edited by HENRYA. BENT North Carolina State Univershy
bench remark/
Raleigh. NC 27695
A Burner and a Beaker Experiments for a First Day in a First Course in Chemistry Henry 4. Bent North Carolina State University, Raleigh. NC 27695
Today, using a burner and a heaker, we're going t o see what chemistry is like. We're going to play with a flame. And we are going to talk about what we see and what chemists say and imagine in order to explain the phenomenon of fire. Many animals play. But only humans start fires and talk. Use of fire and laneuaee are mankind's most distinctive traits. Their union Godkced modern chemistry. It's a good d a c e t o beein a studv of chemistrv. Ontoeenv recanitulates phylogeny."~~ get fired up about ;hemist&, study ;ire. A caution (and a note): Playing with fire is almost synonymous with danger-and civilization. The fleas come with the dog. Nothing is more dangerous in modern life-and more essential to it--than motor vehicles.. airolanes. rockets. and other fire-oowered Dower . plants. INuthing vcnrurcd, nothing gninra. If wr don't put our liic on the line, life will pass us right by. Ewlwion of liir itself h% hren n hnnnrdl>!rswnttrre: playing with murntions, most of them dnngerOUS.)
For our exoeriments we need: a n ex~eriencedchemist with flames(chemical experimentation-particularly without orofessional su~ervisionisn't safe.). a Meker hurner, gas jet, flexible tubing to connect burn& to jet, matches to light the hurner,wire gauze, tongs to hold the wiregauze, a large beaker (capacity of about a liter or more; for an experiment where the burner is completely immersed in the heaker, you will need ahout afour liter hkaker), and some way of holding or supporting the heaker upside down (say on the outer edges of three 6 o n rings each clamped to its own ring stand). T h e heaker may be hand-held. I n chemical experiments, however, it's prudent, a t first, not to use hands for tasks that can he handled mechanically. Caution: Handle the heaker gently! Glass at room temperature is strong. It is stronger than the malleable metal of the h m e r or the flexible rubber of the tuhine. It is. also. brittle. Glass cracks under sudden mechanical s h a e k s ~ ~downheavy et glassware on floors, drains, bench tops and other hard, unyielding surfaces with care. Care is a byword in chemistry. Chemistry done carelessly is nothing, except hazardous to one's hhealth-and the laboratory's budget. (Some chemical laboratories are full of cracked pots. Chemists, they say, are strange people. One has to he a hit unusual to do unusual things.) Genlng - Started
First we need tolight the hurncr.That's easy. Naturedoes most of the work fur us. She's an ideal assistant. Caution: Nature always does her thing, even when we are erpedine samethine else. We often talk about eontrolline Nature. as if mancould get Nature todo~omethingthat rsn'l natural; l~utNature is alway nat~ral.That's Nature'a nature. I t isn't cunceivablr for Naturr to he unnatural. All we can du is to give Nature a chance to 890
Journal of Chemical Education
do what comes naturally to Nature. Laboratories are special places for allowing Nature opportunities to he natural. So, open thegas jet and let Nature get started. Next, wait a moment or two for the air in the tubing and burner t o be flushed out. Then hold a burning match above the burner. A flame! Much haonens ouicklv. Let's Dause for a moment to honor has ione. i s ati ire's most intelligent reprewhat sentatives on earth. it's our soecial dutv to understand what Nature has given & the ahiiity to uncierstand. We need to articulate what we've had the ~rivileeeof witnessing. Perceptions not articulated vanishin thesentimental. me's ~ h e n o m e n aaren't understood, thought-reeistered phenomkna until they've been put into words.
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Methane's Principal Properties
The gas in the gas jet is natural gas, chiefly methane. It is under a pressure slightly greater than atmospheric pressure. When the gas jet is opened, the gas in the jet expands into the tubing previously occupied by air a t atmospheric pressure. Caution: Gases always expand from regions of high pressure to regions of lower pressure. The lowest possible pressure is a vacuum. ("Nature abhors a vacuum.") Evacuate vessels-particularly glass vessels--with caution! They may collapse-r implode-under the expansive power of the atmosphere. The silver lining to that cloud is: When we lower our diaphragms, we increase the volume and, thereby, decrease the pressure of the gas in our lungs. Fresh air rushes in. When we raise our diaphragms, we increase the pressure of the gases in our lungs. Spent air rushes out. As we live and breath, we illustrate one of Nature's universal principles: Matter flows spontaneously from regions where its escaping tendency is high to regions where its escaping tendency is lower. One of the first things we see or, rather, don't see is that methane is colorless. It doesn't absorb light in the visihle part of the spectrum. Methane does absorb light in the infrared and in the far ultraviolet. If we could see infrared or far ultraviolet light, we could see methane. It would look colored. Most of the energy from thesun, however, is concentrated in that part of the spectrum sighted people call visible. For over the course of millions of years of biological evolution, natural selection naturally selected for detectors of light (the rods and cones of our eyes) detectors that are sensitive to that part of the solar s p e & u n where most of the sun's light is. We see what is easiest t o see. Caution: Methane doesn't signal its presence visually. Comhustihle methane-air mixtures are invisible. Around sources of methane-gas heaters, for example, and coal mines-out of sight shouldn't mean out of mind.
Anda caution,too,about chemistry. Chemistryisa caution. Many of chemistry's leading images--gases, atoms, heat, for examplegenerally do not signal their presence visually. In chemistry,in mind usually means out of sight. Evidently, too, methane isn't highly toxic. We didn't don gas masks before we opened the gas jet. Methane, to put the matter chemically, isn't active biochemically. Like the chemically inert nitrogen in the air we breath, methane when inhaled isn't changed chemically in our bodies. In other ways, also, we see that methane is inactive chemically, at room temperature. It doesn't appear to react with the metals that the gas jet and Meker hurner are made of. Nor does methane react with the rubber of the eas tubine. or with the nitrogen or oxygen or water vapor in 'the air in%e room. a t room temDerature. At ioom temperature methane is without affinity for almost evervthina. I t is the first member of a series of compounds rknin; t h o u g h gasoline, light oils, heavy oils, greases, waxes, paraffin, and polyethylene, called paraffins-meaning with little affinity. Many paraffins occur in nature as fossil fuels. They hang around for a long time, for they are not oxidized at usual t&nperatures; are no1 readily biodegradable; and are not sensitive LO ordinary light. Caution: Beware of releasing to the environmentinert chemicals. Oil spills are messy, wherever they occur. They don't quickly disappear, on their own. Yet highly volatile, rapidly evaporating and chemically inert, biologically nondegradable chemicals, such as methane (and chloro-fluoroderivatives of methane, called Freons), in the lone run. For out of sight mav be even more of a oroblem. ~,~~~~ . " doesn't mean out of netion. Eventually volatile, inert chemicals diffuse into the upper atmosphere. (That behavior is a manifestat:un of another characterrstir ofgases: All gar^ mix with each other in all proportions, for gases at ordinary temperatures and pressures are mostly empty space.) Once in the upper atmosphere, methane may alter the climate by absorbing infrared radiation that is on its way out to outer space from the earth and reradiating some of that meenhouse effect). And it mav radiation bsr'. to the earth (the . " diminish thc ;votective wrone layer (via absorption of ultraviolet light and self.dirsociatiun into chemirally reactive fragments that camlyre the druumpositiun of ozonr~. ~
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Also, our experiments show that methane is less dense than air. For when we lit the burner, we didn't hold the burning match immediately ahove the top of the burner. atc chi he flame ignition again. combustible methane rises several inches to where the burning match is and ignites. Then the flame-front quickly advances down t o t h e i o p of the burner. Methane in a Beaker With the beaker we can demonstrate in another wav that methane is less dense than air. Turn off the gas to the durner momentarily, so that the flame goes out. Then turn the gas back on. Now, holding a hand around the air inlets a t the bottom of the burner, squirt methane from the unlit burner down into a n upright beaker. Next move the burner away from the beaker, a yardor two, light it, and then bring i t back to where the beaker is and direct its flame across the top of the beaker. But first, i t might be prudent to have some moist thk worst possible scenario. I t towels handy. ~ l w a jimagine s might help t o turn down the lights. O.K. Ready? Hm. That's interesting. Nothing happens! A failed experiment? No. Nature always does her thing. There may be a failure of imagination, however. There aren't failed experiments, only unimaginative responses to unexpected occurrences. Failure of the gas to burn tells us something-as the dog that didn't bark told Sherlock Holmes something. Maybe there wasn't any flammable gas in the beaker. Maybe methane doesn't stay in an upright beaker that is surrounded by air. Maybe methane is less dense than air. Maybe methane bubbles upward in air, as all gases bubhle upward in water.
That image suggests another experiment. (Our images of Nature guide our actions in chemistry.) Support the beaker upside down about a foot or so ahove the bench. hv restine it. for examnle. on three iron rines clamped r L g stan' aAanged in ring. Next squirt me&ane UD into the beaker with a n unlit burner as before. eas on full, air inlets covered by hand. After five or ten seconds, move the burner a couole of vards awav. .. lieht it. and then pass the burner's flame near'the open end of thk inverted beaker. Woof! Is that what you expected? What's our wordregistered conclusion? There was methane trapped in the inverted beaker. Chemists write for what we saw
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a compound of They say: methane gas (g),"C-H-Four", carbon and hydrogen, reacts with oxygen, "0-Two", from and water, air. Produced is carbon dioxide, "C--0-Two", "H-Two--0". Good heavens!, you say. We didn't see that! We saw a beautiful flame. Caution: Don't expect chemical descriptions of Nature to look like the thing described. Chemical symbolism is a language, not a photograph. Chemical models of Nature, verbal, symbolic, or geometric, don't render the visible. Rather, they attempt to make visible our inner visions and insights into Nature's nature. No model of Nature is perfect. The only perfect model of a flame would be the flame itself. A model. to be uneful, must he wrong in some respects. The trick i s t o see in what respects the model is right--i.e., useful. Chemical models aren't like model airplanes. Chemical models aren't scaled down versions of real things, any more than the English language is a vestpocket version of reality. Yet, like any language, the language of chemistry is about reality. I t is bouts with reality: bouts between thoughts and things, from which emerge new things--concepts, images, models. And about such things we don't ask, "Are they right or wrong?", but, rather, "Are they useful?" Caution: Don't expect chemical models to reveal their uses instantly. We didn't learn to use our mother tongue in a day. Learning tounderstand a Languagethat describes Nature takes time. But once we've Learned one natural laneuaee. we're in an .. . such as Enelish. " crrrllrnt p03ilion to learn another natuwl language,ruch asrhemistry. For rhemistry is merely a modern extrnrion of the dcsrriptive poww of ancient languages. First there were substances; then names for substances (proper and improper nouns),other parts of speech, and ordinary languages; later pure substances, names for pure substances (elements and compounds), other chemical terms, and the language of modem chemistry.
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Coming t o terms with chemical terms is like coming to terms with any language. We need to see and hear the terms used in simple situations, over and over again. Oxygen's Role It's easy to show that, as the chemical equation above suggests, our flame to survive needs oxygen. Like a living person, a flame needs fuel (its food) and oxygen-and someway to mix fuel and oxidizer with each other.
Caution: Do not mix chemicals-household cleaners in a toilet bowl.. e.e.. ...cblorinstine.. aeents .. in a swimmine...wol.. antifreezes in a radiator, insectiurdes in a sprayer, drugs in the body---unlessyou know what you are doing. For mining ir a prelude to chemistry. (Fur molecules to react chemically, they must collide physically; i.e., they must be mixed. "Chemistry" means "to mix".) Thoughtless mixing may lead to unexpected chemical transformations and unwanted side effects. Fuel enters the burner through a pinhole at the bottom. It mixes in the burner's barrel with air that enters through the burner's open side ports. The blue flame that we see iscalled a premixed flame. Fuel and oxidizer are mixed before either Volume 63 Number 10 October 1986
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one enters the flame. The flame itself doesn't need additional air. It can burn in an environment that would extinguish many flames, as we'll see. But firstWhat happens if we place a hand completely over the burner's air-entry ports? The character of the flame quickly changes. I t becomes longer and bushier; and there's some luminositv t o it. owine to the Dresence of hot carbon ~ a r t i cles. The flame is now obtaining all of itsoxygen from theair immediate surmundine it.. hv- convection and diffusion. It's called a diffusion flame. A diffusion flame depends on its environment for survival. I t can't survive in oxygen-deficient air any more than humans can survive with plastic bags over their heads. Caution: Keep large plastic hags away from young children. They may suffocate in them. Plastics are almost impermeable to air, owing to the low solubility of air in plastics-ne reason plastica are useful for packaging foods. [A general rule of gas solubility is: The more difficult it is to liquify a gas, i.e., the lower the normal boiling point of the gas, the lower tends to he the gas's solubility in anything-unless it reacts chemically with the solvent, which low-hoiling oxygen doesn't do (rapidly, at room temperature) with most plastica, which are manufactured to he used in air.] Watch what happens when we place the tip of our upright, lit burner. air ~ o r t closed. s into the inverted beaker, therebv surrounding
