Small-Scale Experiments for School Classes ALLAN ADAIR Kelly College, Tavistock, England
T
HE following account will give some idea of the methods which we have been developing in an English Public School in order to effect economy in chemicals and apparatus. Although small-scale experiments are not new, yet their possibilities in school work have not been widely examined beyond precipitation reactions in watch glasses and on microscope slides. Since British Public Schools cater to boys of ages from 14 to about 18, i t seemed desirable to introduce any novel methods into the lower age groups. Older students tend to be more conservative and are used to handling 250-ml. flasks and beakers; moreover much of the apparatns had to be home-made in the beginning glass has been used exand a stock bnilt up. *ex clusively, although oxygen is required for working it. Corks are not used. All glass straights and bends are slightly tapered a t the ends so that a piece of rubber tubing may be slipped along the taper until it is stretched sufficientlyto act as a stopper for the apparatus to which the tube is to be connected. If still too narrow, a piece of wider tube is put over the top of it. The diagrams show the pieces of apparatus with which each student is provided. Bends are all made from 5 to 6-mm. tubing. The flasks are blown from heavy-walled 1-cm. tubing. Microscope slides and stirring rods are provided, and also 15-ml. conical flasks. The evaporator, K, is made by collapsing the bottom of a round 12-1111. flask, using gentle suction and uniform rotation in the blowpipe flame. The small aperture is about 1mm. in diameter and is blown out while heating with a pin point flame. Liquids to be evaporated are placed in the cavity and the whole is mounted in a cork inserted in a 200-ml. conical flask which acts as a steam boiler. The steam issues as a jet through the orifice. Small nickel boats are useful and are made of such size as to fit the 1-cm. bore combustion tube, G. The method for making them is as follows: A nail of suitable size is securely held horizontally in a vise. The nickel is bent round the nail up to the line XX where X represents a smaU cut in the metal. The end
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to the right of XX is turned up and the projecting flaps are hammered to lie along the sides of the boat. The use of this small-scale apparatus will be best illustrated by giving several examples from the many possible experiments. In most cases weights of materials used have been given as a guide; the boys do not weigh their solids, it suffices to show them a sample and to let them estimate their own quantities. Filtration or Rmmul of Solids from Liquids. This is an art in itself and the choice of method gives a fine exercise in judgment to the young student, since i t will depend on the size and quantity of the particles or crystals. Method 1: A hand centrifuge is used, with snbsequent removal of liquid by a 1-mm. or smaller bore siphon drawn out from ordinary tubing. The use of capillary tubes and siphons is limited to some extcrlt by the particle size. Some particles may easily stick.
too wide to be pushed into the jacket, and then ground in with carborundum. Filter papers are cut with a sharp cork borer. The two tubes are held together by a rubber sleeve which also serves as a stopper for plugging it into the side arm suction tube, M. Mouth suction is used. If there is too much solid to be removed it may block a King filter unless the particles are large. Method 4: A plug of cotton is tightly packed into the mouth of the test tube containing the mixture to be filtered and firmly pushed down to the bottom with a glass rod. Some solution is inevitably left absorbed and is lost. The Reduction of Copper Oxide by Hydrogen. Two grams of granulated zinc are put in the round-bottom flask, which is then half filled with dilute hydrochloric acid (1:2). The flask is attached to the left-hand limb of the silica-gel drier. A small piece of cobalt chloride SELF-PRIMING SIPHON RRMOV-paper placed at the further end of the tube indicates PYREX BUNSENMOUNTED IN A SLOT CUTI N A WOODEN BLOCK ING LIQUIDPROM A CLOCK that the gas is dry by remaining blue. The reduction R a rubber sleeve which GLASS tube H contains 0.2 g. of copper oxide (from wire) and is attached to the right-hand limb of the drier by a bridging the block and screw& down to keep the burner in stout piece of rubber tubing. Heating is started as place soon as air has been expelled. Rapid reduction takes place. The drop of liquid condensing in the V of the Method 2: The mixture is poured into a watch reduction tube is removed by capillary and is deglass and allowed to settle. The liquid is removed by posited on cobalt chloride paper to show that it is using a self-priming siphon pipet, the bulb of which water. holds about 3 ml. It is made by drawing out a heavyHydrogen on Burning in A i r Gives Water. The walled pyrex S/&-inchtest tube which is carefully col- same apparatus is used. A fine jet is substituted for lapsed a t about the middle to a thick-walled tube of the reduction tube. A minute hydrogen flame can be about 3-mm. bore. I t is then removed from the flame lighted and if the lighted jet is thrust into a small test and steadily drawn out into a strong capilla~yof 1-mm. tube a film of moisture condenses on the walls. bore or less. The capillary is bent as shown. On inRedudions with Carbon Monoxide. These are carried troducing the tip into a liquid the latter is drawn in out using the 12-ml. flask joined to tube If by tube B. by capillarity and siphons over into the bulb. I t is The gas is made from 1.5 ml. of formic acid and 1ml. of poured out by the wide tube a t the other end of the concentrated sulfuric acid, which are gently warmed bulb. The solid is further dried by pushing it down together. Reductions may also be carried out using into a heap in the center of the watch glass, using a the combustion tube G and a nickel boat. If the piece of thickly folded filter paper. Drying is com- production of carbon dioxide is to be shown, a suitable g., over a water delivery tube is connected to H or G and the gas is pleted by any suitable method-*. bath. passed through a few drops of lime water. Heating Method 3: If there is only a very small amount of is started when the air has been driven out. Othersolid to be filtered from 1 or 2 ml. of liquid, then the wise thereis sometimes a slight explosion. King Micro filter is useful. This filter consists of a Reduction of Carbon Dioxide. by Carbon. Marble piece of heavywalled capillary tube pushed up into a chips and moderately concentrated hydrochloric acid are used in the flask. The gas is led through L into M where it is washed or dried. It passes from the side arm of M into G, which is packed with charcoal in the center. An ordinary bunsen may be required but it may bend the tube slightly. The issuing gas is led by E into lime water. Preparation of Anhydrous Ferric Chloride. The same apparatus is used as for the last experiment. Chlorine is prepared by warming 0.5 g. of manganese dioxide and 2 ml. of concentrated hydrochloric acid. The gas is dried by sulfuric acid in M. About 2 cm. length of 1-cm.pyrex tubing so as to fit it closely. of fine iron wire are enclosed in a short length of ordiIn order to obtain a good fit the opening of the capillary nary 6-mm. pyrex tubing placed in the center of G. tubing may be very slightly enlarged so that it is just On gentle warming the rapid iron wire combination
This serves best for very large particles which will not get drawn in, or for very small ones, a few of which may enter but are too small to stick.
takes place and a crystalline sublimate of ferric chloride is produced in the colder region of G. It may be dissolved out with a few drops of water and tested with drops of reagents on a white tile. Fourteen sets of this apparatus working together produce insufficient chlorine to cause harm, and very little smell. Most likely a good many more could be run a t the same time with impunity. Preparation of Copper Sulfate. To 2 ml. of dilute sulfuric acid (1:10) copper oxide is added, little by little with warming, until some is left undissolved and the liquid appears blackish because of the sediment. About 0.4 g. will be required. After settling for a minute the supernatant turbid blue liquid is poured off the residue, by one gentle tilting movement, into a clean tube. It is warmed and shaken and about three drops of the same acid are added. The liquid now becomes clear (the opacity is presumably due to a basic compound). The solution is filtered, King filter being used to remove any specks of copper oxide. It is concentrated slightly in the evaporator and allowed to cool. Preparation of Hydrogen Chloride and Hydrochloric Acid. The 12-ml. flask is attached to D which leads to the top of M which acts as a safety device in case of suck-back. The side arm connects to the short limb of E, the long - limb of which dips into 2 ml. of water in a test tube. The flask contains 1 g. of sodium chloride with a little concentrated sulfuric acid. The mixture is eentlv warmed to produce a rapid, steady stream of gas. Too much acid causes uncontrollable frothing. The solution of hydrochloric acid is very concentrated and may be tested using a drop of it with a drop of silver nitrate solution. It reacts well with zinc. Preparation of Sulfur Dioxide. The apparatus is similar to the last experiment. Copper and concentrated sulfuric acid are used. Preparation and Liquefaction of Nitrogen Dioxide. A &inch test tube is filled to a depth of 2 cm. with powdered anhydrous lead nitrate. The gas is led to the bottom of the side-arm tube M by way of L. Tube M acts as an air pre-cooler. The side ann is connected to the short limb of E. The long limb passes to the bottom of a test tube surrounded by ice and salt. If this freezing mixture is unobtainable equal weights of sodium carbonate crystals, ammonium nitrate, and water may be used. The lead nitrate is gently heated and a few drops of the liquefied gas are collected. The above experiments by no means exhaust the possibilities of small-scale work. Many precipitation reactions can be carried out in a watch glass or on white tiles. The white tile method of carrying out the "brown ring test" for a nitrate is in every way superior to the test tube method. A small crystal of ferrous sulfate is crushed in a drop of water on the tile. One drop of the solution of a nitrate is stirred into it. A single drop of concentrated sulfuric acid is placed alongside and the two drops are pushed together by a glass rod. A brown line forms where they meet.
The testing of gases for combustibility or for supporting combustion presents a problem when miniature test tubes are used. Suitable microtapers are made by running cotton thread through melted paraffin wax and cutting it into convenient lengths. Finally let it be stated that we have not entirely eliminated work with ordinary apparatus. The teacher must master the miniature technique and know the pitfalls. Quite young boys can learn to fit up the apparatus in a few minutes and we have very few breakages. A fine is imposed for damage caused by carelessness, but money thus collected is spent on materials for the class. The boys are encouraged to design new apparatus and to make suitable stands and racks for tubes.
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Many of the pieces of apparatus mentioned hold together well and can be held in the hand or by ordinary clamps. Miniature clamps are easily designed from spring paper clips and they can be mounted on wood in suitable ways. It is well worth while exploring the possibilities of small-scale reactions using no special equipment other than apparatus usually found in laboratories. Drops of solutions on colored tiles, or better on sheets of glass, are frequently as satisfactory as larger-scale reactions. Sheets of glass may be viewed against a white or black background. Gases such as sulfur dioxide mav be recognized in quite small quantities when made in 5-cm. test tubes, by holding a film of potassium chromate solution in the mouth of the tube. The film is made by dipping a wire loop, twisted on the end of a suitable piece of wire,
S m a - G E LDRYING TUBE P-glass
wool plug
S--strip of cobalt chloride paper
into the solution. The loop can be about 3 mm. in diameter. Platinum wire is suitable. We use Nichrome wire mounted in the end of a short length of glass tube with a plug of picein or sealing wax. APPARATUS
Glass Bends (5- to 6-mm. tubing) : A. 4.5 cm. 120° 4 . 5 cm. B. 5.5cm. 90" 5 . 5 cm. C . 5.0cm. 60' 5.0cm. D.. 4.0 cm. 90" 5.0 cm. 90" 4.0 cm. E. 6.0 cm. 90° 16.0 cm. ~
F. L.
7.0 cm.
60"
9.0 cm.then tapered off for 5 cm. 6 . 0 cm. 110' 5.0 cm. 90' 9.5 cm. The last half of this limb is drawn out so that it delivers small bubbles. It is used in conjunction with the side-arm tube M, which serves as a drying tube when sulfuric acid is the drying agent.
Special Afiparatus: G. Straight combustion tube. Internal bore about 1cm. Length 15 cm. The ends are slightly opened up for the easy insertion of glass bends with their rubber "stoppers."
