How Do Fireworks Get Their Colors?

deymos, iStock / Getty Images Plus
deymos, iStock / Getty Images Plus

Want to impress your friends while you wait for this year’s fireworks display to begin? Wow them with your knowledge of basic chemistry and let them in on the secrets to the rainbow beauty of the night sky on the Fourth of July.

Small pellets, charmingly named "stars," are the key to the fireworks's colors, EarthSky reports. The stars are filled with different combinations of metal salts that each add a bright color to the firework when it explodes.

Different chemical elements correspond with different colors: strontium carbonate for red, calcium chloride for orange, sodium nitrate for yellow, barium chloride for green, and copper chloride for blue. Purple fireworks are created much like you might create purple paint—by mixing red and blue.

According to LiveScience, when you light the fuse on the outside of the fireworks's thick tube, the flame ignites a pouch of black powder inside known as the lift charge, which causes the shell containing the stars to catapult into the air. As it rises, a time-delay fuse begins to burn within it and, by the time it reaches its maximum height, the shell bursts, causing the stars inside to color each strand of the explosion.

Paul Nicholas Worsey, fireworks expert and professor of mining and nuclear engineering at the University of Missouri at Rolla, told LiveScience that red and green are the easiest colors to create, while blue is more difficult. Worsey says gold is best if you want your firework to keep its color for a long time, maybe even until it hits the ground.

The trick behind those especially crowd-pleasing fireworks that change color after they explode is simple: The stars are simply coated in multiple metal salts. Once you see the firework’s second color, that means the stars burned through their outer layer and reached a different metal salt—kind of like licking a Gobstopper.

For another way to impress your friends this fireworks season, learn the names of these 10 fireworks effects so you can call them out as they burst.

[h/t EarthSky]

The Reason Why Stop Signs Are Red

Roman_Gorielov, iStock/Getty Images Plus
Roman_Gorielov, iStock/Getty Images Plus

Why is red the standard color for stop signs? The short answer is this: because the representatives of the First National Conference on Street and Highway Safety in 1924 decided so.

Though stop signs were still a relatively new idea in the United States back in the 1920s—Detroit erected the first one around 1915, Jalopnik reports—the “red means ‘stop’” custom dates back to 1841, when Henry Booth of the Liverpool and Manchester Railway suggested using red to indicate danger on railroads. London then adopted the color for its regular traffic lights in 1868, and the United States eventually followed suit.

The First National Conference on Street and Highway Safety, called by then-Secretary of Commerce Herbert Hoover in 1924, aimed to standardize the color coding of road signage. It established that for all “signs and signals, both luminous and nonluminous,” red should indicate “stop,” green should indicate “proceed,” and yellow should indicate “caution,” according to the report released after the conference [PDF]. It was also decided that distance and direction signs should be black and white.

That all probably sounds familiar if you’ve ever seen a street before, but implementing the mandate for red stop signs posed immediate issues. A red material that wouldn’t fade over time just didn’t exist in 1924, Gene Hawkins, a professor of civil engineering at Texas A&M University, told The New York Times in 2011. So the writers of the 1935 Manual on Uniform Traffic Control Devices chose the next best thing: yellow. The manual also specified that every sign should be octagonal, another idea from the 1920s.

California was first to figure out that porcelain enamel would resist fading and erect red stop signs across the state, a practice noticed and addressed in the 1954 revision [PDF] to the Manual on Uniform Traffic Control Devices. Now that red was more logistically feasible, the committee in charge of updating the manual decided there should be no more yellow stop signs.

We don’t know exactly why Henry Booth and other early industrialists felt that red aptly signaled “stop.” Maybe they thought it was harder to overlook than blue or green, which natural surroundings like water and foliage might easily camouflage. Maybe they felt red, like fire or blood, just went along well with danger.

There may also be a deeper reason. When, as part of a 2011 study, red-, blue-, and green-clad human experimenters offered apple slices to individual monkeys in a free-range facility, the monkeys seemed to have an aversion to taking slices left by the experimenter wearing red. Perhaps our association of danger with the color red has a psychological basis we don’t fully understand yet.

[h/t Jalopnik]

The Surprising Reasons Why Animals Play Dead

SongayeNovell, iStock / Getty Images Plus
SongayeNovell, iStock / Getty Images Plus

Fight or flight are cited as the two most common responses animals have to immediate threats. But there's a third reaction that will seem familiar to anyone who's seen a "dead" possum spring to life: In the face of danger, some animals will enter a tonic state as a last-ditch shot at survival. Assuming a vulnerable, motionless position may seem like the worst way to get out of an emergency situation intact—but "playing dead" can be a life-saving behavior.

According to World Atlas, feigning death, or thanatosis, is most often used as a strategy to avoid becoming a meal. When some animals feel threatened, their systems become overloaded with fear and they enter a coma-like state. If a predator is looking for live prey and finds an apparent—and possibly diseased—corpse instead, it may lose interest, leaving its would-be victim to live another day.

Some animals do more than flop onto the ground to turn off predators. Opossums sell their performance by releasing a foul odor during thanatosis that suggests they've been rotting for days. The southern hog-nose snake uses a similar, smelly defense mechanism while laying motionless, and has also been known to spit up blood, according to National Geographic. Some creatures change their appearance in a tonic state: The undersides of the fire-bellied frogs of Asia and Europe flush bright orange and yellow to signal to predators that they're toxic.

Thanatosis is also used as a way to get closer to prey, though such cases are rare. Livingston’s cichlid, a fish native to Lake Malawi in East Africa, sinks to the lake bed and waits for unsuspecting fish to swim by it before going in for the kill. Playing dead can also be used as a mating strategy. The courtship ritual of the nursery web spider involves a male spider offering a silk-wrapped insect to a female. Sometimes the female attempts to snatch away the present and leave the male behind. To avoid this, the male plays dead and allows himself to be dragged along with the package as the female tries to run. He revives himself when the female starts to feed.

Thanatosis is observed across the animal kingdom, from invertebrates to mammals. It has provided an evolutionary advantage to many creatures, but it isn't always a guarantee of survival. An opossum that freezes up at the sight of an incoming car, for example, is likely to become roadkill.

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