Can't Judge A Powder B

[robotman]

This is a 9v conductivivty tester that appears to be purchasable without needing a school.

[chalker]

Hmmm,
Went to my local radio shack last weekend for that exact purpose... They had no type of probes, no 9-volt battery caps, a whole drawer full of resistors, a whole drawer full of LEDs, and absolutely no help or expertise in pulling the thing together... Years ago, we used to be able to buy a totally self contained unit where the 9 volt battery just snapped onto the back end, but they no longer make them. There are several types available on ebay, but none that are powered by 9-volt batteries... most are powered by 2 of the 3 volt discs... Check clarifications... Those are not allowed, even though they are actually less voltage than what is allowed. Makes no sense to me, but those are the rules we must live with. In any case, after a 3-hour search, I finally found what I was looking for at Flynn Scientific, but they will only ship directly to schools, so I hope that's not a problem for you. Not cheap... they were over $20 each.

Did you try a google search? A few seconds of searching found many online directions of how to make simple conductivity testers with simple parts, such as:
http://www.ehow.com/how_5860874_create- ... meter.html
http://www.instructables.com/id/Conductivity-Meter/
http://www.youtube.com/watch?v=AVzMDGk8_Lk
http://www.juliantrubin.com/encyclopedi ... meter.html

[ya5min88]

So once I wrote "soluble" on an observation sheet, and I got points off, so you should only write observations, not inferences. Please correct me if I'm wrong... I have a tournament soon.

[robotman]

So once I wrote "soluble" on an observation sheet, and I got points off, so you should only write observations, not inferences. Please correct me if I'm wrong... I have a tournament soon.

Correct, the only thing you should write on your observation sheets are things that you see happen, not what you assume happens.

So for solubility, you should write something along the line of "The powder dissolved when placed in water" or "The powder disappeared when placed in water", then when ased if the compound is soluble, the response would be Yes, based on observation #X.

[ya5min88]

So once I wrote "soluble" on an observation sheet, and I got points off, so you should only write observations, not inferences. Please correct me if I'm wrong... I have a tournament soon.

Correct, the only thing you should write on your observation sheets are things that you see happen, not what you assume happens.

So for solubility, you should write something along the line of "The powder dissolved when placed in water" or "The powder disappeared when placed in water", then when ased if the compound is soluble, the response would be Yes, based on observation #X.

Thank you! I just wanted to clarify.

[hippolover]

We are having an issue with the conductivity testing. Distilled water is a conductor, so when the powders are dissolved in distilled water, they will all be conductors, right? Even sugar will conduct in distilled water. Are we doing something wrong? Does anyone have any advice on this?

[robotman]

Water that is truly distilled water should not be conductive, and should not cause a conductivity tester to light up. So when a powder is added to truly distilled water and the solution then lights the conductivity tester you know the powder is conductive.


However if the water given to you does cause the conductivity tester to flash or light up that should be an observation you write down, then if asked if a powder is conductive your response would be "It is impossible to determine due to [OBSERVATION # that says Distilled water caused conductivity tester to flash]"

[computergeek3]

Water that is truly distilled water should not be conductive, and should not cause a conductivity tester to light up. So when a powder is added to truly distilled water and the solution then lights the conductivity tester you know the powder is conductive.


However if the water given to you does cause the conductivity tester to flash or light up that should be an observation you write down, then if asked if a powder is conductive your response would be "It is impossible to determine due to [OBSERVATION # that says Distilled water caused conductivity tester to flash]"

To build on this, not all powders will be conductors in solution. To fully understand this requires an understanding of high school level chemistry...If you want, I can PM you [hippolover] an explanation and see if it helps at all.

[hippolover]

Thank you robotman. I purchased a bottle of distilled water at the grocery store...it's funny that it is conductive. Where would you suggest I get it (we don't have the equipment to distill water at our school)?

[chalker]

Thank you robotman. I purchased a bottle of distilled water at the grocery store...it's funny that it is conductive. Where would you suggest I get it (we don't have the equipment to distill water at our school)?

I just posted this in the Mission Possible thread, but it might help you here:

Yep.. even pure water (which distilled water from the store is NOT) can conduct some electricity. See http://en.wikipedia.org/wiki/DI_water

"Electrical conductivity of ultra-pure water is 5.5 × 10−6 S·m−1 (18 MΩ cm in the reciprocal terms of Electrical Resistivity) and is due only to H+ and OH- ions produced in the water dissociation equilibrium.[8][9] This low conductivity is only achieved, however, in the presence of dissolved monatomic gases. Completely de-gassed ultra-pure water has conductivity of 1.2 × 10−4 S·m−1, whereas upon equilibration to the atmosphere it is 7.5 × 10−5 S·m−1 due to dissolved CO2 in it.[8] The highest grades of ultrapure water should not be stored in glass or plastic containers because these container materials leach (release) contaminants at very low concentrations. Storage vessels made of silica are used for less demanding applications and vessels of ultrapure tin are used for the highest purity applications. It is worth noting that although electrical conductivity only indicates the presence of ions, the majority of common contaminants found naturally in water ionize to some degree. This ionization is a good measure of the efficacy of a filtration system, and more expensive systems incorporate conductivity-based alarms to indicate when filters should be refreshed or replaced. For comparison, .[10] sea water has a conductivity of perhaps 5S/m (53 mS/cm is quoted), while normal un-purified tap water may have conductivity of 5mS/m(50uS/cm) (to within an order of magnitude), which is still some 2 or 3 orders of magnitude higher than the output from a well functioning demineralizing or distillation mechanism, so low levels of contamination or declining performance are easily detected."