevertheless, the reaction goes on, only optical instruments are needed to detect it. Sugars are optically active substances: a beam of polarized light, passing through their solution, changes the direction of polarization. They say that sugars rotate the plane of polarization, and in one direction or another, and at a very definite angle. So, sucrose rotates the plane of polarization to the right, and glucose and fructose, the products of its hydrolysis, to the left. Hence the word "inversion" (in Latin "reversal"). But, since there are no optical instruments at our disposal, let's try to make sure by chemical means that the sugar taken has indeed undergone changes. Add a few drops of a solution of methylene blue (you can take blue ink for fountain pens) and a little weak solution of any alkali to the initial and resulting sugar solutions. Heat the test solutions in a water bath. In a test tube with ordinary sugar, no changes will occur, but the contents of a test tube with invert sugar will become almost colorless. Invert sugar is much less prone to crystallization than regular sugar. If you carefully evaporate its solution in a water bath, you get a thick syrup that looks a bit like honey. After cooling, it does not crystallize. By the way, three-quarters of the beloved bee honey consists of the same carbohydrates as invert sugar - glucose and fructose. Artificial honey is also made on the basis of invert sugar. Of course, our syrup differs from honey, and significantly - mainly in the absence of smell. But if you add a little natural honey to it, then this drawback can be partially eliminated. But why not make more non-crystallizing syrup at home to make jam on it? Alas, its complete purification from foreign substances is difficult, and there is no guarantee that it will be possible to complete it. In any case, it's not worth the risk. Author: Olgin O.M.
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