Comment by Buttons840 | Hacker News Reader

Comment by Buttons840 | Hacker News Reader

Copper sulfate creeps along the surface of the container I've noticed. I was recently growing some crystals from a kit with my daughter and the blue liquid (which I now believe to be copper sulfate) crept out of the container and started growing on surrounding surfaces which had never been wet. I'd like to learn more about why this happens.

Someone asked about this on Stack Exchange: https://chemistry.stackexchange.com/questions/84704/how-can-...

They never got an answer.

To be clear, the container wasn't disturbed or vibrated excessively. The liquid was never spilled outside the container. In my case, there were 3 plastic cups in a single tray, all connected, and the substance grew out of one cup, down into the bottom of the next cup over, and then started back up the other side of the empty cup which had never had liquid in it. This isn't a residue left over by evaporating liquid. A thin film of this substance grows on surfaces above the highest point the liquid ever reached. Clusters of the stuff started forming a couple inches from the only container that ever contained liquid.

I asked about this just today on Reddit actually: https://www.reddit.com/gallery/u85pwz

I have a master degree in chemistry and during my studies have grown quite a lot of crystals. As a lot of the other posters point out it is due to capillary effects; both formed by the new crystals and by microscopic channels and irregularities in the container.

If crystals didn't 'want' to grow we would often cool the solution down and scratch the bottom of the glass container with a metal spatula.

Not an answer, but that looks very cool!

Maybe that type of plastic has a rough surface making this happen? Glass is much smoother so maybe this is why it doesn't happen in glass.

Is capillary action not a satisfying enough answer for you?

Capillary action requires narrow channels. Single surface shouldn't work, at least intuitively. Some might argue that any tiny crystal formed creates narrow channels between the crevices, wicking out more of the liquid and speeding up outward crystal growth. But even that is something I came up with off the top of my head, and I wouldn't find it credible without a better source than my shower of thoughts.

This mechanism is mentioned in the Chemistry Stack Exchange question linked by the OP.

> Indeed, capillary effect is enough. New portions of solution get higher and higher up, and deposit crystals to build more capillaries, much like a horde of guys with ladders storming some medieval fortress.

I suspect that the capillary action is started by microscopic scratches in the plastic formed when the plastic cups are removed from the molds they were shaped over. This may explain all or some of the capillary effect.

Should be easy to verify by suspending some surfaces ~1cm away from the side of the container to see if there is any deposition occurring by vapor.

It happens in glass too, as shown in one of the pictures.

Glass can have microscopic scratches that act as capillaries. Somewhat related: I remember one of my undergrad chemistry practicals where we had a supersaturated solution in a test tube. Scratching the inside wall of the tube produced tiny glass shards that induced precipitation of the solution.

I don't know much about chemistry so the two word answer "capillary action" wasn't enough, but now that I see it I will do more research.

this sounds like a GREAT science experiment for a kid. "We observed ..." leading to setting up controlled conditions with several variables, some microscopes and other ways to see the motion of the crystals, etc. I originally would have said evaporation, other people suggest capillary action- who knows, you might even get a paper out of it.

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