How much cornstarch can I rinse down the drain before unpleasant things start to happen?
—Anna R., Fort Wayne, IN
It depends what you consider unpleasant.
If you watered down the cornstarch enough, you could likely pour it down the drain for years without any ill effects. If it didn’t clog the sink, the rest of the pipes could probably handle it. It might provoke some confusion among the sewage treatment people, but it wouldn’t break anything.
But it’d be terrifically boring (and thus unpleasant) to pour cornstarch down a sink all day, so I guess that’s—
… Oh. Huh.
Well, let’s thicken the cornstarch.
When you mix cornstarch and water in a bit under a 2:1 ratio, you get oobleck, a substance with some odd physical properties. This is a great kitchen science experiment everyone should try at least once.
At low speeds, oobleck behaves like a liquid. If you gently swish your fingers in a bowl of it, it feels like milk or heavy cream. However, if you make a fist and punch the surface, your hand bounces off with a thud—it’s like hitting a slab of raw meat.
If you vibrate oobleck, even stranger things happen.
It wouldn’t take much of the mixture to clog a drain. It depends on your plumbing, but you could probably stop up your sink with a small bowl of it.
Since your drain is clogged, I guess that’s the answer to your question.
Well, ok. I guess some people don’t mind clogged drains.
If you keep at it, after a few minutes, your sink will overflow, spilling goop onto your floor.
Financially, this will be hard to keep up. If you’re mixing as fast as the water will come out of the sink, this project may cost you in the neighborhood of $5,000 per day if you can’t find a good bulk distributor.
At this rate, it should build up on the floor at a rate of an inch or two per day.
If your doors are sealed well, it will continue to build up. Once it reaches knee height (a week or two after you started), you may be able to hold the rising tide at bay for a little bit—at least in one room—by getting someone to flush the toilet repeatedly. The pipes there are larger and harder to clog; the limiting factor would probably be how fast the tank filled with water.
Eventually, after a month or two, the oobleck would reach the living room window. The pressure would quickly rise, and sooner or later the window would burst.
As the deadly Boston Molasses Flood proved, even thick, viscous fluids can flow quickly when in large enough quantities. The oobleck would pour rapidly out onto the lawn, carrying you with it.
At this point, the local officials would probably notice and, at the very least, shut off your water.
But I guess if you really like cornstarch that much, nothing unpleasant has happened.