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ENTERTAINING EXPERIENCES AT HOME
Spoon straightener. Chemical experiments
Entertaining experiences at home / Chemistry experiments for children For this experiment, any aluminum spoon is suitable - a teaspoon or a tablespoon. It must be thoroughly washed and degreased; you know how to do this from experience with aluminum anodizing. The spoon will be the first part of the future rectifier, and the second one will serve us as an empty tin can, about the height of a spoon, in any case, not much lower. Wash the tin can with soap or washing powder, rinse and fill with a solution for anodizing aluminum: 100 ml of sulfuric acid per 20 ml of water (carefully!). The acid can be replaced by ammonium carbonate (NH4)2WITH3 (10 g) or, in extreme cases, baking soda, dissolving it in water until saturated. Water must be distilled, clean rain water is also suitable. Before lowering the spoon into the jar, figure out where the solution will reach on the spoon. At the boundary of the solution - air, aluminum will dissolve intensively, and the spoon will soon fall apart into two parts. To prevent this from happening, cover the area near the border with a layer of varnish or waterproof glue. Now hang the spoon in the jar so that it does not touch the walls; you probably can easily come up with a suspension device yourself. Place a tile or any other non-conductive stand under the jar. This time we will use not batteries or an accumulator, but alternating current from the network, and, of course, we must completely protect ourselves. For the same reason, carefully insulate all bare ends of the wires, and during the experiment do not touch either the spoon or the jar. It is best if you cover them with an inverted plywood box or plastic bucket before turning on the current.
The electrical circuit is simple: connect in series a lamp with a power of about 40-60 W, a switch, a spoon and a jar; if there is an ammeter, you can connect it. When the circuit is assembled and the reliability of the insulation is checked, turn on the current. First, as you might guess, the lamp will light up, because the solution in the jar is electrically conductive. But after about half an hour, it will begin to shine noticeably weaker, and then go out altogether. The spoon has become a straightener. It passes current in only one direction - from the jar to the spoon. It would be easy to verify this if you had an oscilloscope: at the beginning of the experiment, a sinusoid would glow on its screen, and at the end its lower branch would disappear: the so-called pulsed current flows in the circuit. An oscilloscope would help to immediately establish where the positive pole of the rectifier is and where it is negative (this is very important if you are going to do electrochemical experiments with a home-made rectifier). But you can do without instruments: the polarity of the rectifier is easy to establish using a strip of filter paper moistened with a weak solution of common salt with the addition of an indicator of phenolphthalein. Turn off the current, press the sheet to the spoon and to the jar and secure it, for example, with plastic clothespins. Turn on the current, and a few minutes later the filter paper will turn red at one of the poles. This pole is negative. During the electrolysis of water (salt is needed only to increase electrical conductivity), hydrogen is released at the negative electrode (cathode), and OH ions remain in excess. These ions are responsible for the alkaline properties, which is why the indicator paper turns red. The same test with wet indicator paper with salt and phenolphthalein can be carried out if you have reversed the poles of the accumulator or battery. Since the voltage is low here, you can simply press a strip of paper with your hands against both poles of the current source. But why did an aluminum spoon become a straightener? After switching on the current, an aluminum oxide film grows on it, as in the case of aluminum anodization. And this film is a semiconductor: it passes current in only one direction. This property is often used in technology. Using a homemade rectifier, you can set up some of the electrochemical experiments that are described in this book. But in accordance with the conditions of the experiment, turn on the rectifier through a step-down transformer. In no case should the voltage exceed 40 V. And the current that can be taken from an aluminum spoon can reach several tens of amperes. But is it necessary to take a spoon and a tin can for a straightener? Of course no. Instead of a spoon, you can take an aluminum electrode of any shape, instead of a can - an iron, lead or graphite electrode and immerse them in a glass vessel into which an electrolyte solution is poured. Moreover, this is what we advise you to do if you decide to use a homemade rectifier in practice. But if you're going to demonstrate how aluminum oxide rectifies alternating current, then the spoon and jar look much more spectacular... Author: Olgin O.M.
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