Fun with Explosive Gases (Nov, 1937)
Fun with Explosive Gases
Hydrocarbons Are a Subject for Many Spectacular Experiments in the Amateur’s Chemical Laboratory
By RAYMOND B. WAILES
WOULD you like to get gas from coal without heating the coal? To make an inflammable gas that will dissolve in certain liquids as easily as sugar does in coffee ? To produce a gas that burns with a flame you can hardly perceive? Or to create fiery bubbles of gas, jumping about like grasshoppers, from simple everyday chemicals? These are some of the curious and interesting experiments with hydrocarbon gases that any amateur chemist can easily perform.
Hydrocarbon gases are compounds of carbon and hydrogen. A large proportion of all natural gases, including methane, ethane, propane, and butane, belong to this group. Manufactured illuminating gasâ€”both coal gas and water gasâ€”contains hydrocarbon gases, together with non-hydrocarbons such as hydrogen, carbon monoxide, carbon dioxide, and nitrogen.
Everyone knows that heating coal in a closed chamber or retort produces inflammable coal gas. Less familiar is the fact that coal slowly gives off hydrocarbon gases even without being heated. Miners know this, for they often hear the gas escaping with a hissing noise from fissures in underground seams of coal. They call it fire damp and must guard against it, for it forms a dangerously explosive mixture with air. Chemically speaking, fire damp is largely methane.
You can easily collect the same gas from ordinary house-furnace coal. A few days before you want to observe the result, pound up several large double handfuls of hard or soft coal into fragments the size of peas, or finer, and place the pulverized coal in a deep bowl of water. Connect the stem of a glass funnel to the glass part of a medicine dropper with a four-inch length of rubber tubing, and place the funnel, mouth down, over the coal. Suck the water up until it fills the funnel, and close the rubber tubing with a pinch clamp or a spring-type clothespin. Then let the set-up stand. From day to day, you will observe bubbles of gas rising from the coal, particularly when you tap the bowl, and they will be trapped under the glass bell of the funnel. In several days you should have a cupful of methane gas. Then, if the water level outside the funnel is higher than within, the gas will escape from the tip of the medicine dropper when you open the pinch clamp. You can light it, and it will burn with a pale blue flame.
Gas that bubbles up through the water of marshes also is methane. You can collect it by stirring up the muddy bottom of a lily pool, or the ooze at the bottom of a marsh, and trapping the ascending bubbles under an inverted funnel. The marsh gas, too, will burn.
Methane can be manufactured in your home laboratory from sodium acetate. Place an ounce or two of the crystals on the lid of a tin can, or in a porcelain evaporating dish, and heat the material slowly. Soon it will become liquid. Continue heating for at least ten minutes more, to dehydrate or dry the sodium acetate. When it cools, if the heating has been sufficient, it will no longer be crystalline but will resemble powdery snow.
To the dehydrated sodium acetate, add about one tenth of its volume of sodium hydroxide (ordinary household lye will do) and an equal amount of unslaked lime (calcium oxide). Mix the ingredients thoroughly. Now place the mixture in a test tube, or an ignition tube closed at one end. Stopper the tube with a one-hole cork in which you have fitted the glass portion of a medicine dropper.
Heat the tube slowly, and wave the burner flame along its entire length every ten seconds or so. This will keep its whole surface at a high temperature and prevents water vapor from condensing, running back, and cracking the hot glass. Soon the chemical mixture will melt and methane gas will be given off at the end of the miniature retort system. Light it with a match, and it will burn for five minutes or more. Sodium compounds from the chemicals in the tube, borne along as a spray by the gas, tinge the flame yellow.
Methane gas does not react readily with most other chemicals. In contrast, showy experiments can be performed with acetylene, another hydrocarbon gas. This is the gas that produces the fierce oxyacetylene flame used for cutting metals, when it is burned with oxygen in a special torch. Acetylene has also found service in rural-home lighting and in bicycle and miners’ lamps.
To generate acetylene and study its properties, obtain a can of calcium carbide, commonly sold simply as “carbide.” A bicycle shop or a sporting-goods store is your most likely source of supply. Several lumps of the chemical placed in a large jar or can of water will liberate a copious quantity of acetylene. A bottleful of the gas may then be collected for experiment by guiding the stream of bubbles through a funnel into the submerged mouth of an inverted bottle filled with water, the rising gas displacing the water. Before you lift out the bottle of acetylene, cover the mouth with a sheet of glass or close it with a cork, to retain the gas until you are ready to use it.
Hold a match to the mouth of a bottle filled with acetylene. The gas will burn with a brilliant yellow flame, depositing a large amount of soot or carbon. If you let air mix with the acetylene and then apply a match, the gas mixture will explode with a plop. The explosion is harmless, provided you take care to use a bottle with a wide mouth.
WHEN acetylene burns, it turns into carbon dioxide and water. These are the oxides of carbon and hydrogen, of which two atoms apiece make up the acetylene molecule. After you have burned a bottleful of acetylene gas, place several drops of clear limewater in the bottle and shake the liquid about. A white precipitate of calcium carbonate will turn the limewater milky, showing the presence of carbon dioxide, for which this is a test. The black flakes of soot in the bottle will not interfere. They float, due to the fact that they do not become wet with the liquid.
Acetylene will combine with a number of gases, and its reaction with chlorine is a spectacular one. To demonstrate this, place about a teaspoonful of ordinary bleaching powder (calcium hypochlorite) and about a fluid ounce (one eighth of an ordinary drinking glass) of water in a wide-mouthed bottle. Then add ten or twenty cubic centimeters (half a fluid ounce, more or less) of muriatic, or hydrochloric, acid. Chlorine gas is produced, and may be recognized by its yellowish color. When the bottle has become filled with the gas, toss in a few lumps of calcium carbide. Falling into the water in the bottle, the carbide -immediately gives off acetylene gas. As each bubble of acetylene bobs to the surface of the liquid, it comes in contact with the overlying chlorine gas, and the two gases react vigorously with each other. Each contact produces a flash of light, a tiny explosion, and a little cloud of soot. With the bombardment proceeding at the rate of several explosions a second, the bottle resembles a miniature battlefield. Just as a lighted wax taper or a jet of burning hydrogen will continue to burn when it is lowered into a vessel filled with chlorine, so this fiery reaction between acetylene and chlorine provides another reminder of the curious fact that oxygen is not essential to produce a flame.
Confined under high pressure, acetylene may decompose and explode in response to heat or shock. Therefore cylinders of acetylene used for metal cutting are not filled merely by pumping in the compressed gas. Instead, they are first packed with asbestos fibers saturated in acetone, a liquid organic chemical that dissolves and thus stores up acetylene introduced under relatively low pressure.
WHAT makes this practical is the remarkable quantity of acetylene gas that acetone will absorb. You can observe the effect for yourself with the aid of an easily assembled bit of apparatus. Attach a four-inch length of rubber tubing to the top of a burette, or to one end of a glass tube two feet long and half an inch in diameter. Arrange two pinch clamps on the rubber tubing, one near each end. Stand the glass column vertically, with the rubber-tubing end upward and the lower end fitted to the stem of a funnel inverted in a bowl of water. Now suck upon the rubber tubing until the burette or glass tube is filled with water, and close the pinch clamps. Place several lumps of calcium carbide in the water under the funnel.
Acetylene gas will be instantly evolved, and the bubbles will rise in the water-filled column, growing smaller and smaller as they approach its top. The bubbles contain some water vapor that is condensing, and the acetylene itself is dissolving in water. Soon, however, the glass tube is filled with the gas. Now open the upper pinch clamp and fill the short section of rubber tubing with acetone. Close the upper pinch clamp, open the lower one, and squeeze the rubber tubing. This will force the acetone down into the acetylene-filled glass tube. As the liquid runs down the walls, it absorbs acetylene. A partial vacuum is created, and atmospheric pressure forces water up into the glass tube to take the place of the acetylene that has been dissolved. A single cubic centimeter of acetone will absorb about twenty-five cubic centimeters of acetylene gas. For comparison, you can repeat the experiment with other solvents that will dissolve acetylene, such as carbon tetrachloride and turpentine.
ETHYLENE, like methane and acetylene, offers interesting possibilities for home experiments. This gaseous hydrocarbon is used in ripening bananas artificially, and is present in illuminating gas. It can be prepared easily in an amateur laboratory.
To generate ethylene, place about thirty cubic centimeters (one fluid ounce) of grain alcohol or denatured radiator alcohol in a flask and add about twenty-five cubic centimeters of strong sulphuric acid. Heat the mixture, and the acid will dehydrate the alcohol or chemically abstract water from it, liberating ethylene gas.
If you use a flask of the distilling type, fit it with a single-hole cork carrying a thistle tube, through which the acid is added. Lead the gas that issues from the side arm, through rubber tubing, to a turned-up glass tube submerged in a pneumatic trough or gas-collecting basin filled with water. In case your flask has no side arm, fit it with a two-hole cork carrying the thistle tube and also a bent delivery tube of glass leading to the pneumatic trough. Water-filled bottles, inverted in the basin, will collect the gas and may be closed with glass plates until it is used. The gas will burn with a white, luminous flame. You can safely mix ethylene with air in a wide-mouthed bottle and explode it with a lighted match.
STRONG sulphuric acid absorbs ethylene, and you can show this in much the same fashion as you demonstrated how acetone absorbs acetyleneâ€”that is, by letting the action create a partial vacuum and draw up water. If you have assembled the handy little gas generator described in a previous article of this series (P.S.M., May ’37, p. 68), it will prove especially suitable for this purpose. The device consists simply of a small flask with a two-hole stopper, which carries a separatory funnel and a gas delivery tube leading into an open test tube, the whole being mounted on a base for convenience.
Fill the flask with ethylene, letting the gas pass in through the delivery tube while the stopcock of the funnel is open. Then close the stopcock and arrange the delivery tube of the flask to dip in water placed in the test tube. Put some strong sulphuric acid in the separatory funnel, admit several cubic centimeters of it to the flask, and then close the stopcock again. The acid will absorb the gas, and the vacuum will draw water from the test tube into the flask. If you substitute ordinary illuminating gas for ethylene, you will also be able to observe its loss in volume, though it will be considerably less in this case, since illuminating gas contains only a small percentage of ethylene.
This may be a good point for a word of caution to overzealous home experimenters against the temptation to mix things together indiscriminately and see what will happen. Some hydrocarbonsâ€” acetylene, for exampleâ€”form highly explosive compounds with certain chemicals. Likewise it is perfectly possible to prepare dangerous substances from other easily obtained laboratory materials. Plain common sense should warn any amateur chemist to stick to experiments that he positively knows to be safe, which of course include all of those specifically recommended in these articles.