Friday, October 30, 2009

Halloween party physics

"Sure looks like a lot of fun in there, doing all those physics experiments." / CC BY-NC-SA 2.0

If there's one holiday that seems tailor made for the physics enthusiast (besides Pi Day), it's Halloween. You can trick out your home or Halloween party with spooky effects and decorations, courtesy of science and a few readily-available ingredients.


The light coming from these bulbs isn't black at all, but ultraviolet. We can't see ultraviolet light; instead we see a violet glow (ultraviolet light's visible neighbor on the spectrum) from the bulb, and a white glow from teeth and white shirts and socks. That's thanks to phosphor, an element that glows in the visible spectrum when excited by higher-frequency wavelengths (confusingly, this phenomenon is called fluorescence.) Laundry detergents contain phosphor to make white clothes seem brighter in sunlight, and phosphor is second only to calcium as the most abundant mineral in the body and is found in our bones and teeth. Phosphor is also responsible for the fluorescent colors of highlighters. Buy some blacklight bulbs, hang a huge sheet of butcher paper along one wall, and play fluorescent pictionary with highlighters.

Dry-ice burn and cauldron bubble

Here's a recipe for a bubbling cauldron that requires neither eye of newt nor toe of frog, nor wool of bat nor tongue of dog. The main ingredient is dry ice—frozen carbon dioxide. Start with a juice-based punch, and the dry ice will add both carbonation and spooky smoke to your jungle juice as it sublimates.

Concoct your punch as desired at room temperature—this will make the sublimation more dramatic. Add large chunks of food-grade dry ice once your guests arrive for a spectacular smoky effect. (This site recommends 3-5 pounds for a big bowl.) For a floating hand, freeze a latex glove full of tonic water and add it to the mix.

Is using dry ice dangerous? As with anything fun, it requires a bit of caution, but in a word? No. Dry-ice is much colder than regular ice; at standard pressure, carbon dioxide freezes at about -110 Fahrenheit. That's freaking cold, and is likely to burn you if it touches your skin, so wear thick rubber gloves. For that reason, and one other, you should not serve your guests any solid ice when you're ladling out the punch. Even if it's water ice that's formed as a result of cooling, it could enclose a nugget of dry ice which would rapidly expand to enormous volumes once ingested, via PV=nRT. (Dry ice in a closed container also becomes a terrible idea in about two seconds.) So don't serve up any solids. Finally, using food-grade dry ice will guarantee it's free from impurities—your punch won't get any added "flavors" besides the same carbonation found in soda. (Read this "Ask a Scientist" column from Argonne National Lab for more about dry ice safety, and here are a few additional tips on achieving smoky effects at home.

These unsuspecting peeps will soon be at the mercy of your mad physicist whims. / CC BY 2.0

No peeps were harmed in this experiment

This little experiment really brings out the kid—and the evil scientist— in me. You'll need a bell jar and a hand vacuum pump—think of it as investment in endless amusement.

Place an unsuspecting Halloween peep in the bell jar, ask an assistant to hold the lid tightly to the bottom of the bell jar, and start pumping out air. As the air pressure drops in the jar, the air bubbles in the peep expand, bloating the peep to (relatively) monstrous sizes.

At this point you can challenge your brawniest friend to pry the lid off; the air pressure on the outside of the bell jar will likely foil your friend's muscle. Then unscrew a small valve in the pump's tube to let air back into the chamber suddenly—it will crush the peep. Luckily, it will still taste just as terribly, terribly good.

Other physics recipes for Halloween making a statue whose eyes seem to follow the viewer , magic two-way mirrors, and slime.

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