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Science
by Pat Murphy & Paul Doherty

Playing With Fire

If you've read your Greek mythology (and what reader of fantasy hasn't?), you know about Prometheus, the Titan who stole fire from the sun and gave it to us mortals. Zeus, miffed by Prometheus' presumption, had the poor Titan chained to a rock. Every day, an eagle came by and ate Prometheus' liver, and every day his liver grew back so that the eagle could snack on it again the next day. Hercules finally rescued Prometheus and slew the eagle---but only after the poor guy had suffered for helping us mortals out.

The Greeks don't say it, but we figure that about five minutes after Prometheus showed up with a flaming torch, some parent somewhere said, "Don't play with that!" Fire is such fascinating stuff. Who can sit by a campfire without poking at it, rearranging the logs, watching the flames? It's true that playing with fire can get you burned. But that doesn't mean that you can't experiment with it---just a little.

OUR LAWYERS SAY . . .

In our years of working at the Exploratorium, we have developed many activities for children. It's great fun to create science activities for kids---it gives us an excuse to blow bubbles and build structures from toothpicks and make pinhole cameras and fool around with many interesting things

But we run into trouble, every now and then. Whenever we develop activities for kids, we find activities that we can't, in good conscience, suggest that kids do. These activities usually involve sharp objects or toxic chemicals or fire---especially fire. As responsible staffers at an internationally known museum, we can't, in good conscience, encourage kids to play with matches. Although Paul does confess (usually after a few glasses of wine) that he played with fire in his youth. He played with gasoline, thermite, high voltage arcs, and other potentially dangerous materials. In fact there are several neighborhoods around the country that still remember him.

But this column is written for adults---fine, upstanding members of the science-fiction-reading community. Surely all of you can be trusted to experiment without scorching yourselves or torching your homes or setting a forest on fire

If you are under 18, please ask an adult to help you with these activities. If you are over 18, be careful. As we say at the start of every Exploratorium activity book, "These experiments were designed with safety and success in mind, but even the simplest activity or most common materials can be harmful when mishandled or misused." (Yes, our lawyers made us say that.)

Now that we have warned you about something that you already knew (fire is dangerous stuff!), we are ready to begin. We will start with a series of simple experiments with a candle flame. We will then proceed to describe how Pat has put the knowledge that she gained through these experiments to practical use by inventing a beverage called the Flaming Rum Monkey.

READ THIS BY CANDLELIGHT

Way back in 1860, noted British physicist and chemist Michael Faraday presented a series of demonstrations titled "The Chemical History of a Candle."

You'll find all the equipment you need to duplicate Faraday's demonstrations at a romantic candlelit dinner for two. (In fact, these experiments could provide an educational diversion if you ever find yourself at a romantic dinner that's not going well.) To try them, you need an ordinary wax taper, some matches, a metal teaspoon, some ice water, and a paper towel or napkin.

Light the candle and watch the flame. When you first light the candle, the flame is small, but after the candle burns for a few minutes, the flame gets taller. If the air is still, the flame's size and appearance quickly becomes stable.

As you watch the candle, take a moment to think about what exactly is burning. Take a close look at the candle flame. On the candle that we observed, the wick stood in a pool of molten wax, held neatly in a cup of solid wax. There was a very short section of unburned wick between the flame and the molten wax. The flame never traveled down the wick to burn either the pool of liquid wax or the solid wax.

The wick is burning, but that can't be all that's burning. You know that the candle shrinks over time as the flame consumes it, so the wax of the candle must be burning, too. But the wax doesn't burn all at once. The flame stays neatly on the wick, never venturing down the candle. And if you use a burning match to try to light the candle's molten or solid wax, all you get is dripping wax.

Here's an experiment that gives you a clue about what's burning. Light a match, gently blow out the candle flame, then immediately hold the burning match in the smoke that rises from the wick. If you can't see any smoke, hold the match about half an inch above the wick. Poof! Like a magic trick, the flame from the match jumps the gap and re-ignites the wick.

Something is rising from the candle wick, something flammable enough to catch fire and carry that fire back to the wick. That something is wax vapor, the gas produced when wax is heated to its boiling point, about 350 to 400 degrees Celsius. The vapor rises from the candle wick after you blow out the candle and your burning match re-ignites it.

And that's really what's burning in the candle flame. Only the wax vapor burns, not the liquid or the solid wax. That's why the flame always remains a short distance above the pool of molten wax; molten wax extinguishes the flame.

A candle flame is a kind of self-regulating system. When you light the wick, the heat of the flame melts the wax. The wick soaks up the molten wax through capillary action, just as a paper towel soaks up water. The heat of the flame vaporizes the molten wax in the wick---and the wax vapor burns, feeding the flame. More wax melts and the process continues until all the wax is gone.

If you are brave and careful, you can interrupt this self-regulating system with a pair of tweezers. This is a little tricky---you need to position your hand so the flame doesn't lick your fingers with unfortunate results. If you don't trust yourself to be brave and careful, you can just read about the results. (Usually, we insist that everyone try the experiment, but this time we'll let you off the hook.)

Take a pair of tweezers and carefully pinch the wick just below the flame. Keep the wick pinched tight as the tweezers grow warm in your hand. You'll see the candle flame shrink and go out. By pinching the wick tight, you stop the liquid wax that's rising up the wick and you cut off the flame's fuel supply.

CANDLE FLAME COLORS

If you've pinched your candle out, light it again and take another look at the flame. Look closely and you'll see that the flame isn't uniformly bright. Yellow light comes from the flame's brightest region, which starts near the tip of the wick and rises from there, tapering to a point. The yellow region encloses a darker area: a cone that extends from below the tip of the wick to halfway up the flame. Beneath this cone, at the very base of the flame, there's a region that gives off a faint blue light.

Something different is happening in each part of the flame. You can get some clues about what's happening in each region by using the experimental equipment we mentioned earlier: a shiny metal teaspoon, a glass of ice water, and a paper towel or napkin. First, hold the bowl of the spoon in the yellow part of the flame for a second.

Remove the spoon from the flame and take a look at it. You'll see black soot coating the metal wherever it came into contact with the yellow part of the flame. Wipe the spoon clean. (Careful---it's hot.) Now pass the spoon through the dark part of the flame, just above the wick. Examine the spoon again. Chances are, it'll be soot-free. If you passed through the yellow part, you may have a touch of soot.

Wipe the spoon clean again. Now hold it just above the yellow part of the flame, not touching the flame at all. When you examine the spoon, you'll see that it's free of soot. Only the yellow part of the flame makes the spoon sooty.

To understand what's going on in various parts of the flame, you also need to know a little bit about what candles are made of. To make a candle, you need something that is solid at room temperature, melts in the heat of a flame, and produces a flammable vapor. Candles can be and have been made of beeswax, tallow from animal fat, spermaceti from the sperm whale, waxes from various plants, paraffin wax derived from petroleum, and combinations of these things. Today, most candles are made of a composite of paraffin wax and stearic acid produced from animal fat.

As a science fiction reader, you probably know a little chemistry---at least enough to throw around terms like "carbon-based life forms." Though the waxy substances listed above all have different chemical compositions, they are all carbon-based, composed primarily of chemical compounds made up of carbon and hydrogen and oxygen.

When a candle burns, the chemical bonds that hold these complex molecules together are rearranged. Some bonds are broken and new ones are formed. Some of the atoms combine with oxygen from the surrounding air to make new molecules. The reaction produces heat and light because more energy was stored in the original chemical bonds than is contained in the chemical bonds at the end of the reaction. The excess energy is released as heat and light.

Rather than looking at the complex molecules that make up candle wax, let's begin by considering a simpler situation (a strategy that scientists love to use). Consider the natural gas that produces the beautiful blue flame of your gas stove. Natural gas is the simplest possible combination of carbon and hydrogen; each molecule is made up of one atom of carbon and four atoms of hydrogen. Chemists write this formula in an abbreviated form as CH4.

When natural gas burns, it reacts with oxygen from the air. Since oxygen atoms join together in pairs to make molecules of oxygen, chemists write oxygen as O2.

Each molecule of natural gas reacts with two molecules of oxygen to produce a molecule of carbon dioxide (CO2) and two molecules of water (H2O), along with heat and light. You could write this reaction as:

CH4 + 2O2 = CO2 + 2H2O + heat + light.

The substances used to make candles undergo a similar reaction when they burn. Like natural gas, these waxes burn to make carbon dioxide and water. However, the reaction of these molecules is not as straightforward as the reaction of natural gas. Along the way, the burning wax vapor produces a number of intermediate products that react with each other. The end result is the same, but it takes a little longer to get there.

In the blue part of the flame, wax vapor meets oxygen from the air and burns. This is the hottest part of the flame, generating most of the candle's heat and reaching temperatures of about 1400 degrees Celsius. Wax vapor burns to make some carbon dioxide and water, but the reaction also breaks many of the wax molecules into smaller, unstable fragments. Radiation from some of these fragments is what makes the blue light you see.

The dark cone near the wick is the coolest part of the flame, ranging from 600 to 1000 degrees Celsius. There's wax vapor in this area, but there's not enough oxygen for the vapor to burn properly. That's why this area looks dark: since the wax vapor can't burn, it doesn't produce light.

In this dark area of the flame, the wax molecules break into fragments that react with one another, producing soot particles comprised mostly of carbon and gases made mostly of hydrogen. The soot and gases rise and continue burning, producing the yellow area of the flame.

You know from your experiment that there are plenty of soot particles in the yellow part of the flame, but none above the flame and none down in the blue part of the flame and the dark cone of the flame. Soot is the product of incomplete combustion. In the yellow tip of the flame, the soot and hydrogen burn to produce carbon dioxide, water, and bright yellow light. The glowing particles of soot produce most of the candle's light.

When you stuck your spoon into the flame, the metal intercepted some of the soot before it could burn completely. You can see the same effect at work in a kerosene lamp when you make the lamp's wick too long. Too much wick and too little air make for incomplete burning and a sooty lamp chimney.

Now we've been saying that the products of combustion are carbon dioxide, water, heat, and light. Maybe you're wondering where the water is. You can find it with that fine piece of testing equipment: your spoon.

Dip your spoon in the ice water to cool it to below room temperature. Dry the spoon carefully. Then hold the bowl an inch or two above the yellow part of the flame for a just second.

Examine the bowl of the spoon and you'll find that steam has condensed on the cool metal. The steam clouds the shiny metal, as if you had held the spoon to your mouth and breathed on it. If you don't see any steam, cool the spoon and try again, leaving the spoon above the flame for less time. The spoon has to be cold to condense the water vapor, and if you leave it over the flame too long, it warms up.

The steam on the spoon is water vapor produced by the complete combustion of the wax. There's no soot on the spoon because it burned up in the yellow part of the flame, becoming carbon dioxide that leaves no trace on the spoon.

BEYOND FARADAY

In his role as a scientist (as opposed to his role as a scientific juvenile delinquent), Paul has continued to study fire. (All those experiments as a child finally paid off!) He studied methane flames by zapping them with ultraviolet laser pulses, a technique known as laser-induced fluorescence. A powerful ultraviolet laser pulse excites molecules in the flame (usually OH molecules on their way to becoming H2O). As the molecules decay they emit spectral lines of light that reveal how many of molecules there are and also how fast they are rotating. The rotation rates reveal the temperature of the flame. A fast laser pulse can measure the temperatures in the flame in a billionth of a second, giving a precise picture of combustion.

Laser pointers make great cat toys, which is why Pat and Paul both have them around the house. If you have a laser pointer, you can follow in Paul's footsteps by shining it onto the flame from a candle.

But before you do, predict what will happen. Will the laser be visible, scattering from the flame? Will the flame block or dim the laser? Now, try the experiment and find out for yourself.

The laser goes right through the yellow flame without any noticeable dimming. Paul finds it surprising that the laser beam can easily go through something that looks opaque. In the yellow flame, a few very small soot particles give out a lot of light, yet they don't interact very strongly with light from the laser. The soot particles are smaller than a wavelength of light and so don't scatter much light.

While you have the laser pointer out, shine it on the candle itself. You'll see lovely shimmering dots of laser light on the candle. That's called laser speckle. The speckles are produced by interference of the laser beam with itself as it scatters from the candle wax.

THE FLAMING RUM MONKEY

Experimenting with candles is a fine excuse for a candlelit dinner. If you drink white wine with your candlelit dinner, we recommend that you take a look at the shadow of your wine glass. The rounded sides of a full wine glass cause it to act like a lens, focusing an image of the candle flame. It's fun to play with these images, but we'll save the discussion of optics for another column. We have just enough space left to talk about the Flaming Rum Monkey---an experiment, a literary device, and a cocktail that Pat thinks will take the world by storm.

Mary Maxwell, a pseudonym under which Pat sometimes writes, is also a character in the novel that Pat is currently writing. And Mary Maxwell's favorite drink is the flaming rum monkey, a drink that didn't exist until Pat (collaborating with fellow sf writer Ellen Klages) invented it last week.

Before we provide you with the recipe, we must reiterate our earlier admonitions. Fire is dangerous stuff. Alcohol (with or without fire) can be even more dangerous. If you are not yet of drinking age, we suggest you limit your experimentation to burning alcohol under adult supervision. If you are of drinking age, please experiment with caution and do not drive after experimenting.

Now, here's the recipe. Put a teaspoon of brown sugar and a sprinkling of cloves, nutmeg, and cinnamon, and a teaspoon of coconut syrup (the kind used in pina coladas) in a warm mug. Put in a little boiling water---just enough to dissolve the sugar and let it steep for a minute. Add two ounces of dark Jamaican rum and one ounce of dark creme de cacao. Fill the mug with boiling water and stir.

Put a pinch of brown sugar in a big spoon. Fill the spoon with 151 rum. Hold the spoon over the mug filled with hot water to warm the rum in the spoon.

Now you're ready for the flames. Light the rum in the spoon. Tip the spoon into the mug. The mixture in the mug will burn with a lovely blue flame.

We suggest you experiment with these lovely blue flames just as you experimented with the candle. (Did you think we were just giving you an excuse to drink rum? Hey, we're scientists! First, we experiment. Then we drink rum.) Once again, you'll need a metal spoon, some ice water, and a towel or napkin.

Pass the spoon through the blue flames, then check it for soot. (When we tried this, the spoon came out clean.) Chill the spoon in the ice water and check for water vapor. (We found it in abundance.)

Now here's a question to see if you've been paying attention. Why do you suppose we asked you to warm the 151 rum before you lit it?

Here's a hint: what's burning when you light the rum? The vapor, of course! The 151 rum is about 75% ethyl alcohol or ethanol. The molecular formula for ethanol is C2H5OH, and its vapor burns hot and clean. The flame is blue because the combustion is quick, not producing soot as an intermediate product.

Now, if you like, you can blow out the flames and try a sip of your Rum Monkey. Hot, sweet, and touched with coconut---Pat says it tastes a bit like an alcoholic Mounds bar. (Be warned---the Rum Monkey is a potent drink. We recommend that you complete your experimentation with fire before you consume any Rum Monkeys.)

Being a scientist at heart, Paul is now wondering about the temperature distribution in the flames from a flaming rum monkey. He guesses he'll have to write a grant for "Laser-Induced Fluorescence Studies of the Combustion Byproducts of Organically Derived Ethanol." Any excuse for playing with fire!

****

Note: For more about Pat Murphy's and Paul Doherty's work, check out their web sites at: www.brazenhussies.net/murphy and www.exo.net/~pauld.

===THE END===

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