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ENTERTAINING EXPERIENCES AT HOME
Let's silver the mirror. Chemical experiments
Entertaining experiences at home / Chemistry experiments for children Is it possible? Mirrors are silvered in special factories, in mirror workshops. This craft is ancient, with traditions and professional secrets - it is not easy to make a good mirror. And yet - let's try! Of course, a mirror surface can be prepared without difficulty - the reaction of the "silver mirror" is set at school in the classroom. But a good mirror will not work, it will shine a little - and that's it. No, this is a real mirror. And you will certainly succeed if you work carefully and do not neglect the little things. Important Warning: Use only distilled water. Powder "crocus" - specially prepared iron oxide (III) - sift through a fine sieve and dilute the resulting dust with distilled water. If you do not have "crocus", you can take any ready-made polishing liquid, they are sold in hardware stores. Put the even glass that you are going to make mirrored on the table. Check with a level that the table surface is level, and if not, place folded pieces of paper under the legs to level the surface. Do not take a large glass, it is better to start small. In order not to accidentally crush it, put on the table, for example, an old blanket. Pour the polishing liquid onto the glass and wipe it thoroughly in a circular motion with felt, felt (you can use an old hat), or a piece of soft leather. Rub the glass with gauze moistened with a suspension of finely ground and sifted pumice in water, rinse again with distilled water, wipe with a damp sponge, and then with a gauze swab dipped in a 0,15% tin (IV) chloride solution, rinse again and wipe with a squeezed swab . Glass preparation is complete. This is a very important procedure. The quality of the future mirror depends on how carefully you completed it. The treated surface must be silvered immediately. If for some reason you did not have time to prepare a solution for silvering, then lower the glass into warm distilled water and do not remove it from it until everything is ready. By the way, such an operation is useful in all cases: it is good if the glass is 8-10 degrees warmer than the silvering solution. This solution must be prepared only with rubber gloves. It is obtained by mixing two solutions, each of which is prepared separately. We will indicate the amount of substances per liter of solution, and you can estimate for yourself how much solution you need. First solution: 4 g of silver nitrate, 10 ml of 25% ammonia solution, 4 g of sodium hydroxide. The procedure for preparing the solution is not quite common. Dissolve all silver nitrate in 300 ml of water, pour 9/10 of the solution into a clean glass and add dropwise ammonia solution, stirring the liquid all the time with a glass rod. The cloudy liquid will become more and more transparent, and finally the color will disappear. Add a little silver nitrate solution - the solution will again become cloudy. Add a solution of caustic soda, then the solution will take on a light brown tint. Again, add the ammonia solution drop by drop, and the solution will brighten again, now it seems slightly bluish. Pour in the remaining silver nitrate solution and ammonia, stir well and add distilled water to a liter. If you have to store this solution, then pour it into a bottle or vial with a well-fitted stopper. Do not store the solution in an open container! Second solution: per liter of solution - 100 g of refined sugar and 10 ml of dilute (approximately 10%) sulfuric or nitric acid. Dissolve sugar in distilled water in advance, add acid, boil for a quarter of an hour and add water to the calculated volume. Mix both solutions: for one milliliter of the second solution (with sugar), take about 100 ml of the first solution (with silver nitrate). The exact ratio will have to be found by experience. If there is an excess of sugar solution, then flakes will begin to fall out during silvering; if, on the contrary, this solution is in short supply, then silvering will go too slowly. Stir the resulting mixture quickly and thoroughly; it will first turn orange-red and then turn black. This is a signal: it's time to start silvering. Don't miss the moment! Pour the mixture immediately onto the glass. It will spread over the entire surface, and the glass will become dark, but then it will quickly begin to brighten, a layer of metallic silver is formed on it, which is restored from nitrate. After 5-10 minutes, carefully drive the mixture off the glass with gauze (or even better - a piece of suede) dipped in distilled water, pour the mixture again and hold it for another quarter of an hour. Rinse the already silvered surface with distilled water. If dark spots appear on the glass, they should be wiped with a swab with a pumice mixture, then with a solution of tin (IV) chloride, pour the mixture over these places again and rinse with water. To check if enough silver has deposited on the glass, look through a mirror at a 60W lamp - it should be barely visible through the silvered glass. The silver layer is not yet firmly attached to the glass. To strengthen it, put the mirror in a vertical position for an hour or two to heat up at a temperature of 100-150 ° C. Use a drying cabinet, in extreme cases - a slightly heated oven. Once the mirror has cooled down, coat the silver film with a waterproof clear varnish from a spray bottle (the brush can damage it). After drying, apply a thick layer of opaque paint or black bituminous varnish over the varnish. Move with a brush or spray from a spray gun in only one direction: either from top to bottom or from left to right. The mirror is almost ready. It remains only to put in order its front, non-silvered side. There may be streaks of silver on it; remove them with a swab moistened with a weak solution of hydrochloric acid. If you get your hands dirty, then remove the stains with a slightly warmed hyposulfite solution and wash your hands thoroughly with warm water. How much silver went into making the mirror? And how much silver is in a real mirror? The questions seem to be simple, but they are not so easy to answer. The film of silver is so thin that even if you have a micrometer, you can't measure it... In order not to spoil a good mirror, take some fragment, remove a layer of varnish and paint with cotton wool soaked in acetone, and place a small crystal of iodine on a silver-plated surface. Already at room temperature, iodine evaporates rather quickly, its vapors spread over the glass, because they are much heavier than air. To prevent them from being dispersed by an accidental draft, cover the crystal with an inverted glass. When iodine interacts with silver, silver iodide is formed, and a transparent spot slowly spreads near the crystal: iodide in a thin layer is transparent. And at the edges of the transparent spot, the silver film does not disappear, but becomes thinner. And as a result, colored rings appear on the mirror, which are seen especially well in reflected light. Rings appear colored for the same reason soap bubbles and oil stains on water appear rainbow-colored to us. This phenomenon is called the interference of light in thin films, it is studied in the course of physics. For us, the most important thing is this: the more rings, the thicker the silver film. If there are two of them, then the film thickness is about 0,03 microns, three rings correspond to 0,06 microns, four - 0,09, five - 0,12, six - 0,15, seven - 0,21 microns. Knowing the thickness of the silver layer, it is easy to calculate the amount of silver: you just need to multiply the thickness by the area of the mirror and multiply the resulting volume again by the density of silver (10,5 g / cm3). Here is a guideline for checking the calculation: a mirror with an area of \uXNUMXb\uXNUMXbabout a square meter contains a little more than a gram of silver. Author: Olgin O.M.
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