Thrilling Stunts with a Glass-Eating Chemical (Jan, 1938)
UPDATE: As reader carmarks points out in the comments below, these experiments can be extremely dangerous and you should not actually try to perform any of them. Hydrofluoric Acid can kill you so, be warned.
Thrilling Stunts with a Glass-Eating Chemical
Etching your laboratory glassware is only one of the many possibilities offered by compounds of the active element fluorine
By RAYMOND B. WAILES
NOT long ago, a noted chemist told of a solvent powerful enough to dissolve nearly every known material. If the water on the earth were replaced with a liquid called selenium oxychloride, he said, we should have to carry umbrellas made of glass, platinum, or tungsten whenever it rained, for those are about the only substances that the fluid does not attack. There is a more familiar chemical, however, so corrosive that it could even eat its way through a glass umbrella. Its name is hydrofluoric acid, and it is one of the interesting compounds of the highly active element fluorine with which you will enjoy experimenting in your home laboratory.
Probably you already have at least a speaking acquaintance with the chemical family to which fluorine belongsâ€”the “halogen group” including chlorine, bromine, and iodine. Pure fluorine, like chlorine, is a greenish-yellow gas. You will usually encounter it, however, in the company of other elements with which it forms salts called fluorides, just as chlorine commonly is found in the form of salts called chlorides. The crystalline mineral variously called fluor spar and fluo-rite consists of calcium fluoride, a compound of calcium and fluorine. From natural or artificially prepared fluorides you can make the glass-eating chemical, hydrofluoric acid. This substance in turn will yield a strange gas, silicon tetrafluoride, which has properties that you will find worth while investigating.
Pioneers of chemical research kept their crude apparatuses busy distilling all manner of concoctions to see what they would obtain. When they tried a mixture of fluor spar and strong sulphuric acid, distillation produced a strange substance that ate away the glass of the vessels. The great French physicist and chemist, Andre Ampere, rightly suggested that the substance was a combination of hydrogen and a new chemical element, which received the name of fluorine. Subsequent investigators used distilling apparatus made of metal, usually lead or platinum, which the distilled productâ€”hydrofluoric acidâ€”does not so readily attack.
You can easily duplicate their experiments with a homemade retort and condenser, fashioned from a piece of lead pipe about an inch, or a little more, in inside diameter. Crimp one end to close it securely and place in this end about half an ounce of powdered fluor spar or manufactured calcium fluoride, together with a fluid ounce of strong sulphuric acid. Bend the pipe carefully to form an inverted U. Melt some candle wax and coat the inside of a small bottle with it. Fill the bottle with water and pack it in ice and salt, to keep the water cold. Then immerse the open end of the lead pipe, to a depth of not more than half an inch, in the water in the bottle. Heat the crimped end containing the chemicals with an alcohol lamp, or a low Bunsen-burner flame, for about an hour. Hydrofluoric acid will be produced in the form of a vapor, and will dissolve in the water in the bottle, yielding a solution of the acid. The wax coating of the bottle prevents the glass from being attacked by the solution, which always has to be kept in containers lined with paraffin, rubber, or lead.
To watch the acid actually dissolving glass, first hold a piece of glass tubing in the blue flame of your Bunsen burner and draw it out into a filament. If you break off a piece of this glass thread and hold one end of it in the solution of hydrofluoric acid, you will see the glass disappear before your eyes.
TAKING advantage of the way hydrofluoric acid attacks glass, you can etch lettering, markings, and fancy designs on clear-glass tumblers, vases, microscope slides, and various pieces of chemical glassware. First coat the glass with molten paraffin wax, obtained from a candle, and let the wax cool. Then, with a needle or a keen-pointed tool such as engravers use, cut away some of the wax to expose parts of the glass surface in the pattern you wish. This forms a sort of stencil; the cutaway portions will allow the acid to etch the glass, while the parts on which wax remains will be unaffected. After the article has been exposed to the vapor of hydrofluoric acid or to a liquid solution of itâ€”which may be produced locally on the surface of the glass by one of several methodsâ€”all the wax may be removed and you will find the design permanently imprinted.
To produce a frosted (rough-grained) initial or ornamental design on a plain beverage glass, for example, gently heat a little fluor spar or calcium fluoride with some strong sulphuric acid in a lead dish or tray. The glass, with the letter or pattern cut in its wax coating as just described, is supported just above the dishâ€”so that the vapor of hydrofluoric acid circulates around itâ€”within a can or cardboard carton open at top and bottom. Examine the glass from time to time to determine the progress of the etching, which will require from ten to thirty minutes.
A microscope slide may be titled, when the letters have been cut in the wax coating, by placing upon them a pinch of powdered fluor spar or calcium fluoride. Touch this with several drops of strong sulphuric acid, and let the slide stand about ten minutes before cleaning off the chemicals and the wax. This chemical mixture, which gives smooth-etched lines, may also be used to put volume and capacity markings on your test tubes and beakers. The sharpest and clearest lines are obtained by heating the wax on the glass and letting it cool before you mark it.
Other chemicals that you can use with sulphuric acid for etching glass by direct contact include- sodium fluoride, potassium fluoride, ammonium fluoride, and ammonium bifluoride. An alternative preparation consists of a mixture of barium sulphate, sodium fluoride, and strong hydrochloric acid, made into a paste which should be left on the glass for an hour or so.
Instead of securing the fluorides for your experiments from a chemical supply house, you may, if you wish, obtain fluor spar or fluorite from a dealer in minerals. There are purple, green, and colorless varieties of the crystals, the colorless ones usually appearing blue when light shines through them. You can purchase a remarkably good single crystal for twenty or thirty cents and use it in a beautiful and showy experiment, for it will glow in the dark when gentle heat is applied to it.
To observe this striking phenomenon, known as “thermolu-minescence,” the experiment is best performed in a totally darkened room. Place the crystal on an improvised sand bath consisting of a half inch of sand in a tin-can lid, and heat it gently from below. After each minute or so, turn off the heat and observe the crystal as it slowly warms up. At a certain temperature, it will begin to shine of its own accord. The effect can be seen most plainly with the gas flame extinguished, or with an electric heater that does not glow visibly. A purple or blue crystal works best for the experiment, which may be performed repeatedly without damage to the crystal if you do not heat it so fast as to cause internal strains that would shatter it.
FLUORIDE in a homelier form may be more readily accessible, for roach poison usually contains this chemical. If it does, you can mix the insecticide powder with strong sulphuric acid and perform experiments like etching glassware with it. Even without reading the label of the carton, you can readily detect the presence of a fluoride in the preparation.
To do this, place some of the powder in a test tube with a pinch or so of sand. Add some strong sulphuric acid. Heat the test tube, doing so gently, since the preparation may contain ingredients that would otherwise cause frothing and spattering. A drop of colorless ammonium molybdate solution, suspended meanwhile at the mouth of the tube on a rod of glass or hard rubber, will turn yellow if the compound contains a fluoride. An even simpler test, however, can be made merely by holding a drop of plain water on the rod. When a fluoride is present, the drop of water will turn milky.
This results from an interesting chemical reaction. First, the fluoride in the insect powder reacts with the sulphuric acid, and hydrofluoric acid is formed. The hydrofluoric acid thereupon interacts with the sand, which is an oxide of the element silicon. Fluorine from the acid and silicon from the sand combine to form the rather remarkable gas called silicon tetrafluo-ride. The same gas, incidentally, is formed when hydrofluoric acid attacks glass, which is chemically a mixture of silicates of calcium and sodium.
Can you stretch your imagination for a moment and visualize, if you are able, such a thing as “sand gas”? Chemically speaking, silicon tetrafluoride might answer to such a name, for when the colorless gas comes in contact with water it promptly forms silicic acid, a compound closely akin to sand. This white substance, silicic acid, is what turns the drop of water milky in the test for a fluoride.
YOU can manufacture “sand gas” or silicon tetrafluoride on a larger scale and explore its odd properties with a little simple apparatus. To generate the gas, place in a chemical flask some fluor spar, calcium fluoride, or sodium fluoride. Add about the same quantity of ordinary river or building sand, or the purer white bird-cage sand or gravel. Powdered glass may be substituted for the sand if you prefer. When all is ready, pour in about a fluid ounce of strong sulphuric acid and heat the flask gently with a Bunsen burner, an alcohol lamp, or a small electric heater. The hydrofluoric acid that is liberated will react with the sand or glass to form silicon tetrafluoride gas. You need not fear that the acid will ruin your glassware or damage it in any way. To make sure that all of it is transformed into silicon tetrafluoride, a wad of glass wool or glass fiber, of the kind sold for ornamenting Christmas trees, may be placed in the flask above the reacting mixture.
FROM a one-hole stopper in the generating flask, lead the silicon tetrafluoride gas through bent glass tubing into a second flask or bottle. This empty vessel serves as a trap to catch any water, froth, or foam that might be carried along with the gas. An ordinary flask or bottle with a two-hole stopper, or a flask with a side outlet and a one-hole stopper, may be used. Finally, the gas is led from the outlet of this vessel to a third one, a wide-mouth bottle through which the silicon tetrafluoride circulates and then escapes into the air.
As the silicon tetrachloride meets the outside atmosphere, it will be seen to fume. The white cloud that forms around the outlet of the wide-mouth bottle is actually a miniature sand storm, consisting of particles of silicic acid. Floating lazily in the air as they do, these particles must be almost inconceivably small. They are produced because the air contains moisture or water vapor, and the silicon tetrafluoride gas turns to the sandlike white material when it comes in contact with water.
In case you have failed to put together your apparatus carefully enough, and it is not gas-tight throughout its length, the silicon tetrafluoride gas will turn detective and locate the leaks. Wherever the gas escapes into the air, during an experiment, you will see a telltale white sandstorm; and if tubing is not well fitted to bored corks, the leaks will became encrusted with the white deposit. Here is a picturesque test, then, by which you can check your skill at handling the tools of chemistry.
YOU can perform a curious and striking experiment with silicon tetrafluoride gas by gluing a small glass vial to a square of cardboard, so that it will-stand upright when set down. Attach a little wire bail or handle to the vial, so it can be handled with a hook bent from a fragment of wire, and fill it with water. Then lower the vial into the wide-mouthed bottle of your gas apparatus, and let the bottle become filled with silicon tetrafluoride gas. In a few seconds, you will see the surface of the water becoming coated with a frosty layer of white silicic acid. After a short time, remove the vial with a wire hook. Try to pour out the contents. You will find the vial so tightly corked with the plug of silicic acid that the water will not spill out, even when you hold the vial upside down!
If you let silicon tetrafluoride gas bubble from glass tubing immersed in water, the end of the tubing, likewise, soon becomes tightly sealed by the precipitated silicic acid. This may be avoided by discharging the gas into a little pool of mercury, at the bottom of the dish, so that the bubbles of gas will rise through the mercury before they come in contact with the water above it. Then you can collect the silicic acid, which is quite pure, after the gas has been passed through the water for some time.
OPEN the mouth of an inverted, wide-mouthed bottle of silicon tetrafluoride gas under water, and at once a partial vacuum will be created within the bottle. The speed with which water rises shows how readily the gas dissolves in and interacts with it.
The same reaction you have been using to prepare silicon tetrafluoride gas makes fluoride compounds valuable in metallurgical operations. To free ores of undesired silica, or sandlike compounds of silicon and oxygen, they may be “fluxed” or fused with calcium fluoride. The silicon combines with the fluorine to form silicon tetrafluoride, which escapes as a gas, while the calcium and oxygen form lime. From the pasty slag or residue of lime and other impurities the metal is easily separated.