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EXPERIMENTS WITH CARBOHYDRATES

 

Carbohydrates - one of the "three pillars" of our food (the other two are proteins and fats). Glucose and fructose, starch and cellulose, dozens of other carbohydrates are formed continuously and "burn" (oxidize) in plant and animal cells, serve as the major energy material body.

All the dissimilarity of individual members of the carbohydrates they have, of course, General, mandatory for all properties. This allows us to detect carbohydrates even in very small quantities. Faithful and also a beautiful way to recognize them - colored reaction MOLISA.

Pour into a test tube approximately 1 ml of water and throw a few grains of sugar (sucrose), some glucose tablets or a piece of filter paper (cellulose). Now add 2-3 drops of alcoholic solution of resorcinol or thymol (these substances are sold in the pharmacy). Tilt the test tube and gently pour on the wall 1-2 ml of concentrated sulfuric acid. Be careful with the acid, make sure that it does not fall on the skin! Secure the tube in a vertical position. Heavy acid will sink to the bottom, and on the border with water appears bright beautiful ring - red, pink or purple.

If the substance, the composition of which is unknown, will give the reaction Melisa this ring can be no doubt that the carbohydrate is obvious. Just remember that this reaction is so sensitive that it can cause even a speck of dust or volokna on the tube wall. So utensils which conduct the reaction, we need to thoroughly wash and rinse better than distilled water.

Now, by learning to recognize carbohydrates turn to starch is one of the most famous of carbohydrates. For a start, learn to properly cook the starch paste is a colloidal solution of starch in water. Pour into the pan a little cold water and add the starch, about two teaspoons per Cup (and given the water that you will add later). Mix well stir - the so-called starch milk. Under stirring add the boiling water and continue to stir, heat on the fire until, until the solution becomes transparent. Let it cool down. This is the starch paste, which is so well glues paper; therefore it is often used, for example, for gluing of wall-paper.

Of course, you know that in the presence of free iodine starch turns blue. This property we still need; just note that the iodine solution should be very weak. By the way, using this solution (and to make it to pharmacy dilute the solution with water), you can explore on the starch content of different foods.

Having prepared a test tube with a weak solution of iodine, observe transformations of starch. Let's try to make starch paste glucose.

Huge molecules of starch under the action of water hydrolyzed, broken down into smaller molecules. First formed, soluble starch, then "stumps" smaller - dextrins, then disaccharide, but not all the usual sucrose, and the other maltose, or malt sugar. Finally, when the collapse of maltose formed glucose and grape sugar. The ready hydrolysis product often contains all transitional substances; in this form he is known as molasses.

Half Cup of starch paste, add 1-2 teaspoons of diluted approximately 10%sulfuric acid. Don't forget: when dilute sulphuric acid is necessary to pour acid into water, not Vice versa!

Mix the paste with acid put to boil in a saucepan, gradually adding water as evaporation. From time to time take a spoon of liquid samples and slightly cooled, drop into them diluted iodine solution. Starch, as you remember, gives a blue coloration, but dextrins - red-brown. Regarding maltose and glucose, they are not painted. As hydrolysis color samples will change, and when staining with iodine will disappear, the heating can be stopped. However, for a more complete decomposition of maltose makes sense to boil the mixture for a few minutes.

After boiling the liquid to be cooled and slowly add with stirring 10 g of powder of chalk, to completely neutralize the sulfuric acid. The mixture will froth, because during the reaction of the acid with chalk carbon dioxide. Once the foaming stops, put the obtained yellowish liquid on low heat so that it is evaporated by about two thirds, then another hot strain it through several layers of gauze, then ApARTe liquid again, but now more gently, not on an open fire, and in a water bath (blend easily burn). You get a thick sweet syrup, which is based on glucose. About as molasses is available in large quantities on starch factories.

Glucose is required, it is one of the main energy suppliers. But in bread, potatoes, pasta contains mainly starch and in the body it is converted into glucose by the action of enzymes.

In our experience sulfuric acid in the reaction was not spent. She played the role of catalyst, i.e., substances, dramatically accelerating the course of the reaction. The catalytic action of natural enzymes is much stronger, it is more purposefully. Enzymes are very many, and each of them has own narrow area of work. For example, contained in the saliva enzyme amylase can turn the polysaccharide starch into the disaccharide maltose. Make sure to experience the action of this enzyme.

Distilled water (and if not, then boiled for a minute rinse mouth - you will get the solution of saliva. This solution strain and mix with equal quantity of starch paste. The tube containing the mixture was put in a glass of warm (about 40 °C, water. From time to time take samples with iodine - staining will be exactly the same as in the case of hydrolysis with sulfuric acid, but the reaction will go faster. Not later than the quarter of an hour the starch is hydrolyzed to maltose, and color reaction with iodine will disappear.

There is absolutely simple experiment: try for a long time to chew a piece of white bread. You will notice that the taste is sweet. It works the enzyme amylase into maltose starch contained in the bread.

O. Holguín. "Experiments without explosions"
M., "Chemistry", 1986

 

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