Monday, January 04, 2016

The Physics of Bleach

In a recent viral video from YouTube user Crazy Russian Hacker, the eponymous hacker Taras Kul makes the bewildering decision to top off a glass of coca-cola with an (un)healthy helping of bleach. The effect is almost instantaneous, and visually striking enough to be surprising, as you can see starting at the 28 second mark in the video below.

As the soda reacts with the bleach, its characteristic brown opacity begins to vanish, fading quickly to a sickly yellow-green before eventually turning completely clear, visually indistinguishable from water or club soda. While it's no surprise that bleach can take the color out of a beverage stain, it's still remarkable to see it happen in the glass, and it makes a great jumping-off point for a discussion of light and color.
When a photon, an electromagnetic wave, encounters a molecule, a number of things can happen. If the photon's wavelength is longer than the molecule, it will be transmitted, passing through as if that molecule wasn't even there. This is why radio waves and cell phone signals can pass through our bodies all the time without interacting and losing fidelity. 
However, if a molecule is large enough and has the proper structure, its electrons can absorb photons of certain wavelengths, like a tiny antenna. When this happens, the energy of the photon kicks the electron into an excited state, changing the path it takes as it orbits the nuclei of the atoms that make up the molecule. (While the Bohr model of the atom isn't precisely correct, it's close enough for the purposes of this explanation.) This higher-energy electron will eventually return to its ground state, and the energy it absorbed is often re-emitted as light of another wavelength, such as infrared. This is why shining visible light on a black object can cause it to heat up.
When a molecule appears as a certain color, that usually means it's reflecting photons in a certain range of energies, and absorbing or transmitting everything else. Chlorophyll, for example, is the pigment that gives plants their green color. Chlorophyll absorbs red and blue light, but green light "bounces off", because it's too short in wavelength to be transmitted, but not of the right energy to excite an electron into a higher-energy state. 
Lycopene is another molecule which reflects photons of a specific energy; it's responsible for the red color of tomatoes! When you get a ketchup stain on your shirt, it's because there are molecules of lycopene embedded in the fabric, reflecting red light—and this brings us back to bleach.
Bleach contains highly reactive molecules, like the loosely-bound sodium hypochlorite that functions as the active ingredient in standard chlorine bleaches like Clorox. When it's poured onto a stain, (or into a glass of coke) the chlorine-oxygen group sticks to the pigment molecules, bending them out of shape and changing which wavelengths excite their electrons. Like a bent radio antenna, those molecules can no longer absorb or reflect the electromagnetic energy they were previously sensitive to, and the photons pass on through them. When we bleach a ketchup stain, it may look like it comes out of the fabric, but the reality is that the lycopene molecules are still there—they've just been rendered invisible by all the extra atoms that have been stuck onto them!
In the video, the bleach Taras adds to the soda sticks to the molecules of "caramel color", (a series of carbon rings haphazardly melted together in the caramelization process) disrupting their absorptive properties and leaving behind a crystal-clear glass of liquid. He jokingly says "Don't drink this!", an admonition so obvious it's comical, but perhaps it's worth thinking twice before laughing.
When chlorine molecules react randomly with organic compounds like caramel color, they can create dioxin-like chemicals, which are highly carcinogenic to humans and unusually resistant to degradation. As we just learned, those molecules aren't "washed out"; they stay behind in whatever's been bleached. Consequently, bleaching things like flour could lead to the creation of similar dioxin-like compounds that consumers would then be at risk of ingesting. The fact that the EU has banned the use of chlorine-based bleaches for flour makes the practice all the more worrisome, but ultimately it's probably not worth fretting over—things like farmed meat end up being the main source of exposure for most people.  However, given that those compounds stick around in the body, building up over long periods of time, it might be worth buying unbleached flour next time it's on your grocery list. Cigarette smoke has also been shown to contain dioxin-like compounds and, given what we know about bleaching, the whitened paper that wraps the tobacco seems to be a likely source (just in case you needed another reason not to smoke!) 
From the kitchen to the classroom to the laundry room, physics is everywhere, and this is just one more example of how it can inform our decision-making.

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