Science Crime Busters B

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haven chuck
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Re: Science Crime Busters B

Post by haven chuck »

There was a question on there- "What does sodium chloride do to the pH of diH2O?" Does anyone know the answer for sure? I said it made it more basic on a whim, but was curious what it actually was. Also, this was the third year in a row of the same blood spatter diagram(according to the test booklets from the last 2 years). :lol:
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Re: Science Crime Busters B

Post by Phenylethylamine »

haven chuck wrote:There was a question on there- "What does sodium chloride do to the pH of diH2O?" Does anyone know the answer for sure? I said it made it more basic on a whim, but was curious what it actually was. Also, this was the third year in a row of the same blood spatter diagram(according to the test booklets from the last 2 years). :lol:
It shouldn't do anything to the pH of diH2O- NaCl is a neutral salt (in case you're wondering, this is true because both NaOH and HCl are "strong"- i.e., dissociate completely- and therefore neither "overpowers" the other, so the resulting salt is neutral).
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Re: Science Crime Busters B

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Anybody knows how exactly the Water Testing worked at Nationals? Our coach gave us a lot of information on Water Testing tests but she didn't exactly explain how we're suppose to solve the crime using the results from the tests. Also, she said that we wouldn't have to use probes but it turned out that we did.
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Re: Science Crime Busters B

Post by frogzorz »

those probes were REALLY confusing. our calculator was messed up and i think one probe (the DO) was kind of dysfunctional. i wouldn't know: i did qualitative analysis.
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Re: Science Crime Busters B

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melody2k6 wrote:Anybody knows how exactly the Water Testing worked at Nationals? Our coach gave us a lot of information on Water Testing tests but she didn't exactly explain how we're suppose to solve the crime using the results from the tests. Also, she said that we wouldn't have to use probes but it turned out that we did.
Basically, you got four probes (conductivity, DO, pH, and temperature) and a TI graphing calculator (already set up to receive and display information from any probe if you plugged it in; no preparation was needed to understand how it worked [they gave written and verbal instructions] if it was working properly, though apparently they all weren't). You then had an observation/conclusion sheet to record information. You wrote down the probe used and the numerical value received (0.5 points for each one of these pairs) and then had a line to write a conclusion based on the information (0.5 points for each of these). To get full credit (5 points), you needed 5 data entries and 5 conclusions based off those entries. Here is what we had (I don't remember the numbers)-

DO- ### (low)- There is very little dissolved oxygen in the liquid
Temperature- ### (room temp.)- The liquid is about room temperature
Conductivity- ### (high)- The liquid conducts electricity very strongly
pH- ##.# (high)- The liquid is very basic
Temperature- ### (same as before)- the liquid does not change temperature over time
*I'm not sure if the last one was what they wanted, but there were only 4 probes and you needed 5 tests, so we put that down.

On the crime part of the test, they said "Liquid S (the one you tested) was found around the area of a freezer, dishwasher, and sink (or something to this extent). Do the necessary tests to figure out exactly where it came from". Since it was very basic and a strong conductor, we said it was from the freezing coils in the freezer (again, I don't know for sure if that's right). They didn't ask any supplemental questions about water testing, like they do for the powders, liquids, fingerprinting, DNA, etc.

If your curious about water testing at other competitions, this is what we had-

Regionals (Southeast PA)- A few strips very similar to pH paper that you just dipped in and compared to a chart that they provided. You then had to match you data (the crime scene liquid) to data provided for the liquids that the suspects were carrying (their water bottles) and say which it came from. There were 4 strips (so 4 tests total) if I remember correctly, similar to these- http://www.omega.com/pptst/WTS_Series.html. All in all, it wouldn't take you more than 5 minutes max.

States (PA)- Four liquids were given at a shared station, and you had to do two tests on each: pH and some sort of titration (so 8 tests total). For the pH, i think it was done using indicator dyes, though I have almost no clue, because they said we could use pH paper instead if we wanted. As far as the titration, they had someone there to explain how to do it, and written instructions. You filled a container with the liquid, then added a certain number of drops of an indicator and a certain number of drops of another chemical, mixing continuously. You then had a tiny syringe filled with another chemical and you slowly added it, mixing constantly until the solution turned purple (it was pink beforehand, i think). I was terrible at this, so it took me around 20 minutes :shock:, but you could probably have done it in 10 if you had done a titration before (I would recommend practicing this at least once, because it seems like something that they give at a fair number of competitions.)

Hopefully that helps.
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