Don't Cross the Streams!

I crossed this “bridge” while I was out geocaching on Saturday. It connects the Rachel Carson trail with the Northwest Branch trail just north of the Burnt Mills dam in Maryland.

There are geocaches along both sides of the stream and I didn’t want to hike back to the dam and cross . At the midpoint — where the rope is least effective at providing a restoring force and you are at an antinode of the tree it tends to bounce a bit — is a bad place to get dizzy. But since you are on a curved surface, you have to watch where you step, and there’s no way not to look at the water rushing by a few feet below, which I found to be disorienting. Turning around wasn’t much of an option. Falling in would not have been disastrous, since I was only a few feet up, there were no protruding rocks, the water was not going to sweep me away and it was warm enough that getting wet would not have been more than an inconvenience, but I’m glad I didn’t.

It wasn’t a full blown vertigo attack — I didn’t start talking like Jimmy Stewart, by golly — but enough to make me swear off any future attempts at this sort of thing unless my life is in danger.

Stay! Staaaay!

I’ve been growing salt crystals. Unlike a pet, salt crystals won’t tear up the furniture, do their business on the carpet or need to be walked in the rain. Really I was testing to see if I could do this as a time-lapse project and wanted to test how long it would take. And it has fulfilled Hofstadter’s law: it always takes longer than you expect, even when you take into account Hofstadter’s law.

I took the standard approach of heating some water and dissolving a bunch of (uniodized) salt in it, letting it cool and pouring it into a beaker. And I waited. And waited. Finally, after a few weeks:

One thing I should have anticipated is how long it takes. There was some salt left in the pot when I poured the solution into the beaker, so I thought the crystallizing would begin quickly, but it didn’t. Plus, the evaporation was slow. I knew that boiling point elevation and freezing point depression are colligative properties (they depend on the number of dissolved atoms) so I reasoned that evaporation rate should be as well. And it is — there is Raoult’s law

The vapour pressure of an ideal solution is dependent on the vapour pressure of each chemical component and the mole fraction of the component present in the solution.

The vapor pressure of salt is very low, so as its concentration rises the total vapor pressure of the solution drops, and so does the evaporation rate. Rather than evaporating fully, one would expect it to reach an equilibrium with the atmosphere which would depend on the humidity. In fact, if the salt concentration were high enough, one might expect it to dehumidify the air, which is precisely what some people do. Salt concentrations are used in dehumidifiers — you expose the solution to the air and let is “grab” some water, then heat it up (often solar, for a completely passive system) to let the excess water evaporate, and cool it again in a cycle. Or you can have a solution with some solute left in the container, and as you “grab” the water, you dissolve more of the salt, so it can continue doing its job as long as there is more salt that can dissolve.

Another unexpected event in all of this is that I was getting salt crystallizing on the surface of the water. A small “raft” would float there until it grew massive enough that it would sink (or someone poked it). I had thought the crystallization would just build on any crystal that started up, but there are lots of small cubes rather than just a few large ones. Still, the biggest cubes are perhaps 10-20x larger on a side than the original grains.

Physics is Everywhere

NYC water towers

The distance between the metal bands holding the cylindrical structure together decreases from top to bottom because the pressure the water exerts increases with depth. The top band only needs to fight against the water at the very top of the tower but the bottom bands have to hold the entire volume from bursting out.