Dragonshark wrote:Phenylethylamine wrote:Try using a smaller amount of powder, using transparent well plates, and stirring more thoroughly. If you think the amount of powder in a well has decreased, keep stirring vigorously until you can tell whether the amount of undissolved powder is actually decreasing or remaining constant.
Then look back five or so minutes later, after having performed some of your other tests. The insoluble ones should be the only ones with any undissolved powder still visible in the wells at that point, so you can check that you correctly identified soluble vs. insoluble before.
The problem is that if I check back some time later, I may have lost track of which powders I'm testing, especially during the pressure of the competition. I mean, solubility is a pretty basic test, but I've found it somewhat unreliable from my (limited) experience, and even moreso during the competition when mistakes are inevitable. Also, I haven't done that much testing, but when added in a smallish amount of water, will only calcium sulfate, calcium carbonate, and cornstarch form a milky white mixture? If it is, that could be used in place of a proper solubility test.
P.S. Does anyone have a way to distinguish magnesium sulfate?
My best advice for how to keep track of which powders you're testing is to make each row of your well plate one powder (bring multiple well plates, so you can do this even if they give you a whole bunch of powders) and do them in the order that the powders are numbered/lettered on the event. That way, you only have to keep track of where you're putting the powders in the first place, and from that point on, you can just check the list on the test sheet and the rows will be in that order. Most well plates have rows of four, and rarely do you need more than four tests to identify a powder (especially since some of the tests – e.g., pH, flame test – don't require a new well).
I used to dislike using solubility as well, but seriously:
1. Use less of the powder.
2. Transparent well plates.
3. Stir thoroughly.
4. Put your powders in order, one to a row, so you can look back later and know what's what (which is just a good idea in general, makes it easier to find any mistakes you made).
It works. I recommend trying it before the competition, at least with a small number of powders (some you know to be soluble, and some you know to be insoluble), to see how it looks.
Also, I think that in the chaos of competition, your estimation of what constitutes the "milky white mixture" that you saw beforehand is not likely to be any more accurate than your estimation of whether a powder has dissolved or not.
