Crime Busters B

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SilverBreeze
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Re: Crime Busters B

Post by SilverBreeze » February 27th, 2020, 4:35 pm

dholdgreve wrote:
February 27th, 2020, 9:01 am
jllooi01 wrote:
February 26th, 2020, 5:29 am
SilverBreeze wrote:
February 22nd, 2020, 10:59 pm

Could it be sand(if pH were messed up) or chalk? Without a description of the powder, it's hard to say. Check your pH paper?

Sodium acetate is very soluble, so doesn't seem like the first choice, and pH is usually closer to 8.

EDIT: Also remember to wait before deciding a powder is insoluble, as it can take time.
I don't think it was sand, it was white with fairly consistent particles. Also, it was the first powder I tested, and I let it sit the entire time. It did not dissolve.
So... Just curious... If it wasn't soluble, how did the pH go from neutral to that alkaline? Something is amiss I think!
The pH change in itself is not odd. Sometimes competitors will use paper that turns darker green at pH 7 by design and forget to read the chart, or the "distilled" water will be a bit off, but it is definitely possible for a powder to change pH without fully dissolving. Every powder dissolves slightly in water, even "insoluble" ones like chalk. Most of it stays as a precipitate once at equilibrium, so we treat it as though none of it had dissolved. Sometimes, such as for an ID tool, this works. However, some of the ions that do dissolve(specifically the anions) will take up H+ ions because the negative and positive attract, forming different ions. The removed H+ ions cause lower acidity and thus increase pH. Some ions are a very strong base, so even if only a little dissolves, it still has a non-negligible effect. Yes, the pH of chalk in water would be much higher if it fully dissolved, but it still rises nonetheless.
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Re: Crime Busters B

Post by dholdgreve » February 28th, 2020, 6:06 am

From 7 to 10?
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Re: Crime Busters B

Post by Locoholic » February 28th, 2020, 7:52 pm

SilverBreeze wrote:
February 27th, 2020, 4:35 pm

The pH change in itself is not odd. Sometimes competitors will use paper that turns darker green at pH 7 by design and forget to read the chart, or the "distilled" water will be a bit off, but it is definitely possible for a powder to change pH without fully dissolving. Every powder dissolves slightly in water, even "insoluble" ones like chalk. Most of it stays as a precipitate once at equilibrium, so we treat it as though none of it had dissolved. Sometimes, such as for an ID tool, this works. However, some of the ions that do dissolve(specifically the anions) will take up H+ ions because the negative and positive attract, forming different ions. The removed H+ ions cause lower acidity and thus increase pH. Some ions are a very strong base, so even if only a little dissolves, it still has a non-negligible effect. Yes, the pH of chalk in water would be much higher if it fully dissolved, but it still rises nonetheless.
I didn't know that the pH change is that drastic when something dissolves...good to know.
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Re: Crime Busters B

Post by dholdgreve » June 25th, 2020, 11:04 am

So I'm curious... If we need to go to a satellite type competition (based on latest posing at SOinc.org), how exactly would you be able to ID the chemicals without any liquids... and why would they not be allowed liquids?
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Re: Crime Busters B

Post by CPScienceDude » June 25th, 2020, 11:08 am

dholdgreve wrote:
June 25th, 2020, 11:04 am
So I'm curious... If we need to go to a satellite type competition (based on latest posing at SOinc.org), how exactly would you be able to ID the chemicals without any liquids... and why would they not be allowed liquids?
The test results would be provided to you. So if the unknown was cornstarch, you might be provided with “Turns black in I2, is a very fine powder, hardens in water”. Not sure about the liquids thing though. Probably a liability thing.
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