[Eudoxus]

(sorry for the blurriness, it was dark and I had the focus on manual)



Take a piece of limestone (very common rock, often used for gravel) and put it in a small dish. Fill the dish with vinegar to just over the top of the rock. Leave it for a week. Come back, and you'll have pure white crystals growing on top of the rock.

This is because limestone is made up of calcium carbonate (aka calcite and/or aragonite -- CaCO₃), which dissolves in a weak acid, like vinegar (acetic acid -- CH₃COOH). Calcium carbonate in aqueous acetic acid creates calcium acetate (Ca(CH₃COO)₂). The reaction looks like this:

CaCO₃ + 2CH₃COOH -> Ca(CH₃COO)₂ + CO₂ + H₂O

As the water evaporates, the concentration of calcium acetate increases until the solution becomes supersaturated. The calcium acetate then crystallizes on whatever surfaces are available, starting with the rock and the walls of the container. Scraping the crystal that forms on the sides of the container back into the vinegar can speed up the process, by the way. It will also go faster if slightly heated.

SCIENCE!!!!!!




Now here's the question: What the heck can you do with calcium acetate?  

[vmelkon]

I made acetone with it.
I was considering using some natural minerals for the CaCO3 but I had a bag full of the powdered CaCO3 and purer.

[SirRoderick]

A nice little experiment actually. I'll set one up at home for my, sadly chemistry illiterate, family.

[Eudoxus]

Dang!




I read on wikipedia that mixing calcium acetate with an alcohol would create a sort of gel like substance. Well I put some of the calcium acetate I made in a dish with some isopropyl alcohol and left it, stirring and checking viscosity nightly. I've been busy the past few days so haven't been able to check, until tonight. All the liquid has evaporated out, and now I see these fibrous structures at the bottom of the bowl.

I had two hypotheses: first, simplest, these were just calcium acetate crystals that formed in this pattern due to rapidly becoming supersaturated as the last of the water evaporated. Perhaps the isopropyl helped that process along? I don't know.

Second hypothesis was that the isopropyl and the acetate ions had formed a polymer (which I'd read as an explanation for why Ca-acetate and alcohol supposedly form a gel). Well, if that's the case then the calcium should have formed calcium hydroxide.


What do y'all think?

[Eudoxus]

Could it be possible I've made calcium propanoate?

[vmelkon]

The "propanoate" structure is different from isopropanol (isopropyl alcohol).

You need the carboxylic acid know as propanoic acid to make calcium propanoate.

[zaphraud]

Crystals can grow throughout a mixed alcohol/water solution in a way they just wont grow in a pure water solution. The result is often very pretty.

[Eudoxus]

I found two good uses for calcium acetate.

1. Preservative in sourdough breads. Add a tsp or so to the flour when making sourdough bread, and it will keep the sour flavor of the bread intact longer. Good thing I also enjoy baking.

2. Bug trap. Ca-acetate is very hygroscopic, it's a dessicant. I left a container of Ca-acetate out while I went away for a month; when I came back there were three or four dead gnats or fruit flies in it, which had apparently landed on it, then gotten stuck and dried out. That gave me an idea, so I put a a tablespoon of sugar into the container and mixed it thoroughly with the Ca-acetate. Every week I find several dessicated bugs in it, and almost none around the apartment. Success!

3. You can use it to make acetone, if you're really into home chemistry and have the glassware for it.

Quick observation: powdered Ca-acetate with gradually recrystallize as water is absorbed, then evaporated.

[zaphraud]

Its true, vinegar is in fact the correct substance to use when catching flies. Honey, not so much.

[trtlman]

is there any other stone you can do this with, I herd as a kid that you can do this with Granite?

[zaphraud]

is there any other stone you can do this with, I herd as a kid that you can do this with Granite?

granite is a big maybe because its just a conglomerate of other stuff, so yeah, maybe.

[vmelkon]

Granite and acetic acid, I doubt it. Granite is pretty much silicates.
Try hydrofluoric acid.

[zaphraud]

He's right about the granite, the odds of finding a granite with significant carbonate are almost zero; it'd have to have emerged along a coastal region, then undergone significant erosion followed by submersion and redeposition. Not terribly likely, but not impossible to find if you live in an area where that happened (coastlines in seismically active areas where granite can also be found).

Perhaps a breccia can provide a granite-look that you like, with the necessary carbonate content?

The beauty of using a carbonate is that erosion can make it porous; porous carbonates are able to take advantage of the CO₂ generated within in order to bring more solution to the upper levels of the stone - even above the liquid surface level - by the action of the bubbles themselves. This helps ensure that the crystals grow on the stone instead of on the surface of the liquid or the edge of the container, if it sticks up a little it and has large surface area it probably also has a greater rate of evaporation.

check http://en.wikipedia.org/wiki/Carbonate_minerals for other ideas - there are many, some are almost too soluble, others are so tightly packed that the reaction rate will be much too close to zero for human patience.

[Eudoxus]

Granite by definition is a coarse-grained, silicic/felsic, intrusive igneous rock. That means it contains in some proportion: quartz (silicate), feldspar (aluminum-silicate), and mica (phyllosilicate). No carbonates, sorry, and it would be difficult to see how granite could be metamorphosed to include carbonates.

The only igneous rock I know of off-hand that is carbonate based is the rare carbonatite, which is theorized to form when a bed of carbonate rock like limestone or marble is melted by an upwelling of magma beneath it, then extruded as a volcano and resolidified on the surface. It's very rare; the only active carbonatite volcano on Earth is in Africa's rift valley.

[opsomath]

Please 'splain how you make acetone from calcium acetate. I am intrigued.