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ENTERTAINING EXPERIENCES AT HOME
Crystals - big and small. Chemical experiments
Entertaining experiences at home / Chemistry experiments for children So much has been written about growing crystals, and these experiments are so spectacular and easy to perform, that you have probably set them up at least once and know what the principle is. Actually, there is nothing tricky here: you need to prepare a hot saturated solution of any salt (sodium chloride, copper or iron sulfate, alum, potassium bichromate, etc., the list is very long), cool it carefully so that the excess of the dissolved substance does not fall out into the precipitate (such a solution is called supersaturated), and, finally, introduce a seed - a crystal of the same salt, suspended on a thread. After that, it remains only to cover the vessel with a piece of paper, put it in a secluded place and wait until a large crystal grows, which can take weeks or even months; the only thing that will occasionally have to be done is to add a little saturated solution as it evaporates. All this is really known. But there are a lot of options for experience, and we will choose not the most common ones, for example, with lead nitrate and potassium iodide. Mix equal volumes of 10% solutions of these salts, and lead iodide will precipitate in the vessel. Drain the liquid carefully. Boil water in a transparent vessel, acidify it with vinegar, and while it boils, add more wet precipitate of lead iodide by shaking it. When the liquid cools slowly, golden crystals will grow in it. A variation on the same theme: pour solutions of lead nitrate and potassium iodide into a test tube, boil the contents along with the precipitate so that it dissolves, and then quickly cool under a tap. In this case, tiny gold crystals are formed, suspended in the liquid. In general, the crystal size strongly depends on the cooling rate. Pour 20 g of potassium nitrate in small portions into a vessel with 25 ml of water. After adding the next portion, shake the mixture so that the salt dissolves, and then pour the next portion. When the salt ceases to dissolve, heat the vessel a little, pour in another portion, shake, heat again. And so on until all the salt taken is dissolved. Now pour the solution into two vessels, and leave one to cool in the air (for even slower cooling, you can cover it with several layers of dense fabric). In this vessel, several large crystals are formed, and with a successful combination of circumstances, one crystal. Place another vessel immediately in a pot of cold water, and many small crystals will stand out in it. This is a general rule. The next two experiments are so impressive that they can be safely shown to the audience, of course, after carefully preparing everything. The first of these is the Peligo experience. Wash the cylinder with a height of 25-30 cm from the inside with hot water and pour a hot, very concentrated solution of hyposulfite into it through a funnel along the wall so that it fills the cylinder by 1/3. This solution is prepared as follows: 450 g of hyposulfite are dissolved by heating in 45 ml of water. The second solution - sodium acetate (300 g per 45 ml of water) is also poured hot through the same funnel for another 1/3 of the cylinder. Pour very carefully, this solution should not mix with the previously poured solution. Finally, just as carefully fill the top third of the cylinder with hot water, which will protect the saturated solution from premature crystallization. There are three layers in the vessel: water, a supersaturated solution of sodium acetate, a supersaturated solution of hyposulfite. Cover the cylinder with glass, let it cool to room temperature, and then you can start the experiment. Attach a small, inconspicuous crystal of hyposulfite to the end of a glass rod with a piece of wax (melt the wax slightly by heating it over a flame). In front of the audience, quickly lower the wand into the bottom layer. The concentration of salt is so high that immediately a lot of new crystals will pile up around the crystal, forming a kind of flower. And in the middle layer, the "alien" substance around the hyposulfite crystal will not crystallize. Dip another, exactly the same stick with wax, but with a small crystal of sodium acetate (the audience should not notice the difference) into the middle layer - a flower will also grow here, but completely different! The cylinder, if handled with care, can be used several times. Another trick-like experience is with sodium acetate alone. Dissolve 100-150 g of salt in hot water (preferably in an enamel bowl) and slowly evaporate, trying to accurately catch the moment when you need to stop evaporation: blow from time to time on the surface of the hot solution, and as soon as a film resembling fat begins to appear, this means that the salt concentration is that which is required for the formation of a crystalline hydrate of the composition CH3COONa*3H2O. Pour the liquid into a clean thin glass, close it and leave to cool. It is enough to add an insignificant amount of sodium acetate seed to the cooled liquid so that it instantly crystallizes and turns into a solid mass resembling ice. If you slightly underexposed the liquid on fire and there is too much water in it, then there will be a little water above the frozen mass, which must be drained. If there is not enough water, then a salt coating will appear on the surface. It makes no sense to remove it, it's easier to add a little water. By melting the crystalline hydrate in a water bath and cooling it, the experiment can be done many times, including in front of an astonished audience - and who does not be amazed to see how the water freezes before our eyes without cooling? On the contrary, the glass even warms up - this is the heat of crystallization released. The glass can be turned upside down, and not a single drop will spill out of it. Showing the experience as a trick, try to shake off a grain of salt imperceptibly - say, from the tip of a "magic wand". And be sure to close the glass tightly between experiments, otherwise even a random speck of dust can cause unplanned crystallization. The reagent for this experiment - sodium acetate can be obtained from acetic acid and soda. If you cook it yourself, then dilute the acetic acid with water about three times and pour soda into it in small portions, gradually, waiting for the foaming from the previous portion of soda to stop. Without this, the reaction will go so violently that the liquid can be thrown out of the vessel. And more unusual crystals - metal, We will grow copper crystals. You have already received small copper crystals when you lowered a nail into a solution of copper sulfate. They are so small that the copper film on the surface seems almost continuous. And in order to prepare large crystals, it is necessary to somehow slow down the reaction so that the copper released in the reaction has time to settle on the crystals and complete them. Slow cooling is a possible way, but in the case where there is no chemical reaction... Salt will act as a brake on the reaction. Put some copper sulfate crystals on the bottom of a vessel (for example, a glass jar) and cover them with common salt, if possible fine. Cover them with a circle cut out of blotting or filter paper; this circle should touch the walls of the vessel. From above, directly on the paper, put an iron circle of a slightly smaller diameter. Wipe it in advance with sandpaper and rinse. Pour a saturated solution of table salt into the jar so that it completely covers the iron circle. Everything will go on without your participation. It is impossible to say exactly how long one will have to wait - much depends on the conditions of the experiment. In any case, not an hour or two, but several days. So, after a few days you will find beautiful red crystals of copper in the vessel. By changing the size of the vessel, the size of the crystals of copper sulphate, the thickness of the salt layer and the temperature of the experiment, it is possible to obtain copper crystals of various shapes, sometimes extremely unusual. And sometimes dendrites grow - unfinished crystals in development, similar to tree branches. If you leave copper crystals in the same vessel in which they were received, then they will not last long. Remove them, rinse with water, transfer to a test tube with dilute sulfuric acid and stopper. Now nothing will happen to the crystals. Author: Olgin O.M.
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