The Halloween Science FAQ

What is dry ice and how does it make that awesome fog?

Dry ice is the colorless, odorless, solid form of carbon dioxide, first reported in 1834 by the French chemist Charles Thilorier, who opened a container of liquid carbon dioxide needed for an experiment and observed that most of the liquid CO₂ quickly evaporated, leaving a solid form on the bottom of the canister.

The surface temperature of dry ice is −109.3 °F. As it warms up, it sublimes, or transitions from the solid to gas form with no intermediate liquid form (a process called sublimation). These two characteristics make it an excellent coolant and since 1925, when solid CO₂ was trademarked and sold as "Dry ice" by the DryIce Corporation of America, it's been used to flash freeze and refrigerate food and biological samples, make ice cream, bait mosquito traps (they're attracted to CO₂) and make fog for theater productions, Sunn O))) concerts and haunted houses.

That fog is made by quickly changing the CO₂ into its gas form. In an ice chest, dry ice sublimes at an average rate of 5-10 pounds every 24 hours. But placing dry ice in hot water accelerates sublimation considerably and turns the solid CO₂ into CO₂ gas. The cold CO₂ gas meets the surrounding air and drops its temperature enough for condensation to occur and tiny droplets of water to form in the air and, voila, you have fog. Because carbon dioxide is heavier than air, and cold air is denser than warm air, the fog stays low to the ground for that extra creepy effect.

Why do we get goosebumps?

Goose bumps, also called goose flesh or goose pimples and known to medical professionals as cutis anserina ("cutis," skin + "anser," goose = goose skin) involuntarily develop on our skin when we become cold or experience strong emotions in a reflex called horripilation or piloerection. Whether we're freezing or getting the bejesus scared out of us, our sympathetic nervous systems pick up on a fight-or-flight situation and release adrenaline, muscles at the base of our body hairs contract, pull the hair erect, and create a shallow depression on the skin surface that causes the surrounding area to protrude. A goose bump is born.

In mammals with plenty of body hair or fur (chimps, otters, mice, cats, etc.), horripilation serves two purposes. One, erect hairs trap air, create insulation and aid heat retention. Two, erect hairs make an animal appear larger and helps intimidate enemies. In humans, horripilation as a response to cold or fear provides no known benefit since we lost most of our body hair some time ago.

What's the best candy container for trick-or-treating?

First, the researchers accounted for the wide variety of candies available to the average trick-or-treater. They divided candy into three categories: ""˜premium' (fun-sized candy bars), "˜meh' (chewy boxed candies like Milk Duds), and "˜bottom of the barrel' (hard candy, gumballs, Dum Dum pops)," mixed roughly equal amounts by weight of top, middle, and bottom tier candies, and threw them into the containers by the handful, in order to give the candy a natural spatial distribution.

Each container was filled to a capacity where it could be reasonably carried without spilling and then weighed on a hanging spring scale (adjusted to account for the weight of the container).

Their results"¦

A 10-quart bucket held a total of 9.5 lbs of candy, consisting of 375 pieces. * A standard white 5-gallon plastic bucket allowed for 20 lbs of candy in 675 pieces. * A double-bagged, regular brown paper grocery bag held 25 lbs of candy, consisting of 885 pieces. The researchers found that the bag's unreliable handles were problematic once the bag was full. * A standard size pillow case, allowing enough empty room at the top so that it may be grasped and picked up with two hands, held a whopping 47.75 lbs of candy in the form of 1690 pieces.

Next, they wanted to know if it would be possible to even collect that much candy in one night of trick-or-treating. How far would one need to walk and how many houses would they have to hit?

The researchers picked two different middle-class residential areas representative of suburban America at large to use in the experiment. Campbell, California, in Silicon Valley is an older area with dense housing, and St. Peters, Missouri, a suburb of St. Charles, is more rural and contains many newer developments. The researchers used data from City-Data.com to approximate the number of houses per square mile and constructed several different trick-or-treating scenarios, varying the values for the number of candies received at each house, and the percentage of houses distributing candy. In their worst case scenario, they figure a trick-or-treater would have a 50% success rate and receive an average of 2.5 pieces of candy per house, while a decent trick-or-treating run would see a 75% success rate and 3.5 pieces of candy per house.

They researchers then used Google maps to work out what sort of mileage a candy hunter would have to clock. Assuming the first scenario, a trick-or-treater would have to visit approximately 1352 houses and cover .42 square miles in Campbell, given the housing density, to fill their pillowcase. Under the more favorable conditions of the second scenario, it would take visits to 644 houses and .2 square miles to fill a pillowcase. Looking at the their map, the researchers estimated roughly 1 linear mile of street distance per every .036 square miles, meaning one would walk about 11 miles to fill their candy bag in the worst case scenario.

In the better scenario in St. Peters, the lower density of housing necessitates that someone cover .6 square miles to fill a pillowcase. That's more walking than in the worst case scenario in Campbell—and since the researchers' housing densities are based on statistical averages and don't account for undeveloped land, a trick-or-treater would likely need to cover a lot more ground.