Alum in an aluminum bowl - chemistry question

Factual Questions
1

In attempting to make a large alum crystal, I heated alum in an aluminum bowl in the oven to try and dissolve it. The bowl is now discoloured and the alum solution coloured a blue-green. I don’t care about the bowl, I just want to know what happened. Can I still make a crystal?

2

I’ll assume you used an aluminum bowl with the notion that this would help with some end of the PKa in the solution. Actually, all you did was ruin the possibility of large crystal formation AND corrode your bowl. Same thing happens when a car battery wears out - the ions are all the same, and jump back and forth as the mood (and conditions) suit them. In this case, Al3+, which will wind up with a saturated Al3+ solution and an aluminum bowl slowly turning into solid aluminum precipitate.

Can I be a snotty pedant or what? :wink:

Use glass and a low temperature. And distilled water.

3

I used aluminum because it was the first bowl I could grab.
Your saying that the aluminum came out of the bowl and went into the water to form a saturated Al3+ solution. The aluminum also precipitated out of solution and stuck to the surface of the bowl. That makes sense but can you explain

  1. why my bowl is not a shiny anymore
  2. why there is black material stuck to the bowl that is difficult to scrub off
  3. why my alum solution is blue green ??

Thanks

4

  1. bowl not shiny

    It’s been abraded

2)black gunk

precipitate aluminum

3)blue-green

that’s what color such a solution is, though there may well be impurities

Thank you, selkoe, for giving me a rare opportunity to strut my stuff! :slight_smile:

5

Abraded means to wear down by friction. I assume that you mean that the surface has be roughened through the addition of aluminum from the solution.

As for the black gunk, why would precipitated aluminum be black and not silver.

In addition, if you could help, if alum contains aluminum how can it be used as a food product? I thought aluminum was associated with diseases such as Alzheimers.

6

There’s no conclusive evidence to support that (scroll down to Toxicity).

7

By the way, where does one buy alum? I’ve been looking for some.

8

I’ve seen it in the spice aisle at the supermarket.

9

Precipitated aluminum is black (and a sticky mess) because it is. Light reacts differently depending on particle size, and aluminum specks can get pretty small. There’s also “nickel black”, a big catalyst in the petroleum refining industry.

ETA:solkoe, you’re right. I meant corroded, not abraded.

10

I also seem to recall that pure silver precipitated out of a nitrate solution is inky black. That got a big :confused: out of my 13-year old self.

11

Yeah, it seems that inorganic chemistry just isn’t that pretty. That’s why I loved organic, where we always got pretty and carcinogenic crystals.

Edit: okay, inorganic chem is pretty, just not many precipitates. That yellow lead precipitate is gorgeous, though, as are the colours of many (deadly?) metal-ion solutions.